Hyperthyroidism is a group of disorders in which the thyroid gland produces and releases more thyroid hormone than the body needs, raising the rate of metabolism across nearly every organ system. Because the term refers to overproduction by the gland itself, it is usually distinguished from thyrotoxicosis, the broader clinical state of excess circulating thyroid hormone, which can also arise from leakage of stored hormone during inflammation of the gland or from hormone taken as a drug. The most common causes are Graves disease, an autoimmune condition, and autonomously functioning thyroid nodules. Its symptoms, including palpitations, tremor, heat intolerance and weight loss, can be mistaken for anxiety or other conditions, so diagnosis rests on laboratory testing of pituitary and thyroid hormones and on imaging.
The condition matters for public health because untreated disease can lead to atrial fibrillation, bone loss, and in rare cases a life-threatening crisis called thyroid storm, while treatment itself carries risks of its own. Management has three main routes: antithyroid drugs, radioactive iodine, and thyroid surgery, with beta-blockers used to control symptoms in the meantime. The choice depends on the underlying cause, the patient's age and pregnancy status, and the presence of complications such as eye disease.
This entry first defines hyperthyroidism and places it within normal thyroid physiology, then surveys its epidemiology and causes, including Graves disease, toxic nodules, thyroiditis and rarer conditions. It next describes the clinical picture, extrathyroidal manifestations and complications, and the diagnostic approach. The final sections cover subclinical disease, the three treatment routes, special populations, the history of the field, and current guidelines and open questions.
Definition and terminology
Hyperthyroidism refers to excess synthesis and secretion of thyroid hormones by the thyroid gland itself. The broader term thyrotoxicosis describes the clinical and biochemical state of excess circulating thyroid hormone, whatever its origin. The two terms are often used interchangeably in clinical practice, but the distinction matters for diagnosis. Thyrotoxicosis can arise without the gland overproducing hormone. In painless (silent) thyroiditis and subacute thyroiditis, for example, an inflamed gland releases hormone it has already stored, so the circulating level rises while synthesis does not. Such conditions are classed as thyroiditis rather than hyperthyroidism, and exogenous sources, such as excess levothyroxine, produce thyrotoxicosis in the same way. Accurate terminology therefore depends on identifying where the hormone comes from, not only on measuring its level.
Biochemically, overt hyperthyroidism is defined by a suppressed serum thyrotropin (TSH) together with raised free thyroxine (FT4), free triiodothyronine (FT3), or both. Subclinical hyperthyroidism is defined by a suppressed TSH with free hormone concentrations still within the reference range, and the patient typically has no characteristic symptoms. The threshold for "suppressed" varies with assay sensitivity. Many authorities regard a TSH below 0.1 mIU/L as fully suppressed, and a value between 0.1 and 0.4 mIU/L as low but detectable, a distinction that matters because the two groups differ in cause and in risk of progression. Measurement of FT3 is also needed when TSH is suppressed and FT4 is normal, since isolated T3 elevation, sometimes called T3 toxicosis, can represent an early or mild form of the disease.
Older literature used a number of names that do not map neatly onto modern categories. "Exophthalmic goiter" and "Basedow disease" both describe what is now called Graves disease, the name Graves being taken from Robert Graves and Basedow from Carl von Basedow, who described the condition in the nineteenth century. "Toxic goiter" was applied loosely to several forms of enlarged, overactive thyroid, and it is now avoided because it does not specify whether the gland is diffusely or focally involved. "Plummer disease," named after Henry Plummer, denotes toxic multinodular goiter, in which several autonomously functioning nodules coexist. A solitary autonomous nodule is termed a toxic adenoma. Modern classification instead separates diseases by mechanism, such as autoimmune stimulation, autonomous nodular function, thyroiditis, exogenous intake, and rare central or tissue-resistance causes, and the older labels are retained mainly for historical reference.
Thyroid hormone physiology and feedback control
The thyroid gland synthesizes thyroxine (T4) and triiodothyronine (T3) within follicles, using iodine taken up from the blood and the amino acid tyrosine, which is incorporated into the thyroglobulin protein. The gland secretes mainly T4, which is largely inactive, and most active T3 is produced by conversion of T4 in peripheral tissues. Synthesis is regulated by the hypothalamic–pituitary–thyroid axis. Thyrotropin-releasing hormone (TRH) from the hypothalamus stimulates pituitary release of TSH, which acts on the thyroid through the TSH receptor to drive hormone production and gland growth. Circulating T4 and T3 feed back on both the pituitary and the hypothalamus to suppress TSH secretion. Because the pituitary responds sensitively to small changes in free hormone concentration, a suppressed TSH is usually the earliest laboratory sign of excess hormone, often appearing before free hormone levels rise beyond the reference range.
At the cellular level, T3 binds nuclear thyroid hormone receptors, which act as transcription factors and alter the expression of many genes that control heat production, heart rate and protein turnover. This genomic action explains the broad, systemic effects of excess hormone, which extend well beyond the thyroid gland.
Thyroid hormone physiology and feedback control
The thyroid gland builds its hormones inside follicles, spherical units lined by epithelial cells and filled with a protein-rich colloid. Its main products are thyroxine (T4), which carries four iodine atoms, and triiodothyronine (T3), which carries three. Both derive from the amino acid tyrosine. Follicular cells concentrate iodide from the blood through the sodium-iodide symporter and deliver it into the colloid, where thyroglobulin, a large glycoprotein, is iodinated at tyrosine residues. Coupling of the iodotyrosines produces T4 and T3 while they remain attached to thyroglobulin. When TSH stimulates the gland, follicular cells take up the colloid, and proteolysis releases the free hormones into the circulation. The gland secretes mostly T4, which is comparatively inactive and has a long half-life of about eight days, so it acts largely as a reservoir. Most circulating T3, the more potent form, is generated in peripheral tissues when deiodinase enzymes remove one iodine atom from T4. Because iodine and selenium-dependent deiodinases are both required, the synthetic chain depends on nutritional status as well as on the gland itself.
The hypothalamic–pituitary–thyroid axis links these steps. Neurons in the paraventricular nucleus of the hypothalamus release thyrotropin-releasing hormone (TRH), which reaches thyrotroph cells of the anterior pituitary and stimulates them to secrete thyrotropin (thyroid-stimulating hormone, TSH). TSH is a heterodimer with a common alpha subunit and a hormone-specific beta subunit. It binds the TSH receptor, a G protein–coupled receptor on the follicular cell surface. Receptor activation raises intracellular cyclic AMP and drives iodide uptake, thyroglobulin synthesis and hormone release, and it also sustains growth of the gland. The result is an inverse relationship between TSH and circulating free hormone levels. Thyroid hormones feed back on both upstream levels. In the pituitary, T3 acts through the TRβ2 isoform to reduce transcription of the TSH subunit genes, partly by dampening the activity of the transcription factor GATA2, and in the hypothalamus it suppresses TRH gene expression. Through this loop the free T4 and T3 concentrations are held within a fairly narrow range in healthy people.
The feedback design explains why suppressed TSH is the earliest sensitive sign of excess hormone. Pituitary thyrotrophs respond steeply to small changes in free T4 and T3, so TSH secretion falls well before hormone concentrations rise far enough to be measured as clearly abnormal. In overt disease, TSH is typically undetectable or very low while free T4 and/or T3 is elevated. In subclinical disease, TSH is low but free hormone levels remain within the reference range, a pattern that is discussed in its own section. Because the axis depends on feedback, any lesion that bypasses it, such as a TSH-secreting pituitary tumour, can produce high hormone levels with TSH that is not suppressed.
At the tissue level, T3 acts mainly through nuclear thyroid hormone receptors, chiefly TRα and TRβ. These are ligand-dependent transcription factors that bind DNA at thyroid hormone response elements, usually as heterodimers with the retinoid X receptor. When T3 is bound, the receptor recruits coactivators and increases transcription of target genes; in the unliganded state it can repress the same genes. The two isoforms have different tissue distributions, with TRα predominating in the heart and skeletal muscle and TRβ in the liver and pituitary, which accounts for part of the tissue-specific response to hormone. Among the genes regulated are those for Na⁺/K⁺-ATPase and other components of the energy-consuming machinery of the cell. Increased expression of these genes raises oxygen consumption and heat production, which is the basis for the elevated basal metabolic rate of excess hormone. T3 also increases the expression of beta-adrenergic receptors and of cardiac contractile proteins, which sensitizes the heart to catecholamines and raises heart rate and contractility. In skeletal muscle and other tissues, altered transcription of genes controlling protein synthesis and degradation shifts the balance toward protein breakdown, which underlies the muscle wasting seen in untreated disease.
Epidemiology and risk factors
Prevalence
Reported rates depend on the thresholds, the assays and the population sampled. In a community study of older adults, overt hyperthyroidism was 0.26% and subclinical hyperthyroidism was 0.78% among untreated participants. For the general population, overt hyperthyroidism is estimated at about 0.2–1.3% in iodine-sufficient areas, and the prevalence in the United States is approximately 1.2%. Subclinical disease is harder to pin down. Pooled figures lie between 0.5 and 3.0% of the population, other reviews settle near 2 percent, and typically quoted prevalences of 0.7 – 2% are used for older people.
Sex and age
Women are affected much more often than men. Graves disease, the most common cause of hyperthyroidism in developed countries, is 5-10 times more frequent in women and is most common between 30 and 60 years of age. In a Danish study defined by low TSH, the annual rate was 38.7/100,000 in women and 2/100,000 in men, a ratio of roughly nineteen to one.
