1. A chemical released into the body by a gland or tissue that has a specific effect on tissues elsewhere in the body. 2. A substance that originates in an organ, gland, or part, that is conveyed through the circulation to another part of the body, to stimulate a function or the release of another hormone. 3. Any various chemical messengers, produced by the adrenal, pituitary, thyroid, ovaries, testes, and other glands, that have far-reaching effects throughout the body. Hormones regulate everything from growth and tissue repair to metabolism, reproduction, blood pressure, and the body’s response to stress.