Thyroid hormones are two hormones produced and released by the thyroid gland, namely triiodothyronine (T3) and thyroxine (T4). They are tyrosine-based hormones that are primarily responsible for regulation of metabolism. T3 and T4 are partially composed of iodine. A deficiency of iodine leads to decreased production of T3 and T4, enlarges the thyroid tissue and will cause the disease known as simple goitre.

The major form of thyroid hormone in the blood is thyroxine (T4), which has a longer half-life than 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 cells by deiodinases (5′-deiodinase). These are further processed by decarboxylation and deiodination to produce iodothyronamine (T1a) and thyronamine (T0a). All three isoforms of the deiodinases are selenium-containing enzymes, thus dietary selenium is essential for T3 production.

American chemist Edward Calvin Kendall was responsible for the isolation of thyroxine in 1915. In 2018, levothyroxine, a manufactured form of thyroxine, was the second most commonly prescribed medication in the United States, with more than 105 million prescriptions. Levothyroxine is on the World Health Organization's List of Essential Medicines.

 

Note:   The above text is excerpted from the Wikipedia article Thyroid hormones, which has been released under the GNU Free Documentation License.