Older adults carry a larger share of subclinical disease. One review of elderly populations places it at 5% to 10% of elderly subjects and notes that it is especially prevalent in elderly women. The prevalence also depends on age, gender, and iodine intake. In later life, nodular disease becomes a more important substrate, since suppressed TSH in people with nodular goitre is the earliest biochemical marker of thyroid autonomy and onset of hyperthyroidism. Graves disease, subacute and silent thyroiditis also occur in older people, though these are less common.
Iodine status and contrast exposure
Iodine status is the main regional determinant of which cause predominates. Where intake is low, suboptimal iodine intake induces nodular goitre, and, by time the nodules become autonomic, hyperthyroidism develops. Danish data assign 70–80% of hyperthyroidism cases to Graves disease in areas with adequate iodine intake, while in iodine-deficient areas it accounts for about 50% of cases. Iodine excess can also trigger disease in people who already have autonomous tissue, a process known as the Jod-Basedow phenomenon.
Iodine fortification has produced a measurable side effect. Adding iodine to staples such as bread and salt has caused a transient increase in the incidence of hyperthyroidism during the first years.
Iodinated contrast media are a second source of abrupt iodine excess, and a single CT injection can deliver approximately 45 times the recommended daily intake. Reported rates vary widely, with prospective studies showing a reported risk of hyperthyroidism ranging from 0.2% to 9%, while a meta-analysis put overt cases at roughly 0.3%. Susceptibility is concentrated in people with autonomous nodules or multinodular goiter, and in iodine-deficient regions the incidence has been estimated at up to 1.7% per year.
Other risk factors
Smoking is the best-documented lifestyle factor. In a cohort of women, the hazard ratio among current smokers was 1.93, and among past smokers it was 1.27, and among current smokers, the hazard ratio increased with the intensity of smoking and was 2.63 among women who smoked 25 or more cigarettes daily. Obesity, by contrast, was associated with a decreased risk of Graves' hyperthyroidism.
Family history carries substantial weight. A family history of hyperthyroidism, Hashimoto's disease, or a number of other autoimmune disorders is associated with a significantly higher risk. Genetic predisposition is thought to dominate: one review attributes 79% of the risk for GD to genetic predisposition, while environmental factors account for 21%. Among environmental associations, smoking, iodine excess, selenium and vitamin D deficiency, and the occupational exposure to Agent Orange have been associated with GD. Radiation exposure is also listed among factors that increase autoimmune thyroid disease risk. Consumer-level sources additionally list pregnancy as a factor that may raise the risk of Graves disease.
Graves disease: autoimmune pathogenesis
Stimulating antibodies and the TSH receptor
Graves disease is driven by autoantibodies against the thyrotropin receptor (TSHR), a G protein–coupled receptor on the surface of thyroid follicular cells. The TSHR is the principal autoantigen in the disease, and the same receptor is also targeted in the orbit. The antibodies that matter pathogenically are thyroid-stimulating antibodies (TSAb). They bind the receptor and mimic TSH, so the follicular cells keep synthesizing and releasing thyroxine (T4) and triiodothyronine (T3) even when circulating hormone levels are high. Because this stimulation does not depend on the pituitary, TSH is suppressed. This is the defining biochemical feature of the disease.
Continuous stimulation also drives growth. Thyroid tissue from patients shows increased expression of the TSHR gene and hyperplasia of the follicular epithelium, which together explain the diffuse enlargement of the gland. In the antibody class most relevant to disease, the stimulating antibodies are of the IgG1 subclass, which is typically associated with an early, T-helper-1-dominated response. Antibodies against thyroglobulin and thyroid peroxidase are also common in these patients, but they are regarded as non-pathogenic markers of the broader thyroid autoimmune process.
Immune mechanisms and genetic susceptibility
The disease is thought to arise when peripheral tolerance fails. Self-reactive lymphocytes that escape central deletion in the thymus are normally held in check by regulatory mechanisms; when these fail, T cells and B cells generate sustained antibody responses against the receptor. Histologically, the thyroid of affected patients shows lymphocytic infiltration and inflammation, and the immune response involves both T-helper-1 and T-helper-2 pathways. This mixed pattern has been used to explain why the disease can be intense in some patients and modest in others.
Heritability is substantial, and the genetic component is best characterized at the TSHR locus. Fine-mapping studies narrowed the disease-susceptibility region to a sequence of about 40 kilobases in intron 1 of the TSHR gene, which contains several disease-associated single-nucleotide polymorphisms in strong linkage disequilibrium. Other susceptibility loci, including immune-regulatory genes, contribute as well, and environmental triggers such as smoking and stress interact with this background. The exact functional effect of the TSHR variants on receptor expression or thymic selection remains under study.
Remission and relapse follow from the nature of the immune process. Antibody production waxes and wanes, and antithyroid drug therapy suppresses hormone synthesis without necessarily eliminating the underlying autoimmune clone. Relapse occurs in roughly 20–34% of patients who stop drug therapy even after the stimulating antibody has become negative. The pattern of antibody change during treatment also matters: in one series, remission was reported in 89% of patients with a smooth fall in antibody levels, 37% with a fluctuating pattern, and 20% with a smoldering pattern.
Clinical markers
The clinical picture is shaped by the antibody mechanism. A diffuse goiter, usually confirmed by ultrasonography, is characteristic, and in practice the combination of hyperthyroidism, a diffuse gland and positive antibodies is often enough to diagnose the disease without further testing. Eye involvement is the most common extrathyroidal manifestation. Orbital fibroblasts and preadipocytes express the TSHR, and persistent stimulation of these cells, possibly in synergy with the insulin-like growth factor 1 receptor, leads to glycosaminoglycan deposition and tissue expansion behind the globe. Skin changes, particularly pretibial myxedema, are attributed to the same process. Their detailed staging and management belong to the discussion of Graves orbitopathy and dermopathy.
Antibody measurement in diagnosis, prognosis and withdrawal
Assays for TSHR antibodies (TRAb) fall into two broad groups: binding inhibition immunoassays, which measure whether antibodies displace TSH from the receptor, and cell-based bioassays, which measure functional stimulation. Because the biological activity of the antibody, not just its presence, determines clinical effect, bioassays add information about the risk of orbital disease and the potency of stimulation. Clinical utility is still debated, however. In one pilot study, a commercial assay showed a sensitivity of 100% and a specificity of 97% for confirming the diagnosis in 99 patients, but the cut-off values differ between manufacturers, and a positive result at one laboratory cannot be read directly against another.
For prognosis, the initial antibody level is informative. In the RISG cohort of patients treated with antithyroid drugs, 63.3% of those with a baseline TRAb below 10 IU/L reached remission at 24 months, compared with 39.4% of those above that threshold. In a Chinese series of patients with high antibody titers at diagnosis, remission after drug withdrawal fell from about 60% in the lowest titer group to about 30% in the highest. Single measurements taken during treatment have been used to guide decisions. In the RISG study, a measurement at 6–8 months identified a subgroup in whom treatment could be stopped sooner, whereas measurements at other times were less informative. Guidelines currently use antibody results as one element of the decision to stop drugs, alongside the duration of therapy and the thyroid hormone profile, and not as a stand-alone test.
Toxic nodular goiter and toxic adenoma
Toxic nodular disease arises when one or more thyroid nodules begin to make thyroid hormone without regard to the pituitary's signal. Their excess output raises serum T4 and/or T3 and suppresses TSH, producing clinical thyrotoxicosis. Because the nodules run on their own, the rest of the gland is switched off. On radionuclide imaging, iodine accumulates in one or more discrete nodules while uptake in the remainder of the gland is usually suppressed. Suppression of the non-nodular tissue reflects low TSH rather than any direct effect of the nodule, since exogenous thyroid hormone produces no further suppression, while TSH stimulates iodine uptake in the previously inactive areas. Nodules that are not hyperfunctioning often appear cold on the same scan, so a multinodular gland can contain both kinds of focus. One or more nodules in this condition are often cold and show no uptake on routine scintiscan.
Toxic multinodular goiter is the older name for this entity, and it is also called Plummer disease. TNG, or Plummer's disease, was first described by Henry Plummer in 1913. In the classical literature, the term "toxic goiter" was applied to both multinodular and solitary forms, and in modern usage the two are separated by the number of hyperfunctioning nodules. A solitary toxic adenoma is a single autonomous nodule, which is typically a single large thyroid nodule accompanied by clinical and biochemical hyperthyroidism. Hyperthyroidism tends to appear predominantly when single nodules are larger than 2.5 cm in diameter.
The molecular basis of autonomy is best understood for activating mutations of the TSH receptor, a G-protein-coupled receptor encoded by a gene. In one surgical series, five out of six (83%) hyperfunctioning nodules within toxic multinodular goiters harbored a TSH receptor mutation. Among toxic adenomas, 8 out of 11 (72%) nodules harbored a TSH receptor mutation. These mutations were somatic and heterozygous, and they conferred constitutive activity, so the receptor signals even without TSH. Reported frequencies vary widely across studies, ranging from 10-80%. Notably, only about 60% of toxic nodules have TSHR mutations, and only a very few have G-protein mutations. The remaining nodules have autonomy of undetermined cause. Multinodular disease is thought to start with iodine-driven hyperplasia: low iodine levels cause hyperplasia of the thyroid gland through increased TSH levels, and when iodine becomes adequate, the gland enters a resting phase. Repeated cycles of proliferation then raise the chance that a somatic change in the TSH receptor fixes the nodule in an active state.
