ABSTRACT
The thyroid gland of mammals synthesizes and releases three hormones: thy roxine (tetraiodothyronine [T4]), triiodothyronine (T3), and calcitonin. Structurally related and iodine-containing, T4 and T3 are the products of epithelial cells that make up the spherical thyroid follicles. These hormones influence the metabolic level in most tissues and are required for normal growth and differentiation. The single-chain polypeptide calcitonin, the product of the parafollicular C cells of the thyroid gland, is involved in the regulation of serum calcium homeostasis. The focus of this chapter will be on the physiology of thyroid gland secretion of T4 and T3 and common disorders affecting the thyroid gland.
