Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: September 3, 2004
Publication Date: N/A
Technical Abstract: There are two thyroid hormones produced and secreted by the thyroid gland, these are thyroxine (T4) and triiodothyronine (T3). Thyroxine and triiodothyronine are produced from tyrosine both having either 4 (T4) or 3 (T3) iodine atoms on the thyronine ring. Low dietary intake of iodine will lower the production of these two hormones. The production and secretion of these hormones are initiated by the hypothalamus which secretes thyrotrophin releasing hormone (TRH) which enters the hypophyseal portal system to reach the anterior pituitary gland to bind to membrane receptors on thyrotrophs. The anterior pituitary gland then produces and secretes thyroid stimulating hormone (TSH). Upon thyroid stimulation, thyroglobulin which is the storage form of T3 and T4, is transported from the lumen of the follicle by pinocytosis, into the follicular cell of the thyroid gland. In this cell the large protein thyroglobulin is hydrolyzed into T3 and T4 and secreted into peripheral circulation. The majority of thyroid hormone produced and in circulation is T4. The majority of T3 and T4 concentrations are then bound by thyroxine-binding globulin, the remaining 30% is bound to albumin or thyroxin-binding pre-albumin. Only a fraction of 1% remains unbound in the peripheral circulation. The large percent of bound hormone supplies the body with a large pool to draw from and prevents its degradation until it reaches its target tissue. Of the two thyroid hormones, it is T3 that is considered the biologically active hormone, although T4 to a lesser extent can bind to thyroid receptors. After entering the peripheral circulation, T4 is converted into T3 in organs such as the anterior pituitary gland, the kidney and the liver. Most tissues in the body have receptors for thyroid hormone, which are nuclear receptors, and by activating these receptors thyroid hormones set the basal rate of heat production and oxygen consumption for an animal. Thyroid hormones also influence the rate of lipid, protein and carbohydrate metabolism, increasing these rates when thyroid hormone concentrations are elevated and decreasing these rates when thyroid concentrations are low. Overall secretion of thyroid hormone is regulated by a negative feedback system in which high concentrations of circulating hormone cause the anterior pituitary gland to decrease production of TSH and thus decrease the production of thyroid hormone.