Epidemiologically, toxic nodular disease is the second most common cause of hyperthyroidism in most settings and the most common in some. TNG is the second most common cause of hyperthyroidism in the Western world, after Graves disease. In elderly individuals and in areas of endemic iodine deficiency, TNG is the most common cause of hyperthyroidism. In the United States it accounts for approximately 15-30% of cases of hyperthyroidism, second only to Graves disease. In endemic iodine deficiency, the balance shifts: TNG accounts for approximately 58% of cases of hyperthyroidism, while Graves disease accounts for 40% of cases. A survey from Spain illustrates the regional contrast. Among thyrotoxic patients studied there, 58.6% of the cases were diagnosed of nodular goitre, 30.3% of Graves' disease, and the proportion of nodular disease was higher in the inland zone than on the coast. Prevalence also rises with age, since the prevalence of toxic nodular goiter increases with age and in the presence of iodine deficiency.
Iodine supplementation changes this picture in a complicated way. Correction of iodine deficiency diminishes the incidence of toxic nodular goitre. Early in correction, however, hyperthyroidism can increase. A high prevalence of nodular autonomy usually results in a further increase in hyperthyroidism when iodine intake rises through salt iodisation. This increase is transient, because iodine sufficiency normalises thyroid activity which, in the long term, reduces nodular autonomy.
Unlike Graves disease, toxic nodular disease is generally not self-limiting. Hyperthyroidism caused by toxic adenomas generally does not remit spontaneously. Hyperthyroidism from toxic multinodular goiter and toxic adenoma is permanent and usually occurs in adults. Because the autonomy is fixed by the nodule's own biology rather than by an antibody that waxes and wanes, antithyroid drugs control hormone levels without eliminating the cause. Antithyroid drugs are typically used first to restore euthyroidism, and radioactive iodine ablation usually is recommended as the definitive therapy. Long-term, high-dose antithyroid medication is not recommended, and toxic multinodular goiters and toxic adenomas probably will continue to grow slowly during antithyroid pharmacotherapy.
Definitive treatment differs somewhat between the two forms. For a solitary toxic adenoma, toxic adenoma cure rates with RAI therapy have been reported as high as 95%. Radioiodine is taken up by the hot tissue, which the radioactive isotope then destroys, while the suppressed remainder of the gland is largely spared. Surgery remains an option; the standard operation for a solitary adenoma is hemithyroidectomy or isthmusectomy. Because hot nodules are rarely malignant, FNAC is not recommended routinely for them, although a cold component in a multinodular gland still requires evaluation. Scintigraphy is also the main tool for distinguishing these conditions from Graves disease, which typically shows diffuse rather than focal uptake, a distinction that can become critical when antibody results are negative.
Thyroiditis and exogenous causes
Painless and postpartum thyroiditis
Silent thyroiditis produces thyrotoxicosis without any increase in hormone production. Painless or silent thyroiditis is a syndrome of thyrotoxicosis due to release of preformed thyroid hormones from disrupted thyroid follicles. The circulating excess is therefore stored thyroid hormone that escapes from damaged follicular cells, not newly made hormone, which is why the radioiodine uptake test is low during this phase. Postpartum thyroiditis has the same biochemical pattern and differs mainly in timing, since the diagnosis is made when the condition arises within 12 months of delivery.
Reported frequency varies widely. Prevalence estimates range from 1.1 to 21.1%, reflecting differences in study populations and how often patients were monitored. One North American study reported 6.7%. Current thinking treats the postpartum form as an acute expression of an underlying chronic autoimmune thyroid disease that becomes visible after pregnancy. Recurrence is common: after a first episode, a subsequent pregnancy carries about a 70% chance of another. The course is usually triphasic, with a thyrotoxic phase of one to three months, a hypothyroid phase that can last up to six months, and then recovery. A substantial minority develops permanent hypothyroidism; one textbook estimate places this at 30–40% within a decade. Antithyroid drugs do not help, because they block synthesis rather than leakage.
Subacute granulomatous (de Quervain) thyroiditis
This form is an acute inflammatory disease that often follows a viral respiratory infection. The infection is thought to trigger inflammatory destruction of thyroid follicles, and the gland usually resumes normal hormone production after several months. Pain is the defining feature: it typically starts in the front of the neck and may spread toward the jaw and ear, alongside flu-like malaise. Histology shows multinucleated giant cells within granulomas, which gives the disease its name. Inflammatory markers rise sharply, with significantly increased blood sedimentation and massively increased CRP, while radioiodine uptake falls below about 5%. Doppler imaging is a further clue: vascularity is decreased or absent in this condition, whereas it is increased in most cases of Graves disease.
The hyperthyroid phase usually lasts several weeks. Full recovery is often reported at four to six months from onset, though recurrence of the initial symptoms can extend the course. Estimates of permanent hypothyroidism differ: one review calls progression rare, while another gives a figure of about 15%. Treatment is symptomatic. Salicylates and NSAIDs can be used in mild or moderate cases. In severe cases, corticosteroids generally relieve symptoms within 24–48 hours, with prednisone started at 40 mg daily and tapered over several weeks. Beta blockers can control thyrotoxic symptoms. Thyrostatic drugs are not advised, because the thyrotoxic phase reflects release rather than production, and treatment would only worsen the subsequent hypothyroid phase.
Amiodarone-induced thyrotoxicosis
Amiodarone, an antiarrhythmic drug, contains a large amount of iodine and resembles thyroxine structurally, and it can disturb thyroid function in several ways. Amiodarone-induced thyrotoxicosis develops in about 3% of treated patients in North America.
It is divided into two types with different mechanisms. Type 1 occurs when the iodine load drives excess hormone synthesis in a gland that is already abnormal. It occurs in subjects with an abnormal thyroid, such as goiter or latent autoimmune disease, with the iodine load triggering autonomous hormone production. Type 2 is a destructive inflammatory process in an apparently normal gland, releasing stored hormone. Mixed forms, in which both mechanisms appear to contribute, are also described.
The two types are distinguished by combining history, imaging and isotope studies. A careful history and examination for pre-existing thyroid disease come first. Radioiodine uptake is usually very low or suppressed in type 2, but normal or increased in type 1 despite the iodine load. Color flow Doppler sonography shows absent hypervascularity in type 2 and increased vascularity in type 1. Some studies have also used antithyroid peroxidase antibodies to support a type 1 classification. Treatment follows the classification: type 1 is best treated with antithyroid drugs and potassium perchlorate, while type 2 is typically self-limited and can be treated with steroids. When the type cannot be settled, it is sometimes considered safe to treat for both with a combination of therapies.
Factitious thyrotoxicosis and drug or iodine triggers
Factitious thyrotoxicosis results from taking thyroid hormone preparations such as levothyroxine or liothyronine, whether intentionally or by accident. Its biochemical picture resembles Graves disease, with suppressed TSH and raised free hormone, but the gland is not stimulated. Radioiodine uptake is therefore low, and serum thyroglobulin, which is released in parallel with hormone in thyroiditis, is typically suppressed. A disproportionately high T3 relative to T4 can suggest liothyronine use.
Iodine excess can also provoke hyperthyroidism. The Jod-Basedow phenomenon occurs when an iodine load, from iodinated contrast media, supplements or certain drugs, triggers hormone synthesis in a gland with autonomous nodules or latent Graves disease. Certain drugs cause inflammatory thyroiditis instead: immune checkpoint inhibitors used in cancer treatment and interferon-alpha can both produce painless thyroiditis, which follows the same leak-and-recover course described above.
Rare and special causes
Most hyperthyroidism arises from the thyroid gland itself, but a small group of conditions produces excess thyroid hormone or excess thyroid stimulation through other routes. Several of them matter mainly because they mimic common disorders and are easily misdiagnosed. Two of them, TSH-secreting pituitary adenoma and resistance to thyroid hormone, are often grouped together as inappropriate secretion of TSH, because both produce a pattern of raised thyroid hormone with a TSH level that fails to fall as expected.
Central cause: TSH-secreting pituitary adenoma
TSH-secreting pituitary adenomas (TSH-omas) are rare tumours of the thyrotroph cells of the anterior pituitary. Estimates place them at about 0.5% to 2% of pituitary adenomas, and they are the least common of the functional pituitary tumours. Because the tumour secretes TSH autonomously, the diagnostic hallmark is elevated or high-normal free T4 and T3 accompanied by a TSH that is not suppressed. An elevated alpha-subunit to TSH ratio improves the biochemical sensitivity of the diagnosis.
The condition is easy to miss. Patients often present with the same symptoms as Graves disease, such as heat intolerance, weight loss, fatigue and, in some cases, exophthalmos, so the tumour is frequently treated as primary hyperthyroidism with antithyroid drugs or radioiodine. Mass effects may appear only later, as headache, visual field defects or hypopituitarism, and many tumours are already large and invasive when found. Around 16% of TSH-omas co-secrete growth hormone and about 10% co-secrete prolactin, so signs of acromegaly or hyperprolactinaemia can point toward the correct diagnosis. Pituitary MRI, and in some centres a TRH stimulation test or a somatostatin suppression test, help separate the tumour from thyroid hormone resistance and from assay interference. Treatment is usually transsphenoidal surgical resection, with somatostatin analogues or radiotherapy used as adjuncts. Ectopic TSH-secreting tumours outside the sella are described only in case reports.
