Chapter 3 - Thyroid Gland

Abstract

This chapter describes the structure of thyroid gland and the biochemistry of thyroid hormones; such as their biosynthesis, physiological effects, regulation of secretion, and mechanisms of action. The functional unit of the thyroid gland is the follicle, which is composed of epithelial cells arranged as hollow vesicles of various shapes. Chemically, thyroid hormones are α-amino acid derivatives of tyrosine and include thyroxine and triiodothyronine. Thyroxine contains four atoms of iodine and is abbreviated as T4; triiodothyronine, which has three atoms of iodine, is abbreviated as T3. Thyroxine, the major secretory product, is not the biologically active form of the hormone; however, it must be transformed to T3 at extrathyroidal sites. Biosynthesis of thyroid hormones involves several discrete processes, all of which depend upon the products of three genes that are expressed predominantly in thyroid follicle cells: the sodium iodide symporter (NIS), thyroglobulin, and thyroid peroxidase. The physiological effects of thyroid hormones include growth and maturation of skeletal system, central and autonomic nervous systems, and cardiovascular system. The thyroid gland exerts profound effects on oxidative, carbohydrate, and lipid metabolism in humans. The principal regulator of thyroid function is the thyroid-stimulating hormone (TSH), which is secreted by thyrotropes in the pituitary gland; however, maintaining constant levels of thyroid hormones in blood depends upon negative feedback effects of T4 and T3, which inhibit synthesis and secretion of TSH. Evidence indicates that T3 and T4 produce the majority of their actions through nuclear receptors, but detailed understanding require further research.