Physiology

Abstract

The two principal hormones secreted by the thyroid gland are thyroxine (T₄) and triiodothyronine (T₃). Both hormones are synthesized by the thyroid gland from iodine. The first step in thyroid hormonogenesis is trapping. The thyroid cells are extremely effective in trapping iodine from the plasma against a concentration gradient, since the concentrations of iodine in the plasma are extremely low (0.1 to 1 μg/dl). The trapped iodine is immediately oxidized. Unless oxidized, the iodine cannot be incorporated into tyrosyl residues, a process called organification. The trapped, oxidized iodine undergoes organification instantaneously by a number of enzymes, in particular, the peroxidases. The tyrosyl is provided by thyroglobulin, a large-molecular-weight protein contained within the colloid. The process of iodination of thyroglobulin results in the formation of monoiodotyrosine (MIT). A series of coupling reactions results in the formation of diiodotyrosine (DIT) and tetraiodothyronine (T₄). Similarly, coupling of one DIT with one MIT results in the formation of triiodothyronine (T₃).