Gestational transient thyrotoxicosis
Gestational transient thyrotoxicosis (GTT) is a self-limiting form of hyperthyroidism confined to the first half of pregnancy. Human chorionic gonadotropin (hCG) has a weak thyrotropic action, acting on the TSH receptor, and the resulting biochemical pattern is elevated free T4 with suppressed TSH. The condition is strongly linked to hyperemesis gravidarum. In one series of 143 women hospitalised for hyperemesis, thyrotoxicosis was present in 48.3%, and GTT accounted for most of these cases (45.5%). The severity tracks serum hCG concentration, so the condition is more frequent and more pronounced in multiple pregnancies and in very high hCG states.
Most cases are mild, need no antithyroid drugs, and normalise by the second trimester. Because the thyroid gland is not usually enlarged and TRAb may be negative, the differential diagnosis with Graves disease in early pregnancy can be difficult, and the clinical picture of symptoms and time course is often decisive. Rare severe cases complicated by thyroid storm have been reported, which suggests that clinicians should watch closely in high-hCG pregnancies. A subset of recurrent or unusually prolonged cases reflects a germline activating mutation in the TSH receptor gene that makes the receptor hypersensitive to hCG; in such cases severe, lasting GTT with normal hCG has been proposed as an indication for TSH receptor gene sequencing.
Functional thyroid tissue outside the thyroid gland
Struma ovarii is a rare ovarian teratoma containing mature thyroid tissue, which can secrete enough hormone to cause hyperthyroidism. Thyroid cancer metastases may also produce excess hormone when they retain the ability to synthesise thyroid hormone, most often in invasive or metastatic follicular carcinoma. In both situations the excess hormone comes from tissue that is not the primary thyroid gland, so the diagnosis depends on localising the functional tissue, for example by radioiodine imaging, and on treating the lesion itself, usually by surgical removal with or without radioiodine therapy.
Resistance to thyroid hormone
Resistance to thyroid hormone (RTH) is a genetic disorder caused by germline mutations in the thyroid hormone receptor genes. Most cases involve THRB, which encodes the beta receptor, and the usual pattern is a heterozygous mutation with dominant-negative activity, inherited in an autosomal dominant manner. The condition is rare, with a prevalence reported at about 1 in 40,000 live births, and affects both sexes equally.
The defining feature is raised free T4 and T3 with a TSH that is normal or raised instead of suppressed. Because tissue responsiveness varies, the clinical picture can be mixed: some patients have tachycardia, hyperactivity or goitre, while others are euthyroid and symptom-free, and the pattern can differ between tissues in the same person. The pituitary is often relatively insensitive to thyroid hormone while peripheral tissues respond more normally, which explains why TSH stays elevated. Diagnosis rests on the combination of raised hormone, non-suppressed TSH, absence of a pituitary tumour on MRI, and confirmatory genetic testing. Treatment is directed at symptoms, and ablative therapy is generally avoided, since treating the thyroid as if it were overactive does not correct the underlying receptor defect.
Clinical manifestations of excess hormone
Thyroid hormone acts on nearly every organ system, so the clinical picture of excess hormone is broad. Its features can be grouped by mechanism: a general rise in metabolic rate, an overactive adrenergic-like state, neuromuscular effects, and cardiovascular changes. The mix varies with age, the degree of excess and how long it has persisted, but the pattern is the same whatever the underlying cause.
Hypermetabolic symptoms
The hypermetabolic group is the most characteristic. Weight loss despite increased appetite, palpitations and tachycardia, fine tremor of the fingers, heat intolerance, perspiration, and a warm moist skin are all attributed to increased metabolic rate. Patients often describe poor tolerance of warm surroundings and excessive sweating, which reflects increased cutaneous blood flow. Fatigue and weakness are common, and gastrointestinal changes are frequent: increased appetite, weight loss, or increased frequency of bowel movements may all occur, with the bowel changes attributed to intestinal hypermotility.
Adrenergic and neuromuscular symptoms
Sympathetic-type symptoms are typically prominent in younger patients. A typical patient with moderate disease will complain of palpitations, tremulousness, heat intolerance and excess sweating, increased appetite with weight loss, frequent bowel movements, fatigue, and excess anxiety and irritability. Tremor is among the most frequent findings, seen in 70% of patients, and patients often note poor handwriting or difficulty with fine motor tasks. Sleep disturbance is also common, and many patients also suffer from nervousness, irritation, anxiety, or mood disturbances, especially depression.
Proximal muscle weakness, which affects the hip and shoulder girdles, is a notable neuromuscular feature. In one series 67% of patients had muscle weakness, and it was a presenting symptom in 36%. Because weakness can be the first complaint, it is a useful reason to consider thyroid function in patients who do not report the classic adrenergic symptoms.
Cardiovascular effects
The heart is among the most affected organs. Tachycardia and palpitations are common, and a hyperdynamic precordium may be palpated ... There are brisk carotid and peripheral arterial pulsations. These findings reflect a raised cardiac output and a widened pulse pressure, driven by a larger stroke volume and reduced peripheral resistance. Atrial fibrillation with rapid ventricular response may be found in up to 20% of patients with thyrotoxicosis and it is more likely to be present in men and the elderly. Thyrotoxicosis can also aggravate pre-existing heart disease and can also lead to AF, congestive heart failure, or worsening of angina pectoris. Most of these abnormalities return to normal once a euthyroid state has been achieved, although AF may persist in a minority.
The risk of atrial fibrillation is concentrated in older patients. Paradoxically, patients older than 60 years are oligo- or asymptomatic but are at a higher risk of AF occurrence. Atrial fibrillation is also the most common cardiac complication in patients with hyperthyroidism and may be the initial presentation of hyperthyroidism. Embolic stroke related to atrial fibrillation is among the neurological complications noted in thyrotoxicosis.
Atypical presentations in older adults
In older people, the classic picture is often replaced by a quieter one. Older patients may have "apathetic" hyperthyroidism, in which more typical symptoms and signs reflecting sympathetic activation such as tremor and hyperactivity are absent, and the disorder is sometimes described as apathetic thyrotoxicosis. In this group, fatigue, anorexia, weight loss, apathy, agitation, or cognitive decline are more common, and symptoms may be absent or few in the elderly. Non-specific findings such as muscle weakness, persistent vomiting, hypercalcemia, and worsening osteoporosis should also prompt consideration of the diagnosis.
These atypical features delay recognition. Weight loss, low mood and reduced activity are easily attributed to ageing or to coexisting illness, and symptoms and signs of heart failure and complicating atrial fibrillation (AF) may dominate the clinical picture and mask the more classic endocrine manifestations of the disease. Drugs and acute or chronic illness can also confound the interpretation of thyroid tests, so the diagnosis may therefore be delayed or missed. For this reason, a thyroid evaluation is commonly warranted in older patients presenting with new atrial fibrillation, unexplained weight loss or heart failure.
Graves orbitopathy and dermopathy
Orbitopathy: frequency and mechanism
Graves orbitopathy (GO), also called thyroid eye disease, is the most common extrathyroidal manifestation of Graves disease. Ophthalmopathy is found in 30% of patients, dermopathy in 4%, and thyroid acropachy in 1%. The disease rests on a shared antigen: the thyroid stimulating hormone receptor (TSHR) is believed to be the autoantigen shared between thyroid and orbit tissues. TSH-R/IGF-1R crosstalk represents the principal mechanism of activation of orbital fibroblasts, and TSHR activation enhances the differentiation of orbital preadipocytes into adipocytes, favoring the expansion of orbital adipose tissue.
Three processes are usually cited: inflammation of the periorbital soft tissues, overproduction of glycosaminoglycans by orbital fibroblasts, and hyperplasia of adipose tissue. Hyaluronan is the key glycosaminoglycan. Its polyanionic charge and high osmotic pressure render it highly hydrophilic, so the extraocular muscles swell dramatically. Because the orbit is a bony cavity with fixed volume, the increase in orbital volume and edema in a limited space mechanically explains manifestations of GO including exophthalmos, periocular soft tissue swelling, extraocular muscle enlargement or dysfunction, and optic nerve compression.
Activity, severity and staging
GO follows a biphasic course with an active or progressive phase followed by a quiescent or stable phase, and the active phase generally lasts for 18–36 months. Clinicians therefore grade two separate things: activity, which reflects the inflammatory process, and severity, which reflects quality of life and the risk of vision loss. Activity is commonly assessed with the seven-item clinical activity score (CAS), where GO is defined as active if CAS is ≥ 3/7. A ten-item version adds measures of recent change, such as an increase in exophthalmos of at least 2 mm, which helps track progression.
Severity is graded on the EUGOGO scale as mild, moderate-to-severe, or sight-threatening. Most patients have mild disease, but about 3–7% of GO patients exhibit a severe sight-threatening form of the disease due to corneal exposure or compressive optic neuropathy. Diplopia, from restricted extraocular muscle movement, is a common functional complaint in the moderate and severe categories. The VISA system, which scores vision, inflammation, strabismus and appearance, is an alternative framework. Timing matters for treatment: anti-inflammatory or immunosuppressive treatment is significantly less effective after 18 months of disease duration.
