Levothyroxine is a synthetic form of thyroxine (T4), a hormone normally produced by the thyroid gland. It is biologically equivalent to naturally occurring T4 and is used primarily to treat hypothyroidism, in which the body has insufficient thyroid hormone. Treatment replaces the missing hormone rather than repairing the underlying thyroid disorder. It is available as tablets, oral liquids, and soft-gel capsules. (thyroid.org)
Biological action
Thyroid hormones help regulate metabolism, heat production, and the function of the heart, muscles, and other organs. T4 is the principal hormone secreted by the thyroid. In tissues throughout the body, it is converted to triiodothyronine (T3), the more active thyroid hormone. This conversion continues even when the thyroid gland is diseased or absent, allowing levothyroxine to supply the body's requirements without routinely administering T3 separately. (thyroid.org)
Levothyroxine remains in the body considerably longer than T3. Its prolonged action supports once-daily treatment and relatively stable circulating hormone levels. Changes in treatment do not produce their full effect immediately. (thyroid.org)
Medical uses
The principal use is hormone replacement in congenital or acquired hypothyroidism. Causes include Hashimoto's disease, an autoimmune disease affecting the thyroid; removal of the gland by surgery; and destruction of thyroid tissue by radiation treatment. Hypothyroidism may also result from impaired hormonal signals from the pituitary gland or hypothalamus, rather than a defect in the thyroid itself. Permanent loss of thyroid function generally requires lifelong replacement. (thyroid.org)
A distinct use is suppression of thyroid-stimulating hormone (TSH) in selected patients with well-differentiated thyroid cancer, as an adjunct to surgery and radioiodine treatment. This differs from ordinary replacement, which aims to restore normal thyroid function. (dailymed.nlm.nih.gov)
In children, thyroid hormone replacement supports normal growth and development. Treatment of congenital hypothyroidism is particularly important because thyroid hormone deficiency in early life can impair brain development. (thyroid.org)
Formulations and monitoring
The required dose varies with age, body weight, remaining thyroid function, cardiovascular status, other medicines, and the condition being treated. Treatment is assessed using clinical findings and blood tests, rather than symptoms alone, because symptoms such as tiredness and weight change are not specific to thyroid disease. (niddk.nih.gov)
For primary hypothyroidism, TSH is the main laboratory measure used to assess replacement. Blood testing commonly occurs about six to eight weeks after treatment begins or the dose changes. Once treatment is stable, testing is less frequent. In hypothyroidism caused by pituitary or hypothalamic dysfunction, TSH is unreliable for assessing replacement; free T4 is used instead. (niddk.nih.gov)
Absorption and interactions
Food can affect oral absorption. Iron and calcium supplements, antacids, and some other medicines can reduce the amount absorbed. Other interactions alter hormone binding, metabolism, or physiological effects rather than absorption alone. (dailymed.nlm.nih.gov)
Although products contain the same active hormone, their inactive ingredients differ. Changes between formulations or manufacturers may therefore be relevant when interpreting subsequent thyroid-function measurements. (thyroid.org)
Safety and pregnancy
Most adverse effects reflect excessive thyroid hormone exposure and resemble hyperthyroidism. They include palpitations, tremor, heat intolerance, insomnia, and diarrhea. Overreplacement can increase the risk of atrial fibrillation and reduce bone mineral density. Uncorrected adrenal insufficiency is a contraindication because thyroid hormone treatment can precipitate an adrenal crisis. (dailymed.nlm.nih.gov)
United States prescribing information carries a boxed warning against use for obesity or weight loss. Physiological replacement doses do not reduce weight in people with normal thyroid function, while larger doses can cause serious or life-threatening toxicity. (dailymed.nlm.nih.gov)
During pregnancy, levothyroxine is used to replace deficient maternal thyroid hormone, and requirements often increase. Maternal T4 is especially important early in pregnancy, before the fetus can produce sufficient thyroid hormone independently; the developing fetal brain converts T4 into the T3 it needs. (niddk.nih.gov)
References
- Thyroid Hormone Treatment | American Thyroid Associationthyroid.org
- Hypothyroidism (Underactive Thyroid) - NIDDKniddk.nih.gov
- DailyMed - LEVOTHYROXINE SODIUM tabletdailymed.nlm.nih.gov
- DailyMed - LEVOTHYROXINE SODIUM tabletdailymed.nlm.nih.gov
- Thyroid Hormone Treatment in Children and Adolescents | American Thyroid Associationthyroid.org
- Thyroid Disease & Pregnancy - NIDDKniddk.nih.gov