Physiology of the pituitary, thyroid, parathyroid and adrenal glands

The pituitary gland is made of clusters of cells producing specific hormones that control growth (growth hormone), thyroid function (triiodothyronine (T3) and thyroxine (T4)), adrenal function (adrenocorticotrophic hormone (ACTH)) and gonadal function (follicle-stimulating hormone and luteinizing hormone). In addition, the neurons that join the posterior pituitary (neurohypophysis) secrete vasopressin – the antidiuretic hormone involved in maintaining water balance.The negative feedback loop is the basic mechanism to control the regulation of all endocrine glands. Hypothalamic peptides – releasing hormones (e.g. TRH, corticotrophin-releasing hormone) reach the hypophysis via the portal venous system and induce the secretion of specific stimulating hormones (e.g. thyroid-stimulating hormone, ACTH) that drive the end-target endocrine cells to secrete hormones (e.g. thyroid hormones – T3 and T4 or adrenal hormones – cortisol, dehydroepiandrosterone sulphate). The plasma levels of these circulating hormones inhibit the pituitary (short feedback) or the hypothalamus (long feedback) and limit the further release of releasing and stimulating hormones. The effects of circulating hormones on different tissues are mediated via specific receptors on the cell membrane (e.g. vasopressin receptors), in the cytoplasm (steroid receptor for cortisol) or in the nucleus (e.g. thyroid hormone receptors). Understanding the physiological effects of peripheral hormones helps understanding the mechanisms by which clinical signs and symptoms develop in diseases characterized by excessive hormone secretion (e.g. thyrotoxicosis, Cushing syndrome, phaeochromocytomas) or lack of hormone secretion (e.g. diabetes insipidus).The parathyroid gland and adrenal medulla are not controlled by the pituitary but play important roles in calcium metabolism and the adrenergic (sympathetic nervous system) function respectively.