Hormones are chemical messengers that regulate bodily processes such as growth, reproduction, metabolism, digestion, mineral and fluid balance, and the functioning of various organs. In animals, hormones are secreted by organs, tissues, and glands of the endocrine system directly into the blood by and carried in the bloodstream to target organs. Once there, they alter the activities of the organ or regulate the production of other hormones.

Hormones aid in determining an animal behavior patterns and also the probability that a particular behavior will occur. Hormones exert substantial control over the following behavior patterns: parental care, territorial behavior, metamorphosis (in insects), foraging behavior, and circadian rhythms (behavior patterns that always occur at the same time each day). Most hormones fall into two main categories: peptides (chains of amino acids) and lipids (which include steroids).

The Endocrine System

The endocrine system produces many hormones. The major endocrine glands are the pituitary, located at the base of the brain, the thyroid and the parathyroid in the neck, and the pancreas, adrenals, and gonads (reproductive glands) in the torso. Hormones are also produced by the stomach, the small intestine, and the kidneys.

The Pituitary Gland

The tiny pituitary gland was once considered to be the "master gland" of the body. Today, scientists realize that the hypothalamus modulates the activities of the pituitary. The pituitary gland is composed of two lobes, the anterior pituitary and the posterior pituitary. The anterior pituitary produces six major hormones, and the posterior pituitary stores two hormones originating in the hypothalamus. The pituitary's target endocrine glands are the thyroid, adrenal gland, and the gonads. Through these glands it controls the growth of the skeleton and regulates the functions of the thyroid and the gonads. One pituitary hormone, called growth hormone, must be secreted in just the right amount for normal growth in childhood. If too little is produced, the child will become a dwarf; if too much is secreted, the child will grow to be a giant.

Thyroid Hormones

Thyroid hormones stimulates oxygen consumption and metabolism, regulating the growth of body tissues and the rate at which food is burned to provide body energy. They also increase the sensitivity of some organs, especially the central nervous system. If the thyroid becomes overactive, it produces a condition called hyperthyroidism, which causes nervousness and irritability. Another thyroid condition, cretinism, is caused by a congenital lack of thyroid secretion. It is marked by greatly stunted physical and mental growth.

Insulin and Glucagon

The pancreas produces two important hormones, insulin and glucagon. Insulin affects most cells in the body because it is involved in the metabolism of carbohydrates, proteins, and fat. Too little insulin results in diabetes, a condition of high levels of blood sugar resulting in weakness and dehydration. Too much insulin causes very low levels of blood sugar, resulting in weakness, anxiety, and convulsions. Glucagon raises the blood sugar level. Together, insulin and glucagon help keep a normal level of glucose in the blood.

Adrenal Glands and Gonads

Hormones in the adrenal glands control the concentration of salts and water in body fluids and are necessary for maintaining life. They also produce sugar from proteins and store it in the liver to help maintain resistance to physical and emotional stress.

Hormones found in the gonads control sexual development and reproductive processes. A fetus's sex is determined by genetics, but certain hormones produced by the gonads (under the influence of the pituitary gland) must be present for the fetus to develop appropriate sex organs.

Early Discoveries

The term hormone (from the Greek for "to spur on") was first used by the British biochemists William Bayliss and Emest Starling in 1904. The duo coined the term to describe the action of a digestive substance they had isolated called secretin, which stimulates the flow of pancreatic juice. Scientists later realized that the first hormone to have actually been isolated and synthesized (artificially created) was the adrenal hormone epinephrine, identified by Japanese American chemist Jokichi Takemine in 1901.

Beef insulin hormone. Most hormones fall into two main categories called peptides and lipids.
Beef insulin hormone. Most hormones fall into two main categories called peptides and lipids.

The isolation of the thyroid hormone thyroxine in 1914 by American biochemist Edward Kendall marked another important milestone in understanding how hormones work. Too much or too little thyroxine can cause illness. One of the earliest thyroid disorders diagnosed was Graves' disease (a disease of the thyroid gland resulting in increased size and activity of the gland). Its cause is unknown, but it is believed to be an autoimmune disorder, and it occurs most often in women. Graves' disease often results in bulging eyes, tachycardia (fast and irregular heartbeat), and thickening of the skin.

One of the most well known developments in endocrinology was the isolation of insulin by the Canadian physicians Frederick Banting and Charles Best in 1921. Soon various types of injectable insulin were being used to treat diabetes.

The 1920s also saw the discovery that the pituitary gland stimulates the sex organs and the introduction (in 1928) of the first pregnancy test. Soon after, the relationship between female sex hormones and the menstrual cycle was explained. Working from this relationship, Gregory Pincus would introduce the first oral contraceptives in the 1950s.

In the 1920s and 1930s it was also learned that the adrenal glands contain hormones that control the concentration of salts and water in body fluids and are essential for maintaining life. Adrenal hormones are also essential for sugar and protein formation and storage in the liver. They also help resist physical and emotional stresses In the 1930s, Kendall and the Swiss chemist Tadeus Reichstein both isolated one of these hormones, cortisone, which is a steroid.

American researcher Philip Hench used cortisone to reduce inflammation in rheumatoid arthritis and other connective tissue diseases in the 1940s, making cortisone the first hormone to be used medically.

Synthetic Hormones

Scientists eventually learned to make some hormones in the laboratory. Vincent Du Vigneaud, an American biochemist, synthesized the small pituitary hormone oxytocin, which regulates milk production in the mammary glands and causes uterine contractions. This led to the synthesis of many larger and more complex hormones for medical purposes.

Today, hormone production can be automated, yielding a great deal of synthetic hormone at a rapid rate to meet increasing medical demands. Patients with hormone deficiencies can often be treated effectively with these artificial hormones. Diabetics, for example, receive insulin. Patients suffering from dwarfism are given human growth hormone. Oral contraception combines the use of estrogen and progesterone to prevent ovulation and thus pregnancy. Hormones are also used to treat infertility.

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