Pretibial myxedema and acropachy
Pretibial myxedema, also called thyroid dermopathy, is a localized deposit of glycosaminoglycans in the skin. Its pretibial area was most commonly involved (99%), nonpitting edema was the most common form of dermopathy (43%), and the majority of patients had coexisting ophthalmopathy (96%). Onset generally comes later than eye disease: dermopathy typically follows the onset of ophthalmopathy by 6-12 months, and most commonly occurs 1-2 years after the diagnosis of Graves disease.
Thyroid acropachy is rarer. It is clinically characterized by clubbing of the fingers and the toes, periosteal proliferation of the shafts of the phalanges and other distal long bones, and swelling of the soft tissues overlying affected bony structures. When present, acropachy usually follows dermopathy. Both skin changes are considered markers of severity: the presence of dermopathy and acropachy predicts the severity of the autoimmune process. Reported frequencies of dermopathy vary between sources, from 0.5–4.3% of patients with Graves disease, rising to 13% in those with severe eye disease, to 1 to 4 percent of all patients with Graves disease, approaching 15 percent in severe Graves-related ophthalmopathy. Local corticosteroid application is the standard therapy, and response to therapy is good in mild cases and poor in severe cases.
Interaction with treatment decisions
Smoking is a well-established risk factor. Tobacco use has been shown to be a strong risk factor for the development of Graves ophthalmopathy, pretibial myxedema, and thyroid acropachy. Smoking is thought to affect both humoral and cell-mediated immunity, although the mechanism is not fully established. Cessation is therefore a standard part of management.
Eye disease does not reliably follow the hormone level. Restoring euthyroidism does not always resolve GO, so eye activity and severity are assessed separately from thyroid function. Radioactive iodine can transiently worsen GO in some patients, particularly smokers and those with pre-existing moderate or severe disease, which is why corticosteroid cover is often given around treatment. Intravenous glucocorticoids are used for active moderate-to-severe GO, and their use and timing depend on the activity and severity grading described above. Surgical decompression is reserved for sight-threatening or disfiguring disease.
Thyroid storm and complications
Thyroid storm
Thyroid storm, also called thyrotoxic crisis, is an acute and life-threatening hypermetabolic state that develops in people with thyrotoxicosis when hormone release becomes excessive. It is induced by excessive release of thyroid hormones (THs) in individuals with thyrotoxicosis. The condition is usually accompanied by evidence of multiorgan decompensation, usually occurring in the presence of an inciting trigger. Typical findings include high fever, a rapid pulse, signs of heart failure, vomiting or diarrhoea, liver dysfunction and changes in mental state. Central nervous system symptoms such as agitation, delirium and coma can also occur. Because the picture overlaps with other emergencies, the diagnosis rests mainly on clinical features. The diagnosis of thyroid storm is based on clinical features, not on laboratory test findings. Scoring systems such as the Burch-Wartofsky point scale are used as aids, but they did not always fully reflect clinical severity, particularly in the presence of comorbid conditions.
Published estimates of frequency and mortality vary. One review gives an occurrence of 1-2% of thyrotoxicosis patients with a mortality rate of 10-20%, while another cites a US survey in which the diagnosis was made in 16.2% of thyrotoxicosis admissions and notes a mortality risk of more than 10%, mostly from multi-organ failure and heart failure.
Triggers are common and often identifiable. Abrupt discontinuation of antithyroid medications and infection are the most common underlying triggers. Other precipitants include surgery, trauma, and iodine exposure. Iodine-containing contrast media can also trigger the condition in sporadic cases, and this occurs more commonly in iodine-deficient areas, where thyroid storm is mostly related to iodine contamination in patients with thyroid autonomy. Obstetric events and radioiodine treatment have been reported as precipitants as well.
Management
Treatment starts before laboratory results are available when the clinical picture is convincing. Given the mortality risk of untreated thyroid storm, clinical features such as goiter, high fever, marked tachycardia, and a precipitating illness warrant prompt treatment initiation even before test results become available. Care is supportive first, with cardiovascular stabilization, respiratory support, and temperature control, along with identification and treatment of precipitating factors.
The drug regimen has several parts. Antithyroid agents block thyroid hormone synthesis, beta blockers counteract adrenergic effects, and corticosteroids reduce peripheral conversion and support adrenal function, with iodine usually given only after thionamides have been started, so that the gland cannot use the iodine to make new hormone. Beta-blockers are generally used in the absence of severe heart failure. Methimazole and propylthiouracil are both used, and recent data suggest comparable outcomes with methimazole. Plasmapheresis is reserved for cases that do not respond to these measures. The plasmapheresis should be the last resort if above measures fail.
Response is assessed over days rather than hours. The most relevant indicators include resolution of hyperthermia, reduction in heart rate, improvement in haemodynamic stability, and recovery of mental status. Hormone levels fall gradually, and TSH should not be used to guide early treatment response. If stabilisation is not seen within a day or two, clinicians look again for an ongoing trigger, poor drug absorption, cardiomyopathy or the need for stronger therapy. Failure to demonstrate clinical stabilisation within 24–48 h should prompt reassessment.
Long-term complications of untreated disease
Cardiovascular involvement is the most serious long-term risk. Cardiac involvement also has the potential to be the most serious complication. Atrial fibrillation is the most frequent of these. AF is the most common cardiac complication in patients with hyperthyroidism and may be the initial presentation of hyperthyroidism. Thyroid hormone excess also creates a hypercoagulable state, as elevated levels of different clotting factors have been reported in affected individuals, which adds to the embolic risk. Atrial fibrillation in hyperthyroidism usually resolves once euthyroidism is restored, but a significant number of patients remain in chronic AF and require electrical cardioversion.
Heart failure is typically a consequence of prolonged disease. Heart failure is most commonly seen as a result of longstanding, often untreated, thyrotoxicosis with coexistent atrial fibrillation. The pattern is usually high-output failure, and cardiac dilation (increase in the size of the heart cavities, which actually thins the heart muscle) and congestive heart failure are recognised consequences. Thyrotoxic cardiomyopathy with severe left ventricular dysfunction is rarer. Thyrotoxic cardiomyopathy with severe LV dysfunction is rare.
Skeletal damage develops more slowly. Excess hormone accelerates bone turnover and increases urinary and faecal losses of calcium and phosphate. Long-term high levels cause bone mineral density to drop, especially in women who have gone through menopause, which raises the risk of fracture.
Pregnancy and neonatal hyperthyroidism
Overt hyperthyroidism in pregnancy carries risks for both mother and child. Overt hyperthyroidism may lead to complications, such as miscarriage, pre-eclampsia, premature labor, congestive heart failure in the mother and fetus, thyroid storm in the mother, intrauterine growth restriction, low birth weight, and fetal and neonatal thyrotoxicosis.
The main mechanism of neonatal disease is placental transfer of antibodies. Autoantibodies associated with autoimmune thyroid disease can cross the placenta. Stimulating TSH receptor antibodies are the relevant type. Thyroid-stimulating hormone receptor-stimulating antibodies, which occur and reach high levels in mothers who exhibit a picture of Graves’ disease, cross the placenta and lead to a picture of fetal hyperthyroidism. Because the fetal thyroid responds to these antibodies only after mid-gestation, fetal thyrotoxicosis manifestation is observed with the transition of TSH receptor stimulating antibodies to the fetus from the 17th–20th weeks of pregnancy.
Risk depends largely on antibody levels. The risk of neonatal Graves disease is directly related to the concentration of maternal stimulatory thyrotropin receptor antibodies (TSHR-Ab) during the third trimester of pregnancy. Overall incidence is modest. Neonatal hyperthyroidism (NH) is a rare condition, affecting between 1% and 5% of newborns whose mothers have had either active or previous GD. Affected infants are usually recognised early, since babies who will develop neonatal Graves disease are almost always hyperthyroid at birth or within the first week of life. Antibodies can persist after definitive treatment of the mother. Even if a pregnant mother is hypothyroid after radioiodine therapy or surgery, her TRAbs can still, although rarely, induce fetal hyperthyroidism. Maternal antithyroid drugs also cross the placenta, so the choice of drug during pregnancy has to balance the maternal disease against the fetal effects, as discussed in the pregnancy section of this entry.
Diagnosis and laboratory evaluation
The workup usually starts with serum thyrotropin (TSH). Because pituitary output falls when circulating thyroid hormone rises, a low TSH is the earliest sign of excess hormone, and TSH is considered the most sensitive marker for thyroid conditions. A low value alone does not establish the diagnosis. Free thyroxine (FT4) and free triiodothyronine (FT3) are measured to confirm overt disease and to show how far the hormone is elevated. The American Academy of Family Physicians recommends measuring thyroid-stimulating hormone, free thyroxine, and total triiodothyronine, with radioactive iodine uptake and scanning used to determine the cause.
Free fractions are preferred over total hormone levels because binding proteins shift the total. Estrogen from pregnancy or estrogen therapy raises thyroxine-binding globulin, so total T4 and T3 read high while TSH and free T4 remain normal, and this pattern does not indicate hyperthyroidism (Excess estrogen from pregnancy or estrogen therapy leads to elevated levels of thyroxine-binding globulin). Free T3 is useful for judging severity when TSH is suppressed, and it can be disproportionately high in some patients with normal free T4. Free hormone measurement is also essential when TSH is not suppressed, as in a TSH-secreting pituitary adenoma, where TSH is inappropriately normal or raised despite high free T4 and T3 (Elevated TSH with elevated thyroid hormones, consistent with central hyperthyroidism).
