Thyroid hormones

Thyroid hormones are two <a href="/facts/Hormone/WFnBnINJ">hormones</a> produced and released by the <a href="/facts/Thyroid_gland/bjovTJVu">thyroid gland</a>, <a href="/facts/Triiodothyronine/MPBz6ApV">triiodothyronine</a> (T3) and <a href="/facts/Thyroxine/9k4xJtCq">thyroxine</a> (T4). They are <a href="/facts/Tyrosine/RJhEgE4C">tyrosine</a>-based hormones that are primarily responsible for regulation of <a href="/facts/Metabolism/WxDrm8k5">metabolism</a>. T3 and T4 are partially composed of <a href="/facts/Iodine/JlfMecMf">iodine</a>, derived from food. A deficiency of iodine leads to decreased production of T3 and T4, enlarges the <a href="/facts/Thyroid/bjovTJVu">thyroid tissue</a> and will cause the disease known as <a href="/facts/Simple_goitre/0kNGMcb6">simple goitre</a>.
The major form of thyroid hormone in the blood is thyroxine (T4), whose half-life of around one week is longer than that of T3. In humans, the ratio of T4 to T3 released into the blood is approximately 14:1. T4 is converted to the active T3 (three to four times more potent than T4) within <a href="/facts/Cell_(biology)/bLM9UQvy">cells</a> by <a href="/facts/Deiodinase/HzyLgjXr">deiodinases</a> (5′-deiodinase). These are further processed by <a href="/facts/Decarboxylation/Uugc18fz">decarboxylation</a> and deiodination to produce <a href="/facts/Iodothyronamine/rDqRnAje">iodothyronamine</a> (T1a) and <a href="/facts/Thyronamine/WdsK1Tux">thyronamine</a> (T0a). All three isoforms of the deiodinases are <a href="/facts/Selenium/k975NApC">selenium</a>-containing enzymes, thus dietary selenium is essential for T3 production. <a href="/facts/Calcitonin/E0xGEcr5">Calcitonin</a>, a <a href="/facts/Peptide_hormone/8AdC52vt">peptide hormone</a> produced and secreted by the thyroid, is usually not included in the meaning of "thyroid hormone".
Thyroid hormones are one of the factors responsible for the modulation of energy expenditure. This is achieved through several mechanisms, such as mitochondrial biogenesis and adaptive thermogenesis.
American chemist <a href="/facts/Edward_Calvin_Kendall/8xLnvbfz">Edward Calvin Kendall</a> was responsible for the isolation of thyroxine in 1915. In 2020, <a href="/facts/Levothyroxine/AarfgbeG">levothyroxine</a>, a manufactured form of thyroxine, was the second most commonly prescribed medication in the United States, with more than 98 million prescriptions. Levothyroxine is on the <a href="/facts/WHO_Model_List_of_Essential_Medicines/6kmu81Xm">World Health Organization's List of Essential Medicines</a>.

Thyroid hormones open-in-new

Thyroid hormones