ABSTRACT
The thyroid gland is a butterfly-shaped organ located anteriorly to the trachea at the level of the second and third tracheal rings. It is the earliest endocrine structure to appear during human development. The function of the thyroid gland is to produce and store thyroid hormones and functional unit of the thyroid gland is the thyroid follicle. The thyroid secretes two major hormones, thyroxine and triiodothyronine, commonly called T4 and T3. Thyroid hormone regulates metabolic processes essential for normal growth and development as well as regulating metabolism in the adult. Thyroid hormone actions are mostly, but not exclusively, mediated by nuclear hormone receptors. Thyroid hormone regulates metabolism primarily through actions in the brain, white fat, brown fat, skeletal muscle, liver and pancreas. Thyroid secretion is controlled primarily by thyroid-stimulating hormone (TSH) secreted by the anterior pituitary gland. TSH measurement in serum is the most sensitive test available to clinicians for the diagnosis of most states of thyroid dysfunction. This chapter reviews current understanding of the physiology of thyroid gland.