Thyrotropin receptor antibody (TRAb) assays identify the autoantibodies that drive most cases of Graves disease. Stimulating antibodies (TSAb) act as the causative agent, and their detection provides a specific biomarker of GD. The serum level of these antibodies helps distinguish Graves disease from other causes when the clinical picture is not diagnostic or when radioactive iodine testing is contraindicated (The serum level of thyroid-stimulating immunoglobulins or TSH-receptor antibodies helps distinguish Graves disease from other causes of hyperthyroidism). Current assays report whether antibodies are present and at what concentration, but not whether they predominantly stimulate or block the receptor (These assays are only able to report the presence or absence of TRAbs and their concentration levels but cannot indicate their overall functional predominance).
Antibody levels also guide prognosis and the decision to stop antithyroid drugs. In one cohort, relapse rate was 31% at 1 year and 70% at 4 years, and high antibody levels at diagnosis or at the end of treatment were linked to recurrence, mostly within the first two years. Reported cut-offs vary between studies; a 2025 retrospective cohort suggested that cut-off at 10.90 and 16.01 IU/L may guide prognosis and treatment. The predictive value of antibody subtypes is less clear, since one study found that separating stimulating from blocking antibodies is of no help in the prediction of relapse.
Radioactive iodine uptake (RAIU) and thyroid scintigraphy separate high-uptake disease from low-uptake thyroiditis. In Graves disease the gland is typically diffusely enlarged with very high uptake, whereas in subacute thyroiditis a typical nuclear scintigraphy scan shows no radioactive iodine uptake in the thyrotoxic phase of the disease. Uneven uptake points toward nodules (unevenness in uptake suggests the presence of a nodule). Standard iodine-123 or iodine-131 uptake is measured at 24 hours, which requires patients to follow a low-iodine diet for over 2 weeks beforehand, and iodine-containing food and contrast agents can distort results. Technetium-99m pertechnetate is an alternative: it is not organified, yet it enters the gland through the sodium-iodide symporter (the pertechnetate ion is transported into the thyroid by the sodium/iodide symporter), and its much shorter half-life (T1/2) of 6.04 h compares with about eight days for iodine-131. One study found a technetium uptake cut-off of 1.55% to be useful in separating Graves disease from subacute thyroiditis (the 99mTc uptake cutoff value of 1.55% had an accuracy of 92.9%).
Thyroid ultrasound with Doppler imaging complements these tests by showing gland size, echotexture, nodules and intrathyroidal blood flow, and it can guide biopsy of suspicious nodules. Graves disease usually shows increased vascularity, whereas destructive thyroiditis tends to show reduced flow, but the patterns overlap, so ultrasound cannot always separate Graves disease from destructive thyroiditis. Imaging research is exploring other approaches: one study of 102 Graves patients and 37 patients with painless thyroiditis found that iodine uptake, scintigraphy and antibody levels, together with diffusion-weighted MRI, had diagnostic value (RAIU, ADC and TRAb are of diagnostic value for differentiating between GD and PT). Because the two conditions can coexist, clinicians often combine these tests rather than relying on any single result.
Subclinical hyperthyroidism
Definition and prevalence
Subclinical hyperthyroidism is a biochemical pattern in which serum TSH is suppressed while the concentrations of T3 and T4 remain normal. In the endogenous form, the thyroid gland is the source of the hormone. The threshold used to define a suppressed value matters greatly. In the United States, approximately 0.7% of the population has TSH levels below 0.1 mIU/L, and about 1.8% has levels below 0.4 mIU/L. Estimates vary with the TSH assay used, iodine intake and local screening practices. Across studies, the figure ranges from 0.6 to 16%. Prevalence rises with age, and older women have the highest prevalence. In iodine-deficient regions, prevalence may reach 15% in people over 70.
Whether the pattern represents true disease or a laboratory artifact is still debated. Most affected people are asymptomatic or have non-specific symptoms, and many revert to normal thyroid status over time. The severity also differs. Mild cases have a low but detectable TSH, usually 0.1 to 0.4 mIU/L, and this group accounts for 65% to 75% of people with the condition. A North American panel judged that the consequences of mild subclinical disease are minimal, and it recommended against routine treatment in the 0.1–0.45 mIU/L range.
Cardiovascular and skeletal associations
The strongest association concerns atrial fibrillation. In a cohort of older adults, those with TSH levels below 0.1 mIU/L had a relative risk of 3.1 for atrial fibrillation over 10 years. A pooled analysis of studies found a risk ratio of 1.99 for incident atrial fibrillation in subclinical hyperthyroidism, although the authors rated the quality of this evidence as low. The evidence is weaker for milder suppression. One five-year follow-up of older patients with mild suppression found no increased incidence of atrial fibrillation, and the panel noted insufficient evidence to establish a clear association between this mild degree of hyperthyroidism and adverse clinical outcomes.
Skeletal risk has been examined in a meta-analysis of ten prospective cohorts that included 61,219 individuals. In that analysis, subclinical hyperthyroidism was associated with an increased risk of vertebral fracture (RR 2.20). The broader evidence on fracture is less certain. A review rated the fracture data as limited-quality evidence, and the mechanism is thought to involve bone loss, with the risk of osteoporosis noted as a reason to consider treatment.
Factors favouring treatment
Guidelines differ in their thresholds. The American Thyroid Association and the American Association of Clinical Endocrinologists suggest treatment when TSH is below 0.1 mIU/L in people aged 65 or older with cardiac risk factors, heart disease or osteoporosis, and also in postmenopausal women who are not taking estrogen or bisphosphonates. Treatment is also considered for people with hyperthyroid symptoms. The European Thyroid Association, by contrast, advises that these patients should be followed without treatment due to the low risk of progression to overt hyperthyroidism and the weaker evidence for adverse health outcomes. Cause also shapes the decision. The panel recommended considering treatment for suppressed TSH caused by Graves disease or nodular thyroid disease, whereas subclinical hyperthyroidism due to destructive thyroiditis resolves spontaneously, and treatment apart from symptomatic therapy such as beta-blockers is usually not required. Where the diagnosis is made, the rate of progression is also relevant. A small proportion of patients, around 1–5% per year, progress to overt hyperthyroidism, and those with fully suppressed TSH are more likely to progress than those with partial suppression.
Monitoring and confounding influences
Untreated patients are usually followed with repeat testing. Guidance generally advises repeating serum thyroid function tests in 3 to 6 months to confirm persistence before a diagnosis is accepted, because a single low TSH may be transient. The reading is also prone to interference. Subclinical suppression is common in individuals treated with levothyroxine, being present in 14% to 21% of such patients, and may be as high as 20%, particularly in those taking desiccated thyroid hormone. Acute nonthyroidal illness, glucocorticoid or dopamine exposure, and assay interference can also lower TSH without true thyroid overactivity, so these factors should be considered before a diagnosis is made. Given these confounders, the reliability of a single TSH value is a central point in clinical decision-making, and the question is often examined through evidence-based reviews and randomized trials of treatment.
Antithyroid drugs and symptomatic therapy
Thionamides and their mechanism
The main antithyroid drugs are the thionamides. Methimazole or thiamazole, together with carbimazole and propylthiouracil, belong to this class. Methimazole is the active metabolite of carbimazole and has similar pharmacological activity. Both methimazole and propylthiouracil (PTU) act on thyroid peroxidase, the enzyme that oxidizes iodide, so they interfere with the oxidation and coupling of iodide to thyroglobulin. This reduces new hormone synthesis but does not affect hormone already stored in the gland. Methimazole is about ten times more potent than PTU. PTU has an additional action: it inhibits the peripheral enzyme that converts T4 to the more active T3 (5'-deiodinase), which is why it is sometimes chosen for its faster effect on circulating T3.
Because the drugs depend on the turnover of circulating hormone, the response is gradual. Patients treated with methimazole typically need three to eight weeks to become euthyroid, and PTU behaves similarly. A typical course lasts six months to two years in Graves disease, and the hyperthyroid state may recur after the drugs are stopped. Dosing is usually titrated to the free thyroid hormone and TSH values, with a lower maintenance dose in some regimens. Methimazole is generally the first choice outside pregnancy, while PTU is recommended in the first trimester, after which switching back to carbimazole or methimazole is advised because of PTU's liver toxicity.
Adverse effects and monitoring
The most serious adverse effect is agranulocytosis. It is a potentially fatal reduction in white blood cells. Estimates given in one reference are that it affects about 0.5% of patients, is more common in older people and at high doses, and is rapidly reversible with discontinuation. Other hematologic effects include aplastic anemia and thrombocytopenia. Hypersensitivity reactions are more common: fever, rash, arthralgia, agranulocytosis and hepatitis are thought to be hypersensitivity reactions. Rash is the most frequent problem, with itching and urticaria being typical. Cross-reactivity between the two drugs is well documented, so patients who have immune reactions to PTU are likely to react to methimazole as well. Severe reactions to one drug generally call for a change to a different class of treatment rather than a switch within the thionamides.
The two drugs also differ in organ toxicity. PTU is associated with hepatotoxicity, and methimazole is associated with congenital malformations when used in early pregnancy. Routine monitoring consists of a blood count and liver tests during treatment, and patients are instructed to report sore throat, fever, or mouth ulcers promptly, since these may signal a fall in white cell count.
Remission and relapse
Remission rates after a course of drugs vary across cohorts. One review found that remission rates are variable (30% - 80%), and many clinical and genetic factors serve as predictors. Relapse is common: relapse following treatment is approximately 30% - 40% in the first year, and 50% - 60% in the long term. Most recurrences are reported after a period of remission, and most cases occurring after a period of four years of remission is one commonly cited pattern.
