Mazzone, Theodore Section of Diabetes and Metabolism, University of Illinois, Chicago, Illinois.
Last reviewed:May 2019
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Any of the chemical messengers produced chiefly 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. Hormones are unique chemical messengers produced predominantly by the endocrine system. (There are also plant hormones, that is, naturally occurring, organic substances that influence plant physiological processes at low concentrations; however, these nonendocrine hormones are not discussed in this article.) The endocrine glands involved in the maintenance of normal body conditions are the pituitary, thyroid, parathyroid, adrenal, pancreas, ovary, and testis (Fig. 1). The pineal gland is another important endocrine gland; located in the brain, it secretes melatonin, which is strongly regulated by light stimuli and is a key component of the circadian timing system. However, the aforementioned organs are not the only tissues that are involved with 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 also: Adrenal gland; Biological clock; Endocrine mechanisms; Endocrine system (invertebrate); Endocrine system (vertebrate); Gland; Ovary; Pancreas; Parathyroid gland; Pineal gland; Pituitary gland; Plant hormone; Signal transduction; Testis; Thyroid gland
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