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A hormone is a chemical director of biological activity that travels through some portion of the body as a messenger. On a weight basis, hormones are some of the most powerful of all known biological substances. Hormones can be classified according to their basic chemical structures (such as steroid), or their effects (such as anabolic). All multicellular organisms, including both plants and animals, produce hormones, and each of these substances has major effects in the growth, development and metabolism in the creature that produces them. Some hormones are similar enough in structure that they also have an effect if placed in the systems of beings other than the kind they were created by; and so particular kind of plant hormones, for example, sometimes can affect animals, and certain hormones of one type of animal will have effects even in the body of another class or species of animal. A hormone generally does its work by turning on some sort of receptor that then initiates a chain of additional biochemical reactions. The strength of a hormone's effect depends on both the amount of active hormone that is present (an active form fits the receptor well), and also also on the presence of a functioning receptor for that hormone being available in the biological system. That means that the strength of particular hormonal effects can be regulated by a large variety of methods. Even minor changes to the structure of a circulating hormone can change its "dose effect", if the change makes the hormone either more or less able to properly fit into a receptor. Other means of regulation include increasing or decreasing the amount of production of a hormone, increasing or decreasing its rate of breakdown, so that the overall level of the hormone rises or falls. Another means of regulating hormonal effect is by either blocking or facilitating access of a hormone to a receptor molecule. Additionally, some hormones work on the same effector systems as do other hormones, and so putting a second such hormone into play can change the effects of the first. These effector systems are sometimes called second messengers. In vertebrate animals, the overall regulation of hormones is ultimately by the endocrine system and the brain.

Animal hormones

Vertebrate hormones

The best-known animal hormones are those, like insulin, estrogen, and testosterone, that are made by endocrine glands of vertebrate animals, but there are hormones made by nearly every organ system and tissue type in the body. Many hormones are secreted (released) directly into the bloodstream; some hormones, (sometimes called 'ectohormones'), aren't secreted into the blood stream, but travel by 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. Hormones act as signals to the target cells; their actions are determined not only by the amounts in which they are secreted, but also by their pattern of secretion, and exactly how they act depends on the signal transduction mechanisms of the target 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 hormones can be described as acting to regulate metabolic activity of an organ or tissue. Hormones also control the reproductive cycle of virtually all multicellular organisms.

Human hormones

Hormones in health and disease

Endocrinology is the field within medicine and the health sciences that focuses on the role of hormones in wellness and disease. The first endocrine diseases that were understood to be caused by hormonal imbalance were the result of either too much, or too little, activity of a particular gland. Later, it was recognized that the same kind of imbalances could be caused at the level of the receptors for the hormone rather than the amount of the hormone itself. For example, Insulin Resistance Syndrome, and its frequent sequela, Type II Diabetes, are understood to be an abnormally low activity of the receptor for insulin, rather than a problem with insulin production.


The concept of "internal secretion" was 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). A major breakthrough was the identification of secretin in 1902 by Ernest Starling and William Bayliss as a hormone secreted by the upper small intestine (duodenum) that stimulates [secretions of a major digestive gland, the pancreas. Previously, the process had been considered (e.g. by Ivan Pavlov) to be regulated by the nervous system. Starling and Bayliss showed that injecting duodenal extract into dogs rapidly increased pancreatic secretions, and they hypothesized the presence of a chemical messenger. Starling is also credited with introducing the term hormone, having used it in a 1905 lecture. Later reports indicate it was suggested to him by the Cambridge physiologist William B. Hardy [1].

The concept of hormone receptors gained credence with the identification of human disease states in which there was resistance to the effects of a hormone. In 1941, Fuller Albright and collaborators first described a syndrome of resistance to parathyroid hormone (PTH). These patients had the signs and symptoms of a lack of production of that hormone, but did not improve even when generous injections of the substance were given. Albright coined the jaw-breaking term, pseudohypoparathyroidism to this problem, literally - false (pseudo) low (hypo) parathyroid gland activity. Sometimes the cause of resistance to the effects of a hormone are beyond the level of the receptor, and are due to a defect in the chain of events that normally activate after the hormone properly stimulates a working receptor. "For example, resistance to TSH may rise from inactivating mutations of TSH receptor, but also from inactivating mutations of G protein α-subunit that prevent the transmission of TSH message throughout the cAMP cascade. Moreover, hormone resistance may stem from alterations other than those related to the receptor or second messengers. Indeed, at least in the thyroid field, alterations of a membrane specific transporter, such as monocarboxylate transporter 8, or enzymes involved in thyroid hormone metabolism, such as selenoprotein deiodinases, may cause particular forms of resistance to thyroid hormones." (will translate into simple word explanation) (reference:Paolo Beck-Peccoz MDPages Preface: Hormone Resistance Syndromes.Best Practice & Research Clinical Endocrinology & Metabolism Volume 20, Issue 4 , December 2006, Pages vii-viii )

Types of vertebrate hormones

Vertebrate hormones fall into three chemical classes:

  1. Amine-derived hormones are derivatives of the amino acids tyrosine and tryptophan. Examples are the catecholamines (dopamine, epinephrine and norepinephrine) and thyroxine.
  1. Peptide hormones consist of chains of amino acids. Examples are TRH and vasopressin. Peptides composed of scores or hundreds of amino acids are usually referred to as proteins, and examples include insulin, secreted by the pancreas and growth hormone, secreted from the anterior pituitary. More complex protein hormones have carbohydrate side chains and are called glycoprotein hormones. Luteinizing Hormone, Follicle-Stimulating Hormone and Thyroid-Stimulating Hormone are all glycoprotein hormones secreted from the anterior pituitary. Peptide hormones are all secreted by calcium-dependent exocytosis, and all act via specific, high affinity G-protein coupled receptors that are present on the cell membrane of the target cell.
  1. Lipid and phospholipid-derived hormones derive from lipids such as linoleic acid and phospholipids such as arachidonic acid. The main classes are the steroid hormones that derive from cholesterol and the eicosanoids. Examples of steroid hormones are testosterone and cortisol. Sterol hormones such as calcitriol are a homologous system. The adrenal cortex and the gonads are the main sources of steroid hormones. Examples of eicosanoids are the widely-studied prostaglandins.


Many hormones are 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 adrenaline, while steroid and vitamin D creams are used extensively in dermatological practice.

A 'pharmacological dose' of a hormone is a dose of a hormone that is much greater than ever occurs naturally in a healthy body. The effects of pharmacological doses can be different from responses to naturally-occurring amounts and can be therapeutically useful. An example is the ability of pharmacological doses of glucocorticoid to suppress inflammation.

Invertebrate hormones

Plant hormones


  1. Henderson J (2005) Ernest Starling and 'Hormones': an historical commentary J Endocrinol 184:5–10 PMID 15642778.