hormone

a chemical substance that is naturally produced by the body and acts as a messenger. A hormone is produced by CELLS or GLANDS in one part of the body and passes into the bloodstream. When it reaches another specific site, its 'target organ', it causes a reaction there, modifying the structure or function of cells, perhaps by causing the release of another hormone. Hormones are secreted by the ENDOCRINE GLANDS , and examples are the sex hormones, e.g. TESTOSTERONE , secreted by the testes (see TESTICLE ), and oestradiol and PROGESTERONE , secreted by the ovaries (see OVARY ).