Thyroid hormones

Thyroid hormones are any 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, derived from food.^[2] 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.^[3]

The major form of thyroid hormone in the blood is thyroxine (T₄), whose half-life of around one week^[4] is longer than that of T₃.^[5] In humans, the ratio of T₄ to T₃ released into the blood is approximately 14:1.^[6] 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.

The thyroid hormone is one of the factors responsible for the modulation of energy expenditure. This is achieved through several mechanisms, such as mitochondrial biogenesis, adaptive thermogenesis, etc.^[7]

American chemist Edward Calvin Kendall was responsible for the isolation of thyroxine in 1915.^[8] In 2020, levothyroxine, a manufactured form of thyroxine, was the second most commonly prescribed medication in the United States, with more than 98 million prescriptions.^[9][10] Levothyroxine is on the World Health Organization's List of Essential Medicines.^[11]