Define Biosynthesis and Secretion of Thyroid Hormones?

Histologically, the functional cells of the thyroid gland are arranged in follicles, which surround a central lumen containing a colloid in which the hormones are stored in the form of thyroglobulin. Thyroglobulin is a glycoprotein and is synthesized in the follicular cell as prothyroglobulin and the tyrosine units are iodinated in the intact protein. As you may have noticed in Figure, the iodide actively transported into the cells from extracellular fluid (ECF) is released from the thyroid cells into the colloid follicle where it is oxidized by thyroperoxiduse in the presence of hydrogen peroxide.

The oxidized iodine is then combined with the amino acid tyrosine in thyroglobulin to form mono and di-iodotyrosines (MITIDIT), which are again catalyzed by thyroperoxidase. These are then coupled to form triiodo to thyronine and tetraiodo- thyroxine. The iodinated thyroglobulin is absorbed back into the cells by a process known as pinocytosis. The iodinated thyroglobulin is hydrolyzed within the cells by the cellular proteolytic enzymes to release T₄ and T₃ into the blood circulation. Unutilized mono- and di-iodotyrosines are not released into the blood but axe conserved within the gland for further incorporation into thyroglobulin. In blood, these hormones bind to transport proteins mainly thyroxine-binding-protein and are distributed to the target cells in the peripheral tissues. All phases of biosynthesis and secretion of thyroid hormones are stimulated by thyroid-secretary hormone (TSH), which is secreted by anterior pituitary gland in response to low levels of thyroid hormones.