thyroid-stimulating hormone (TSH): see thyrotropin.

parathyroid hormone or parathormone, a hormone secreted by the parathyroid glands that regulates the metabolism of calcium and phosphate in the body. It has been purified extensively and appears to be a protein containing 84 amino-acid residues, a sequence of which about 33 to 35 are necessary for biological activity. Parathyroid hormone acts to raise the extracellular calcium concentration, that is, the concentration of calcium ions in the spaces between the cells of the body and in the blood plasma; it promotes the absorption of calcium by the intestine, mobilizes calcium salts from the bones, and increases the tendency of the kidney to recover calcium from the urine. The hormone also enhances both the excretion of phosphate by the kidneys and its uptake by the cells. This removes phosphate, which tends to form a relatively insoluble salt with calcium, from the extracellular spaces, allowing more calcium to remain in solution. Calcium is intimately involved not only in the formation of bone, but also in the functioning of the nervous system; thus hypoparathyroidism, the disease associated with a deficiency in parathyroid hormone secretion, is characterized by muscle spasms leading eventually to generalized convulsions and various psychiatric symptoms. This condition is sometimes successfully treated by the administration of the hormone. Hyperparathyroidism, the result of oversecretion of the hormone, often leads to the resorption of bone and can only be treated by the surgical removal of all of the parathyroid glands, which can be found in unusual locations.

lutenizing hormone (LH): see gonadotropic hormone.

human growth hormone (HGH): see growth hormone.

hormone, secretory substance carried from one gland or organ of the body via the bloodstream to more or less specific tissues, where it exerts some influence upon the metabolism of the target tissue. Normally, various hormones are produced and secreted by the endocrine glands (see endocrine system), including the pituitary, thyroid, parathyroids, adrenals, ovaries, testes, pancreatic islets, certain portions of the gastrointestinal tract, and the placenta, among the mammalian species. As lack of any one of them may cause serious disorders, many hormones are now produced synthetically and used in treatment where a deficiency exists. The hormones of the anterior pituitary include thyrotropin, adrenocorticotropic hormone, the gonadotropic hormones, and growth hormone; the posterior pituitary secretes antidiuretic hormone, prolactin, and oxytocin. The thyroids secrete thyroxine and calcitonin, and the parathyroids secrete parathyroid hormone. The adrenal medulla secretes epinephrine and norepinephrine while the cortex of the same gland releases aldosterone, corticosterone, cortisol, and cortisone. The ovaries primarily secrete estrogen and progesterone and the testes testosterone. The adrenal cortex, ovaries, and testes in fact produce at least small amounts of all of the steroid hormones. The islets of Langerhans in the pancreas secrete insulin, glucagon, and somatostatin. The kidneys also produce erythropoietin, which produces erythrocytes (red blood cells). The passage of chyme (see digestive system) from the stomach to the duodenum causes the latter to release secretin, which stimulates the flow of pancreatic juice. The duodenum can also be stimulated by the presence of fats in the chyme to secrete cholecystokinin, a hormone that stimulates the gall bladder to contract and release bile. There is evidence that the upper intestine secretes pancreatozymin, which enhances the amount of digestive enzymes in the pancreatic juice. In addition, the pyloric region of the stomach secretes gastrin, a hormone that increases the secretion of hydrochloric acid into the stomach. The placenta has been shown to secrete progesterone and chorionic gonadotropin. There is evidence that it even contains a substance similar to growth hormone. Insects have a unique hormonal system that includes ecdysone, a steroid that influences molting and metamorphosis, and juvenile hormone, needed for early development. Plants, too, have a hormonal system, which includes the auxins, the gibberellins, the cytokinins, and substances associated with the formation of flowers, tubers, bulbs, and buds. Ethylene is said to function as a hormone in plants, acting to hasten the ripening of fruits.

growth hormone or somatotropin, glycoprotein hormone released by the anterior pituitary gland that is necessary for normal skeletal growth in humans (see protein). Evidence suggests that the secretion of human growth hormone (HGH) is regulated by the release of certain peptides by the hypothalamus of the brain. One such substance, called somatostatin, has been shown to inhibit the secretion of HGH. HGH is known to act upon many aspects of cellular metabolism, but its most obvious effect is the stimulation of the growth of cartilage and bone in children.

See also auxins (plant growth hormones).

Role in Dwarfism and Gigantism

A deficiency of growth hormone secretion before puberty (by the end of which the synthesis of new bone tissue is complete) results in pituitary dwarfism. Pituitary dwarfs, who can be as little as 3 to 4 ft (91-122 cm) tall, are generally well proportioned except for the head, which may be relatively large when compared to the body (this relationship of head to body is similar to that of normal children). Unlike cretins, whose dwarfism is caused by a deficiency of thyroxine, pituitary dwarfs are not mentally retarded; they are often sexually immature. They can be treated by injections of synthetic growth hormone, either somatrem or somatropin, which are produced by genetically engineered bacteria.

An excess of growth hormone in children results in gigantism; these children grow to be over 7 ft (213 cm) in height and have disproportionately long limbs. Excess growth hormone produced after puberty has little effect on the growth of the skeleton, but it results in a disease affecting terminal skeletal structures known as acromegaly.

