Thyroid hormones
Thyroid hormones are two hormones produced and released by the thyroid gland, namely triiodothyronine (T) and thyroxine (T). They are tyrosine-based hormones that are primarily responsible for regulation of metabolism. T and T are partially composed of iodine. 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 approximately 14:1. 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. Edward Calvin Kendall was responsible for the isolation of thyroxine in 1915.
Fetal programming
Thyroid hormones play an instrumental role during the early development of the fetus's brain. Therefore, mothers suffering from thyroid-related issues and altered thyroid hormone levels may inadvertently trigger structural and functional changes in the fetal brain. The fetus is able to produce its own thyroid hormones from the onset of the second trimester; however, maternal thyroid hormones are important for brain development before and after the baby is able to synthesize the hormones while still in the uterus. Due to this, the baby may experience an increased risk of neurological or psychiatric diseases later in life.