What is "thyroid hormone"

n. any of several closely related compounds that are produced by the thyroid gland and are active metabolically

The thyroid hormones, triiodothyronine (T) and its prohormone, thyroxine (T), are tyrosine-based hormones produced by the thyroid gland that are primarily responsible for regulation of metabolism. T and T are partially composed of iodine (see molecular model). A deficiency of iodine leads to decreased production of T and T, enlarges the thyroid tissue and will cause the disease known as simple goitre. The major form of thyroid hormone in the blood is thyroxine (T), which has a longer half-life than T. In humans, the ratio of T to T released into the blood is between 14:1 and 20:1. T is converted to the active T (three to four times more potent than T) within cells by deiodinases (5'-iodinase). These are further processed by decarboxylation and deiodination to produce iodothyronamine (Ta) and thyronamine (Ta). All three isoforms of the deiodinases are selenium-containing enzymes, thus dietary selenium is essential for T production.