I.

Introduction

Hormone, substance in animals and plants that regulates bodily processes such as growth, metabolism, reproduction, and the functioning of various organs. In animals, hormones are secreted directly into the bloodstream by ductless or endocrine glands. A state of dynamic equilibrium is maintained among the hormones, and they are able to produce their effects in surprisingly minute concentrations. Their distribution through the blood results in a response that, although slower than that of a nervous reaction, is frequently maintained over a longer period of time.

Deficiency or excess of any one of the hormones upsets the chemical equilibrium that is essential to health, normal growth, and, in extreme cases, life. The method of treating diseases arising from endocrine disturbances, known as organotherapy, involves the use of preparations of animal organs and synthetic products. It has achieved, and continues to achieve, spectacular successes in treating a number of conditions, including Addison’s Disease, Cretinism, Diabetes Mellitus, Gigantism, Goitre, and Myxoedema.

The principal organs involved in the production of hormones are the pituitary gland; the thyroid gland; the parathyroid glands; the adrenal gland or superadrenal gland; the pancreas; the gonads, or reproductive glands; the placenta, and, under certain conditions, the mucous membrane of the small intestine.

II.

Pituitary Hormones

The pituitary gland, situated below the brain, has three parts; the anterior lobe; the intermediate lobe, which is generally thought to be non-functional or virtually absent in humans; and the posterior lobe. The anterior lobe is considered the master gland of the endocrine system. It controls the growth of the skeleton; regulates the function of the thyroid; affects the action of the gonads and the adrenals; produces substances that interact with those excreted by the pancreas; and may influence the parathyroids. It also secretes the hormone prolactin, except when inhibited by the progesterone secreted by the placenta (see below); prolactin stimulates the formation of milk in mature mammary glands. It also secretes ACTH, a hormone that stimulates the secretion of hormones in the adrenal glands, such as cortisol, which has many important metabolic functions. The anterior lobe also secretes the melanocyte-stimulating hormone intermedin, which stimulates the functioning of pigment cells.

Hormones produced or stored in the posterior lobe increase blood pressure, prevent excessive secretion of urine, and stimulate contraction in muscles of the uterus. Several of the pituitary hormones are opposed in effect to other hormones; for example, the diabetogenic effect inhibits the performance of insulin.

III.

Thyroid Hormone

The hormone of the thyroid gland stimulates general metabolism; it also increases the sensitivity of various organs, especially the central nervous system, and has a pronounced effect on the change from infantile to adult form. The secretion of the thyroid hormone is controlled primarily by the anterior lobe of the pituitary but is also affected by the hormones of the ovaries and, in turn, affects their development and function. The hormone of the parathyroid glands controls the concentration of calcium and phosphate in the blood.

IV.

Pancreatic Hormones

The pancreas secretes at least two hormones, insulin and glucagon, which regulate metabolism of carbohydrates in the body. Insulin, which is a protein, was synthesized by Canadian scientists in 1965, and glucagon was synthesized by German researchers in 1968.

V.

Adrenal Hormones

The adrenal glands are divided into two parts, an outer cortex and an inner medulla. Extracts of the cortex contain hormones, such as cortisol, that control the concentration of salts and water in the body fluids and are essential for the maintenance of life in an individual. The cortical hormones are also necessary for the formation of sugar from proteins and its storage in the liver and for maintenance of resistance to physical, emotional, and toxic stresses. The cortex also secretes hormones that affect secondary sexual characteristics. The medulla, which is functionally and embryologically independent of the cortex, produces adrenalin, which increases blood sugar and acts to stimulate the circulatory system and the sympathetic nervous system, which is part of the autonomic nervous system, and the related hormone, noradrenalin.

VI.

Sex Hormones

The gonads, under the influence of the anterior lobe of the pituitary, produce hormones controlling sexual development and the various processes of reproduction. In the male, the testicular hormones (known as androgens, which include testosterone) control the development of sperm in the testes and the appearance of secondary sexual characteristics. In the female, the hormones of the ovaries are produced primarily in the ovarian follicles. These hormones, known as oestrogens, are produced by the granulosa cells, and include oestradiol, the most important, and oestrone, which is related chemically to oestradiol and is similar in action but far less potent. Oestrogenic hormones interact with those of the anterior lobe of the pituitary to control the cycle of ovulation and menstruation.

