A naturally occuring substance secreted by specialised cells that affects the metabolism or behaviour of other cells possessing functional receptors for the hormone. Hormones may be hydrophilic, like insulin, in which case the receptors are on the cell surface or lipophilic, like the steroids, where the receptor can be intracellular.
Some cancers may be stimulated and others inhibited by hormones (for example breast and prostate cancer). Some hormones can be synthesised and used as replacement in glandular deficiencies or as treatment for some cancers. Hormones may be injected, others taken by mouth.