Other Medical Uses

HGH has been used with some success to combat the weight loss and general wasting characteristic of AIDS and cancer. It is used illegally by bodybuilders and athletes to increase muscle mass. Controversy surrounds its use in normal children who simply want to be taller. In addition, a 1990 medical study that reported the reversal of many of the physiological effects of aging with regular injections of HGH has created a lucrative black market for it and has prompted funding of further trials. There has been no conclusive evidence, however, to support the use of HGH as an anti-aging treatment, and it can cause serious side effects, including diabetes, in older adults.

gonadotropic hormone or gonadotropin, any one of three glycoprotein (see protein) hormones released by either the anterior pituitary gland or the placenta (the organ in which maternal and fetal blood exchange nutrients and waste products) that have various effects upon the ovaries and testes (see testis). Gonadotropin-releasing hormone (Gn-RH) is produced and released from the hypothalamus. Gn-RH release stimulates the secretion of both follicle-stimulating hormone (FSH) and lutenizing hormone (LH) from the pituitary gland. Gn-RH is a peptide composed of ten amino acids which are synthesized in the hypothalamus (that portion of the brain nearest the pituitary). This hormone travels in the bloodstream to the anterior pituitary, where it causes the release of the gonadotropic hormones. The hormones FSH and LH inhibit the amount of Gn-RH released by a mechanism called "negative feedback." In the female, FSH causes an increase in the weight of the ovaries and encourages the growth of Graafian follicles (containing maturing eggs); in males, FSH produces spermatogenesis in the testes. In females, secretion of LH is associated with the maturation of the follicles, the manifestation of heat (or estrus), and the release of the egg from the follicle, which is transformed into a corpus luteum. In males, LH stimulates the testes to release testosterone. Sex hormones released from the ovaries and testes eventually reach the hypothalamus and help to regulate the hormonal cycle. Human chorionic gonadotropin (HCG), produced in the placenta, helps to maintain pregnancy once a fetus begins to develop. It appears in the urine in approximately the first week after the first missed menstrual period, and is the basis for two kinds of pregnancy tests; in the Ascheim-Zondek testm it causes the ovaries of an immature female rat or mouse to gain weight and produce ripened follicles, and in the Friedman test, it stimulates female rabbits to ovulate.

follicle-stimulating hormone (FSH): see gonadotropic hormone.

antidiuretic hormone, polypeptide hormone secreted by the posterior pituitary gland. Its principal action is to regulate the amount of water excreted by the kidneys. Antidiuretic hormone (ADH), known also as vasopressin, causes the kidneys to resorb water directly from the renal tubules, thus concentrating the salts and waste products in the liquid, which will eventually become urine. ADH secretion by the pituitary is regulated by neural connections from the hypothalamus, which is believed to monitor either the volume of blood passing through it or the concentration of water in the blood. Dehydration or body stress will raise ADH secretion and water will be retained. Alcohol inhibits ADH secretion. Failure of the pituitary to produce ADH results in diabetes insipidus. In pharmacological doses ADH acts as a vasoconstrictor. The structure and chemical synthesis of ADH was announced (1953) by Nobel laureate Vincent Du Vigneaud and others.

adrenocorticotropic hormone, polypeptide hormone secreted by the anterior pituitary gland. Its chief function is to stimulate the cortex of the adrenal gland to secrete adrenocortical steroids, chief among them cortisone. The release of adrenocorticotropic hormone (ACTH), also known as corticotropin, is stimulated by corticotropin-releasing factor (CRF), a secretion of the hypothalamus. ACTH secretion is an excellent example of the regulation of a biological system by a negative-feedback mechanism; high levels of adrenocortical steroids in the blood tend to decrease ACTH release, whereas low steroid levels have the opposite effect. ACTH has the same pharmacologic and clinical effects as cortisone when given intravenously or intramuscularly; however, it has no value when applied externally and cannot be taken orally since it is deactivated by digestive enzymes. The action of ACTH is contingent upon normally functioning adrenal glands and is therefore useless in disorders caused by adrenal insufficiency, e.g., as replacement therapy where both adrenal glands have been removed.

or somatotropin

Peptide hormone secreted by the anterior lobe of the pituitary gland. It promotes growth of bone and other body tissues by stimulating protein synthesis and fat breakdown (for energy). Excessive production causes gigantism, acromegaly, or other malformations; deficient production results in dwarfism, dramatically relieved if GH is given before puberty. Genetic engineering techniques now permit large-scale production of adequate amounts of GH for that purpose.

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Organic compound (often a steroid or peptide) that is produced in one part of a multicellular organism and travels to another part to exert its action. Hormones regulate physiological activities including growth, reproduction, and homeostasis in vertebrates; molting and maintenance of the larval state (see larva) in insects; and growth, bud dormancy, and leaf shedding in plants. Most vertebrate hormones originate in specialized tissues (see endocrine system; gland) and are carried to their targets through the circulation. Among the many mammalian hormones are ACTH, sex hormones, thyroxine, insulin, and epinephrine. Insect hormones include ecdysone, thoracotropic hormone, and juvenile hormone. Plant hormones include ethylene, abscisin, auxins, gibberellins, and cytokinins.

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in full adrenocorticotropic hormone

Polypeptide hormone made in the pituitary gland. It regulates the activity of part of the adrenal cortex (see adrenal glands), the production site of important steroid hormones that affect electrolyte and water balance and the metabolism of fats, carbohydrates, and proteins. ACTH is found in vertebrates (except jawless fishes); in mammals it contains 39 amino acids. Overproduction of ACTH is one cause of Cushing syndrome.

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