triiodothyronine

Triiodothyronine, or T3, is one of the two major thyroid hormones produced by the thyroid gland and is the physiologically active form. It contains three iodine atoms and is more potent than thyroxine (T4). The thyroid gland secretes a small amount of T3 directly, but most circulating T3 is generated by deiodination of T4 in peripheral tissues, particularly the liver and kidneys, by the action of deiodinase enzymes (types D1 and D2). In humans, T4 is produced in larger amounts and serves as a reservoir for T3 production.

T3 exerts genomic effects by binding to thyroid hormone receptors (TRα and TRβ) in the nucleus, regulating transcription of genes involved in development, metabolism, growth, and energy expenditure. It also has non-genomic actions at the cell membrane and cytoplasm, contributing to rapid cellular responses.

In the bloodstream, T3 circulates mostly bound to transport proteins such as thyroxine-binding globulin, albumin, and transthyretin; a small fraction is free and biologically active. The half-life of T3 is about one day, shorter than that of T4, which contributes to its more rapid onset and offset of action.

Regulation occurs via the hypothalamic–pituitary–thyroid axis: the hypothalamus releases TRH, prompting the pituitary to secrete TSH, which stimulates thyroid hormone synthesis and release. Circulating T3 and T4 provide negative feedback to the hypothalamus and pituitary.

Clinical notes include the measurement of total or free T3 alongside TSH and T4 in thyroid evaluation. T3 toxicosis is a pattern of hyperthyroidism with elevated T3 but normal T4. Liothyronine, a synthetic form of T3, is used for certain thyroid disease treatments but requires careful dosing due to its shorter half-life and potential for fluctuations.