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 produce hormones (including plants - see article phytohormone).

The best known animal (and human) 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.

History
The concept of internal secretion developed in the 19th century; Claude Bernard described it in 1855, but did not specifically address the possibility of secretions of one organ acting as messengers to others. Still, various endocrine conditions were recognised and even treated adequately (e.g. hypothyroidism with extract of thyroid glands).

The major breakthrough was the identification of secretin, the hormone secreted by the duodenum that stimulates pancreatic secretions, by Ernest Starling and William Bayliss in 1902. Previously, the process had been considered (e.g. by Ivan Pavlov) to be regulated by the nervous system. Starling and Bayliss demonstrated that injecting duodenal extract into dogs rapidly increased pancreatic secretions, raising the possibility of a chemical messenger.

Starling is also credited with introducing the term "hormone", having coined it in a 1905 lecture. Later reports indicate it was suggested to him by the Cambridge physiologist William B. Hardy (Henderson 2005).

The remainder of the 20th century saw all the major hormones discovered, as well as the cloning of the relevant genes and the identification of the many interlocking feedback mechanisms that characterise the endocrine system.

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
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.

Types of hormones
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.