VII.

Ovulation

During this cycle the corpus luteum is produced; this forms at the site of a ruptured follicle following ovulation. The corpus luteum secretes mainly progesterone, and thus controls the cycle of menstruation. Progesterone is also formed in large amounts by the placenta during pregnancy; together with the oestrogens, it causes development of the mammary glands (breasts) and, at the same time, instructs the hypothalamus to inhibit the secretion of prolactin by the pituitary.

A.

Contraception and HRT

The various oral contraceptives used since the 1950s, which are usually a combination of oestrogen and progestogen, are collectively known as the birth control “pill”. They inhibit ovulation and conception. The placenta also secretes a hormone, similar to one produced by the pituitary and called chorionic gonadotrophin, that inhibits ovulation. This hormone is present in the blood in substantial quantities and is excreted readily by the kidneys; it is the basis of some pregnancy tests.

Hormone replacement therapy (HRT) is a growing range of treatments supplementing the hormones, particularly oestrogens, that are no longer produced in women who have started or have been through the menopause, or who have had their ovaries surgically removed. HRT alleviates the symptoms associated with the menopause and helps to prevent osteoporosis (thinning of bones), as without oestrogen calcium is leached out of the bones into the bloodstream to boost calcium levels in the blood.

VIII.

Digestive Hormones

A special group of hormones is secreted by the mucous membrane of the small intestines at a certain stage of digestion. They act to coordinate digestive activities, controlling the motility of the pylorus, duodenum, gallbladder, and bile duct. They also stimulate formation of the digestive juices of the small intestines, of liver bile, and of the internal and external secretion of the pancreas.

The hormone gastrin is produced by one part of the lining of the stomach and is released into the blood by nerve impulses that are initiated by tasting food or by the presence of food in the stomach. In the stomach, gastrin stimulates the secretion of pepsin—a protein-splitting enzyme—and hydrochloric acid, and stimulates contractions of the stomach wall. Gastrin stimulates secretion of digestive enzymes and insulin by the pancreas, and secretion of bile by the liver. When the pancreas produces insufficient insulin to metabolize glucose, or when the insulin fails to act on specific receptor cells in the blood, the result is diabetes mellitus or diabetes insipidus.

IX.

Hormonal Mechanisms

When secreted into the bloodstream, hormones bond with specific plasma or carrier proteins that prevent them from degenerating prematurely and keep them from becoming immediately absorbed by the tissues they affect, called target tissues. Target tissues usually have receptor sites or cells that selectively trap and concentrate their respective hormone molecules, which are then held until the moment they are to react with target tissues.

Hormones are believed to affect target tissues in three basic ways. First, they regulate the permeability of the outer cell membrane and intracellular membranes. The hormone insulin is thus believed to relax the membranes of skeletal muscle cells, enabling them to transport glucose rapidly.

Second, hormones modify intracellular enzymes. Adrenalin, for example, coming from the adrenal medulla, enables the breakdown of glycogen into six-carbon sugars in liver and muscle cells by activating the membrane-bound enzyme adenyl cyclase. This process is mediated by “second messenger” molecules—non-hormone chemicals located in the target cells. When cell receptors bond with hormones from the bloodstream, they alter the activity level of the second messengers, which either stimulate or inhibit the target tissue.

The third way that hormones may affect target tissues is by changing the gene activity of the target cells. Either by directly entering the target cells, or more likely by acting indirectly through second messengers, hormones have been found to cause “puffs” in particular chromosomes, indicating that genes are actively involved in the synthesis of messenger ribonucleic acid (mRNA) molecules (see Genetics; Nucleic Acids). The mRNA molecules, in turn, are translated into specific proteins that are necessary for such diverse hormonally produced processes as moulting in insects, or maintaining secondary sexual characteristics in vertebrates.

X.

Producing Hormones from Bacteria

Through recombinant DNA technology (genetic engineering), researchers have developed techniques for using genetically modified bacteria to produce insulin in quantity for diabetes patients. Similar methods are used to produce growth hormone for treating growth-deficient children. By conventional methods, the growth hormone from 50 donated human pituitary glands is required for only one year’s treatment.