One of the chemical messengers produced by endocrine glands, whose secretions are liberated directly into the bloodstream and transported to a distant part or parts of the body, where they exert a specific effect for the benefit of the body as a whole. The endocrine glands involved in the maintenance of normal body conditions are pituitary, thyroid, parathyroid, adrenal, pancreas, ovary, and testis. However, these organs are not the only tissues concerned in the hormonal regulation of body processes. For example, the duodenal mucosa, which is not organized as an endocrine gland, elaborates a substance called secretin which stimulates the pancreas to produce its digestive juices. The placenta is also a very important hormone-producing tissue. See separate articles on the individual glands.
The hormones obtained from extracts of the endocrine glands may be classified into four groups according to their chemical constitution: (1) phenol derivatives, such as epinephrine, norepinephrine, thyroxine, and triiodothyronine; (2) proteins, such as the anterior pituitary hormones, with the exception of adrenocorticotropic hormone (ACTH), human chorionic gonadotropin, pregnant-mare-serum gonadotropin, and thyroglobulin; (3) peptides, such as insulin, glucagon, ACTH, vasopressin, oxytocin, and secretin; and (4) steroids, such as estrogens, androgens, progesterone, and corticoids. Hormones, with a few exceptions like pituitary growth hormone and insulin, may also be classified as either tropic hormones or target-organ hormones. The former work indirectly through the organs or glands which they stimulate, whereas the latter exert a direct effect on peripheral tissues. See Endocrine system (vertebrate)