A hormone (from Greek horman - "to set in motion") is a chemical messenger from one cell (or group of cells) to another. All multicellular organisms (including plants) produce hormones. The best known hormones are those produced by endocrine glands of vertebrate animals, but hormones are produced by nearly every organ system and tissue type in a human or animal body. Hormone molecules are secreted (released) directly into the bloodstream, other body fluids, or into adjacent tissues. They move by circulation or diffusion to their target cells, which may be nearby cells (paracrine action) in the same tissue or cells of a distant organ of the body. The function of hormones is to serve as a signal to the target cells; the action of hormones is determined by the pattern of secretion and the signal transduction of the receiving tissue.
Hormone actions vary widely, but can include stimulation or inhibition of growth, induction or suppression of apoptosis (programmed cell death), activation or inhibition of the immune system, regulating metabolism and preparation for a new activity (e.g. fighting, fleeing, mating) or phase of life (e.g. puberty, caring for offspring, menopause). In many cases, one hormone may regulate the production and release of other hormones. Many of the responses to hormone signals can be described as serving to regulate metabolic activity of an organ or tissue. Hormones also control the reproductive cycle of virtually all multicellular organisms.
Vertebrate hormones fall into four chemical classes:
- Amine-derived hormones are derivatives of the amino acids tyrosine and tryptophan. Examples are catecholamines and thyroxine.
Peptide hormones consist of chains of amino acids. Examples of small peptide hormones are TRH and vasopressin. Peptides composed of scores or hundreds of amino acids are referred to as proteins. Examples of protein hormones include insulin and growth hormone.
Steroid hormones are derived from cholesterol. The adrenal cortex and the gonads are primary sources. Examples of steroid hormones are testosterone and cortisol. Sterol hormones such as calcitriol are a homologous system.
Lipid and phospholipid hormones are derived from lipids such as linoleic acid and phospholipids such as arachidonic acid. The main class is the eicosanoids, which includes the widely studied prostaglandins.
Physiology of hormones
Every cell is capable of producing a vast number of regulatory molecules. The classical endocrine glands and their hormone products are specialized to serve regulation on the overall organism level, but can in many instances be used in other ways or only on the tissue level.
The rate of production of a given hormone is most commonly regulated by a homeostatic control system, generally by negative feedback. Homeostatic regulation of hormones depends, apart from production, on the metabolism and excretion of hormones.
Hormone secretion can be stimulated and inhibited by:
- Other hormones (stimulating or releasing-hormones)
- Plasma concentrations of ions or nutrients, as well as binding globulins
- Neurons and mental activity
- Environmental changes, e.g. of light or temperature
A large number of hormones is used as medication. The most commonly prescribed hormones are estrogens and progestagens (in the contraceptive pill and as HRT), thyroxine (as levothyroxine, for hypothyroidism) and steroids (for autoimmune diseases and several respiratory disorders). Insulin is used by many diabetics. Local preparations for use in otolaryngology often contain pharmacologic equivalents of adrenalin, while steroid and vitamin D creams are used extensively in dermatological practice.
A "pharmacologic dose" of a hormone is a medical usage referring to an amount of a hormone far greater than naturally occurs in a healthy body. The effects of pharmacologic doses of hormones may be different from responses to naturally occurring amounts and may be therapeutically useful. An example is the ability of pharmacologic doses of glucocorticoid to suppress inflammation.
Important human hormones
One special group of hormones are trophic hormones that act as stimulants of hormone production of other endocrine glands. For example: thyroid-stimulating hormone (TSH) causes growth and increased activity of another endocrine gland--the thyroid--hence increasing output of thyroid hormones.
A recently identified and studied class of hormones is that of the "Hunger Hormones" - ghrelin, orexin and PYY 3-36 - and their antagonists - e.g. leptin.
Current North American and international usage is estrogen, gonadotropin. British usage retains the Greek diphthong in oestrogen and the unvoiced aspirant h in gonadotrophin.