Serological markers are among the strongest predictors. Persistently high TRAb (> 10 IU/L) during ATD therapy is associated with an 80% - 100% chance of hyperthyroidism recurrence. A prospective trial found that patients with baseline TRAb below 10 IU/L attained remission in 63%, compared with 39% of those above that threshold. The pattern of antibody change matters as well: patients with fluctuating TRAb titer had a lower rate of remission (37.2%) than those with a smooth non-fluctuating type (88.9%). Eye disease also influences the outcome; in one retrospective series, 42% of patients with mild Graves ophthalmopathy achieved remission during ATD treatment, compared with 8% of those with severe disease. In a smaller retrospective cohort, male sex, lower body mass index, and higher TSH levels during treatment were associated with remission. Prolonging therapy appears to help: in a randomized study of patients who had reached remission, 96.4% of those assigned to continued treatment sustained remission, compared with 66% in the observation group.
Beta-adrenergic blockers
Beta blockers do not treat the underlying disease. They do not cure hyperthyroidism, but they offer prompt relief of the adrenergic symptoms of this disease, such as increased pulse pressure, tremor, stare, palpitations, nervousness, heat intolerance, and tachycardia. The rationale is that in many tissues, hyperthyroidism is associated with an increased number of beta-adrenergic receptors. The ensuing increase in beta-adrenergic activity is responsible for many of the symptoms. Guidance in one review is that a beta blocker should be started in most patients as soon as the diagnosis of hyperthyroidism is made, even before determining the cause of the hyperthyroidism. They should be continued until resolution of hyperthyroidism.
Propranolol is the most widely studied agent. Nonselective beta blockers such as propranolol should be prescribed for symptom control because they have a more direct effect on hypermetabolism. Typical doses are 80 to 320 mg daily, given in divided doses. Propranolol also blocks peripheral conversion of thyroxine (T4) to triiodothyronine (T3), an effect that is considered a secondary benefit. Cardioselective agents such as atenolol are alternatives for patients with asthma or intolerance to propranolol. Non-dihydropyridine calcium channel blockers such as diltiazem can replace beta blockers in patients who cannot tolerate them, to reduce heart rate.
Beta blockers have a defined role around definitive treatment. These agents are added to antithyroid medications or around treatment with radioactive iodine for symptomatic relief only. Before thyroidectomy, they are commonly used to control heart rate while antithyroid drugs bring hormone levels into the normal range, so that surgery is performed in a euthyroid state.
Radioactive iodine and thyroidectomy
Radioactive iodine (RAI) therapy uses iodine-131, a beta-emitting isotope of iodine. This therapy employs the use of iodine-131, which is a beta-emitting radioisotope of iodine. Because the thyroid concentrates iodide, the radiation is delivered largely to follicular cells, which are gradually destroyed, reducing gland volume and hormone output. RAI has been used to treat hyperthyroidism for more than 70 years and is indicated in cases of hyperthyroidism due to Graves' disease, multi-nodular goitre, and solitary toxic nodules. It may be used as first-line treatment or after antithyroid drugs have failed.
Dosing follows two broad strategies. Fixed-dose regimens are simple and inexpensive, and comparable outcomes to dosimetry-based approaches have been reported. The 2025 Korean Thyroid Association guideline recommends a fixed dose of 10 to 15 mCi. Calculated-dose approaches instead use ultrasound-directed assessments of iodine uptake and volume of the thyroid gland. In these calculations, the dose is usually raised when the gland is very large, there is evidence for very rapid turnover of ^131I (higher 6 h than 24 h uptake, small gland), there has been prior anti-thyroid drug treatment, there is toxic nodular disease. The American Thyroid Association guideline sets etiology-specific targets: a mean dose of 370 to 555 MBq is recommended to achieve hypothyroidism in Graves disease, and doses ranging from 370 to 740 MBq are generally preferred for toxic multinodular goiter and toxic adenoma. Preparation includes limiting iodine intake; the Korean guideline states that iodine-rich foods should be avoided for at least 1 week before treatment. Antithyroid drugs given beforehand can reduce the effectiveness of RAI, which is why they are usually withdrawn for a period and then resumed afterwards.
Radiation exposure is a central concern. Besides the thyroid, the drug is distributed systemically, so guidance limits the activity given and the estimated dose to bone marrow, and nursing and household contact are restricted. Its use in children is limited: low-dose therapy increases the risk for thyroid cancer later in life, and the 2022 European Thyroid Association guidance advises against RAI in children under five. RAI also carries a small but definite increased risk of development or worsening of Graves' ophthalmopathy, a risk that a meta-analysis found to be higher than with antithyroid drugs.
The most common consequence of RAI is hypothyroidism. In one meta-analysis, the hypothyroidism rate in the radioiodine therapy group and antithyroid drug group was 19.7% and 9.3%, respectively. At higher doses the proportion is larger; one summary notes that up to 90% of patients receiving higher radioiodine doses may develop hypothyroidism within the first year. Positive thyroid peroxidase antibodies have been identified as the strongest predictor of early hypothyroidism in one cohort. Because hypothyroidism is itself a risk factor for orbitopathy, some centres give early levothyroxine after RAI, although the evidence for this practice is still being tested. Once established, hypothyroidism generally requires lifelong thyroid hormone therapy.
Thyroidectomy is the third option. The main indications for thyroidectomy are thyroid cancer, multinodular goiter, and Graves' disease. Surgery is also appropriate for large goitres that cause compression and for nodules with suspicious features, and the choice between surgery, RAI and drugs often reflects patient preference and the risk of eye disease; for example, antithyroid medications are appropriate in patients attempting to avoid long-term thyroid hormone therapy and should be considered in those with increased risk of Graves ophthalmopathy, such as smokers. The American Thyroid Association guideline still recommend[s] both total and near-total resections if surgery is chosen as the definitive therapy for GD. Before operation, patients are normally rendered euthyroid with antithyroid drugs and beta-blockers, because uncontrolled hyperthyroidism raises the risk of perioperative thyroid storm.
Surgical complications are more frequent in Graves disease than in other indications. In one large series, there were 84 (15.73%) cases of postoperative hypoparathyroidism (65, 12.17%, temporary and 19, 3.56%, permanent) in the Graves group, and overall postoperative hypoparathyroidism, permanent hypoparathyroidism, unilateral recurrent laryngeal nerve injury, and temporary recurrent laryngeal nerve injury were significantly greater in GD Group. A review of Graves surgery gives wider ranges, with transient and permanent rates between 18.0–20.0% and 3.0–7.0%. Recurrent laryngeal nerve injury is also more likely after operations for Graves disease; one study reported Surgery indication for Graves' disease (OR 14.34, p < 0.001) compared with nodular goitre during hemithyroidectomy. Injury can cause hoarseness, shortness of breath, or the need for intubation in the case of bilateral palsy. Permanent nerve palsy was documented in 1.4% of a German cohort. Thyroidectomy in Graves disease is generally regarded as technically more demanding than in other conditions, and outcomes depend heavily on surgical technique and experience.
Notes on the terms used
Special populations: pregnancy, children and older adults
Pregnancy
Hyperthyroidism occurs in roughly 0.05 to 3.0 percent of pregnancies, and its early signs overlap with normal gestation, which makes recognition harder. Clinicians interpret free T4 or T3 against trimester-specific reference ranges together with a low TSH. Treatment is generally not required for subclinical hyperthyroidism in pregnancy.
The central management aim is to limit fetal exposure to excess maternal thyroid hormone while also limiting the adverse effects of drugs on the fetus. Thionamides are the drugs of choice. The goal of therapy is to keep the mother's free T4 in the high-normal to mildly elevated range on the lowest dose of antithyroid medication. Combined "block-and-replace" regimens with levothyroxine are not recommended. Because of differences in teratogenic profiles, guidelines from the American Thyroid Association and the Endocrine Society favour propylthiouracil in the first trimester and a later switch to methimazole, given the greater frequency of birth defects with MMI. One review of first-trimester exposure estimated that as many as 2-4% of children exposed to MMI in the first trimester of pregnancy may develop birth defects. Beta-blockers can be used for symptom control, but they are used sparingly due to reports of impaired fetal growth with prolonged use. When surgery is unavoidable, the second trimester is considered the safest window.
Preconception planning is central. Women of childbearing age with Graves' disease should be counselled against becoming pregnant until a euthyroid state is achieved. TRAb levels are followed in women who conceive on antithyroid treatment, because persistently positive antibodies signal risk of fetal or neonatal hyperthyroidism. Transient hCG-mediated thyrotoxicosis in early pregnancy, which is associated with hyperemesis gravidarum, is generally managed by supportive care rather than antithyroid drugs, and the principle is to avoid treating a self-limited process that does not require drug exposure.
Children
GD is the most common cause of thyrotoxicosis in children and adolescents and contributes to 10-15% of all pediatric thyroid disease. Antithyroid drugs are the usual first-line therapy, with methimazole generally preferred where it is available. Children differ from adults chiefly in the length of treatment. In adults, the course usually lasts 12-18 months, whereas pediatric remission rates are lower, ranging from 20% to 30%, compared to 40%–60% in adults, and many children relapse after stopping treatment.
