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Hormone definition, Biochemistry. any of various internally secreted compounds, as insulin or thyroxine, formed in endocrine glands, that affect the functions of specifically receptive organs or tissues when transported to them by the body fluids. Pharmacology. a synthetic substance used in medicine to act like such a compound when introduced into the body.
Freebase(4.00 / 1 vote)Rate this definition: Hormone. A hormone is a chemical released by a cell, a gland, or an organ in one part of the body that affects cells in other parts of the organism. Generally, only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one cell to another. All multicellular organisms produce hormones; plant hormones are also called phytohormones. Hormones in animals are often transported in the blood. Cells respond to a hormone when they express a specific receptor for that hormone. The hormone binds to the receptor protein, resulting in the activation of a signal transduction mechanism that ultimately leads to cell type-specific responses.