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A hormone is a chemical substance that is produced in a special tissue within a gland. Hormones are released into the bloodstream and then travel to distant responsive cells where they exhibit characteristic effects. A hormone acts like a key fitting into a lock and this produces a characteristic response. We call the 'locks' receptors. Hormones are essential chemical regulators of body functions. Well known examples include insulin, which prevents our blood sugar rising too high after a meal, and thyroxin, the thyroid hormones, which regulates the speed of body activities. Hormones are extremely potent. Although they are released in tiny amounts, their effects are profound. An oestradiol implant, which is introduced under the skin, usually when the ovaries are no longer functional after the menopause (Q 28.6), is smaller than an airgun pellet and most of it is composed of inert material which slowly releases the hormone. A typical oestradiol implant will contain only one twentieth of a gram of oestradiol. This may be enough for one year to maintain generalised well-being, to relieve menopausal symptoms and, in the long-term, to reduce the risks of cardiac disease and osteoporosis significantly. Endocrinology is the medical discipline involving hormone investigation and treatment.
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