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.
Maternal hypothyroidism
Maternal hypothyroidism is hypothyroidism in pregnant mothers. Even with appropriate treatment, it may pose risks not only to the mother, but also to the fetus. Thyroid hormones, T4 and TSH, diffuse across the placenta traveling from the mother to fetus for 10–12 weeks before the fetus’s own thyroid gland can begin synthesizing its own thyroid hormones. The mother continues to supply some T4 to the fetus even after he/she is able to synthesize his/her own. Infants with sporadic congenital hypothyroidism show T4 concentrations in the umbilical cord suggesting the mother is still providing 25-50 percent of T4. If these infants are not screened soon after birth for their hypothyroidism and treated, the infants can become permanently intellectually disabled, since they can’t meet their bodies demand for T4.