Guidance on duration has shifted. The 2016 ATA guidelines recommend a duration of 1–2 years of ATD therapy for pediatric first-line treatment, whereas the European Thyroid Association has updated its guidelines to recommend a minimum treatment duration of 3 years for children with GD, extending to 5 years or more in cases with a low likelihood of remission. A randomized trial comparing long and short courses reported that children given long-term (96–120 months) MMI treatment had markedly higher remission after withdrawal than those given a short course, with the difference persisting at four years. Pediatric monitoring also includes watching for side effects, and in one cohort MMI-related adverse events occurred in 11.3% of patients, the most common of which were rash and hematologic abnormalities.
Older adults
In older patients the main concerns are cardiac and skeletal. Atrial fibrillation is more frequent with age: in one study, 25% of hyperthyroid patients older than 60 years had atrial fibrillation compared to 5% in patients less than 60 years of age. For subclinical disease, treatment is usually considered in patients older than 65 years with TSH < 0.4 mlU/L, or in younger patients with TSH < 0.1 mlU/L. The ATA has described the data as a strong argument for treating older patients to prevent dysrhythmias and stroke, while the case for younger patients is less clear. Evidence remains contested: in a randomized trial of patients aged 50 and over with subclinical disease, four patients in the surveillance group and one in the treatment group developed AF, a difference that was not statistically significant, and no large randomized prospective study has yet shown that treating SCH may reduce the risk of cardiovascular complications.
Older patients also bring a higher risk of treatment complications and a different disease pattern. Autonomous nodules are more common with age, and the remission of hyperthyroidism is unlikely if it is due to a toxic adenoma. A French consensus statement favours definitive therapy with surgery or radioiodine, reserving long-course antithyroid medication for patients in whom those options are unavailable or unsuitable. Iatrogenic cases also arise in this group: in elderly patients with atrial fibrillation on levothyroxine, adjusting TSH goals is crucial to prevent rapid ventricular response, aligning with American Thyroid Association's recommendation of 4-6 mIU/L for those aged 70-80.
History of discovery and understanding
The condition was described in clinical terms in the early nineteenth century, although its features had been seen earlier. Caleb Hillier Parry, a physician in Bath, England, treated a woman in 1786 who had palpitations, a goitre and prominent eyes, but his notes were published only posthumously in 1825. Giuseppe Flajani reported 2 cases in 1802, and Robert James Graves described the syndrome of palpitations, goiter, and exophthalmos in 1835. Karl Adolph von Basedow published similar observations in Germany in 1840. The eponyms reflect national traditions: the term 'Graves disease' is most common worldwide, though in German-speaking Europe, Basedow disease is preferred. Diagnosis rested on this clinical triad at the bedside. Early physicians, including Graves and his colleague Stokes, thought the disorder was a form of heart disease, mainly because the most prominent and disturbing complaint was cardiac in nature. Later theories shifted toward the gland itself, and in 1909 William Osler proposed that the condition was caused by hypersecretion of an unknown product that produced a chronic toxaemia.
Before effective drug therapy, the principal treatment was surgical. Subtotal thyroidectomy was generally associated with a high mortality in the hands of most surgeons, which made the operation a last resort and encouraged the search for non-surgical options. Iodine was one such approach; Plummer, practicing at the Mayo clinic, reasoned that the secreted product could perhaps be iodine deficient. The pursuit of a safer alternative to surgery also shaped the work that led to radioactive iodine.
The therapeutic use of radioactive isotopes of iodine emerged from a question asked at a lecture. In 1936, from the audience, Hertz asked, "Can iodine be made artificially radioactive?" He was already several years deep into experiments on thyroid iodine physiology at the MGH in his quest for a non-operative solution to hyperthyroidism. With the physicist Arthur Roberts, Hertz undertook an experimental study of 48 rabbits, which demonstrated that the goitrous thyroid glands retained more RAI than the normal control glands. Production was constrained by the short half-life of the isotope, since the 25-minute half-life of ^128^I prohibited transportation of the radionuclide across the Charles River to the MGH. The first patient was treated on 31 March 1941 at Massachusetts General Hospital, although one account gives 1 January 1941 for the same first administration. Dr. Hertz subsequently administered radioactive iodine therapy to another 28 patients over the next two years, and results were published in 1946, when two papers appeared in the same issue of the Journal of the American Medical Association. Results were not reported until several years later, in 1946, as Hertz joined the navy in 1943. Adoption then spread steadily, and radioiodine therapy continues to be a mainstay of therapy for hyperthyroidism and thyroid cancer. Hertz's own recognition lagged; with his early death in 1950, his contributions have often been overlooked until recently.
The autoimmune mechanism was established in the mid-twentieth century. Before 1956, Graves' hyperthyroidism was thought to be due to high levels of TSH. In that year, Adams and Purves identified in patients' serum a stimulator with a prolonged action, which they named long-acting thyroid stimulator (LATS). LATS was only present in the serum IgG fraction, suggesting it was a thyroid stimulating autoantibody. The laboratory of Max McKenzie, together with that of Joe Kriss, provided the evidence that the serum 'long-acting thyroid stimulator' in Graves' disease ... was a TSH-receptor autoantibody. Subsequent work proceeded in stages: in 1964, LATS was identified as an immunoglobulin, the TSH receptor itself was identified in 1966, and in 1974, the target antigen of LATS was shown to be the TSHR. The molecular picture was refined later. Cryo-EM structure of full length TSHR bound to M22 Fab (stimulating autoantibody) has since been reported, and assays for these thyroid stimulatory antibodies have been continually refined and their epitopes on the thyroid stimulating hormone receptor are increasingly well characterized.
Guidelines, current evidence and open questions
Major guidelines and how they differ
The American Thyroid Association's 2016 guideline on hyperthyroidism and other causes of thyrotoxicosis contains 124 evidence-based recommendations, covering initial evaluation, the standard treatments for Graves disease, orbitopathy and less common causes (management of less common causes of thyrotoxicosis). Its most consequential change concerned diagnosis. There is a shift away from starting with a radioactive iodine (RAI) uptake and scan as the first step, and antibody testing can come earlier. The guideline also advises that TRAb be measured after 1–2 years of ATD therapy, with the decision to continue drugs or proceed to definitive treatment depending on the result.
The European Thyroid Association (ETA) places the decision point differently. Its 2018 Graves guideline suggests that low-dose MMI be continued for a further 12 months with repeated TSH-R-Ab testing after 1 year. The ETA's 2022 pediatric guideline goes further in one respect, favouring a prolonged course of methimazole/carbimazole ATD treatment (3 years or more) and advising against propylthiouracil.
For orbitopathy, the 2021 EUGOGO guideline asks clinicians to urge all patients with Graves hyperthyroidism to stop smoking, whether or not eye disease is present (Physicians should urge all patients with Graves' hyperthyroidism, irrespective of the presence/absence of GO, to refrain from smoking). A comparison with the 2022 ATA/ETA consensus found a high degree of concordance as to classification, assessment, and prevention, as well as treatment of mild GO, but the major disagreement regards treatment of moderate-to-severe and active GO.
Related documents address specific exposures. The ETA's 2021 guideline on iodine-based contrast media covers cases where iodine excess can induce hyperthyroidism, and the ETA has also issued a 2026 guideline on interference in immunoassay measurements used in assessment of thyroid function.
Pregnancy-specific recommendations
Recent pregnancy guidance has reshaped drug choice. The 2016 ATA document introduced new paradigms for managing pregnant patients and for preparing them for thyroid surgery. Earlier guidance suggested switching to methimazole for the rest of pregnancy after the first trimester. The rationale for the first-trimester preference is teratogenicity: one ATA patient summary notes that as many as 2-4% of children exposed to MMI in the first trimester of pregnancy may develop birth defects.
A more recent management review describes propylthiouracil as preferred in the first trimester, but if antithyroid drug (ATD) therapy is still required beyond 16 weeks of gestation, the decision to continue on propylthiouracil or switch to methimazole should be guided by shared decision-making. Clinicians also note that thyroid autoimmunity might improve in the second trimester due to pregnancy-induced immune tolerance, therefore pregnant women might require decreasing doses of antithyroid drugs. Because TRAb can cross the placenta and cause fetal hyperthyroidism and goitre when present in high levels., antibody measurement remains part of the pregnancy assessment.
Open questions
Several questions remain unsettled. For mild Graves disease, the best initial treatment is not clear. The ATA notes that appropriate treatment requires an accurate diagnosis and is influenced by coexisting medical conditions and patient preference. Whether a longer or shorter drug course, or early radioactive iodine, gives the best balance of remission and risk in mild disease is still debated.
For subclinical hyperthyroidism, the long-term consequences of low TSH with normal hormone levels, especially for atrial fibrillation and bone health, are not fully established, and it is unclear which patients gain from treatment. Antibody assays raise a similar question. TSH-receptor antibodies are now central to diagnosis, prognosis and decisions about stopping drugs, yet the pediatric ETA guideline notes that measurement of serum TSH receptor antibodies is recommended for all pediatric patients with hyperthyroidism. How assay generations and immunoassay interference affect clinical decisions needs further study.
Unmet needs
Atypical cases, particularly in older adults and in pregnancy, remain difficult to diagnose on clinical grounds alone, so better biochemical and imaging strategies are needed. Therapy for Graves orbitopathy is an equally pressing gap, since guidelines still differ on how to treat moderate-to-severe active disease. Finally, reducing the burden of lifelong hypothyroidism after radioactive iodine or thyroidectomy depends on developing definitive treatments that spare more patients from permanent hormone replacement.
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