Tyroksin

Tyroksin, also called thyroxine or tetraiodothyronine (T4), is a hormone produced by the thyroid gland in vertebrates. It is a prohormone that is converted in peripheral tissues to the more active triiodothyronine (T3).

Chemically, T4 contains four iodine atoms and is derived from the amino acid tyrosine via thyroglobulin. Most circulating T4 is bound to transport proteins such as thyroxine-binding globulin, albumin, and transthyretin; only a small fraction is free to enter cells.

Functionally, T4 regulates basal metabolic rate, thermogenesis, and lipid and carbohydrate metabolism. It also influences growth, neural development, and maturation, particularly in fetuses and infants. The hypothalamus and pituitary regulate thyroid hormone production via thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH).

Clinical use: Synthetic thyroxine, levothyroxine, is the standard treatment for hypothyroidism, myxedema, and as an adjunct in some forms of thyroid cancer to suppress TSH. Dosing is individualized and adjusted by TSH and free T4 levels. In pregnancy, dosing often increases due to higher thyroid-binding globulin levels.

Pharmacokinetics: Orally administered T4 is absorbed variably; the half-life is about seven days in healthy adults, longer in some conditions. Therapeutic effect takes weeks. Potential drug interactions include calcium or iron supplements, proton pump inhibitors, and drugs that alter thyroid-binding proteins or hepatic enzymes.

Adverse effects: Symptoms of excess thyroid hormone, such as palpitations, weight loss, anxiety, and tremor. Over-replacement increases risks including atrial fibrillation and bone loss in susceptible individuals.

See also: levothyroxine, hypothyroidism, hyperthyroidism.