Hypothalamus, Pituitary gland, Thyroid gland, Parathyroid glands, Adrenal glands, Pancreas, Female hormones, Male hormones
Biochemical agents that transmit messages between components of living organisms.
Hormones are biochemical messengers that regulate physiological events in living organisms. More than 100 hormones have been identified in humans. Hormones are secreted by endocrine (ductless) glands such as the hypothalamus, the pituitary gland, the pineal gland, the thyroid, the parathyroid, the thymus, the adrenals, the pancreas, the ovaries, and the testes. Hormones are secreted directly into the blood stream, where they travel to target tissues and modulate digestion, growth, maturation, reproduction, and homeostasis. Hormones do not fall into any one chemical category, but most are either protein molecules or steroid molecules. These biological managers keep the body systems functioning over the long term and help maintain health. The study of hormones is called endocrinology.
Most hormones are released into the bloodstream by a single gland. Testosterone is an exception, because it is secreted by both the adrenal glands and by the testes. The major site that keeps track of hormone levels is the hypothalamus. A number of hormones are secreted by the hypothalamus, and they stimulate or inhibit the secretion of hormones at other sites. When the hypothalamus detects high levels of a hormone, it reacts to inhibit further production. When low levels of a hormone are detected, the hypothalamus reacts to stimulate hormone production or secretion. The body handles the hormone estrogen differently. Each month, the Graafian follicle in the ovary releases increasing amounts of estrogen into the bloodstream as the egg develops. When estrogen levels rise to a certain point, the pituitary gland secretes luteinizing hormone (LH), which triggers the egg's release into the oviduct.
The major hormones secreted by the hypothalamus are corticotropin releasing hormone (CRH), thyrotropin releasing hormone (TRH), follicle stimulating hormone releasing hormone (FSHRH), luteinizing hormone releasing hormone (LHRH), and growth hormone releasing hormone (GHRH). CRH targets the adrenal glands. It triggers the adrenals to release adrenocorticotropic hormone (ACTH). ACTH functions to synthesize and release corticosteroids. TRH targets the thyroid where it functions to synthesize and release the thyroid hormones T3 and T4. FSH targets the ovaries and the testes where it enables the maturation of the ovum and of spermatozoa. LHRH also targets the ovaries and the testes, helping to promote ovulation and increase progesterone synthesis and release. GHRH targets the anterior pituitary to release growth hormone to most body tissues, increase protein synthesis, and increase blood glucose.
The hypothalamus also secretes other important hormones such as prolactin inhibiting hormone (PIH), prolactin releasing hormone (PRH), and melanocyte inhibiting hormone (MIH). PIH targets the anterior pituitary to inhibit milk production at the mammary gland, and PRH has the opposite effect. MIH targets skin pigment cells (melanocytes) to regulate pigmentation.
The pituitary has long been called the master gland because of the vast extent of its activity. It lies deep in the brain just behind the nose, and is divided into anterior and posterior regions. Both anti-diuretic hormone (ADH) and oxytocin are synthesized in the hypothalamus before moving to the posterior pituitary prior to secretion. ADH targets the collecting tubules of the kidneys, increasing their permeability to and retention of water. Lack of ADH leads to a condition called diabetes insipidus characterized by excessive urination. Oxytocin targets the uterus and the mammary glands in the breasts. Oxytocin also triggers labor contractions prior to birth and functions in the ejection of milk. The drug pitocin is a synthetic form of oxytocin and is used medically to induce labor.
The anterior pituitary (AP) secretes a number of hormones, including growth hormone (GH), ACTH, TSH, prolactin, LH, and FSH. GH controls cellular growth, protein synthesis, and elevation of blood glucose concentration. ACTH controls secretion of some hormones by the adrenal cortex (mainly cortisol). TSH controls thyroid hormone secretion in the thyroid. In males, prolactin enhances testosterone production; in females, it initiates and maintains LH to promote milk secretion from the mammary glands. In females, FSH initiates ova development and induces ovarian estrogen secretion. In males, FSH stimulates sperm production in the testes. LH stimulates ovulation and formation of the corpus luteum, which produces progesteronein females, whereas LH stimulates interstitial cells in males to produce testosterone.
The thyroid lies under the larynx and synthesizes two hormones, thyroxine and tri-iodothyronine. This gland takes up iodine from the blood and has the highest iodine level in the body. The iodine is incorporated into the thyroid hormones. Thyroxine has four iodine atoms and is called T4. Tri-iodothyronine has three iodine atoms and is called T3. Both T3 and T4 function to increase the metabolic rate of several cells and tissues. The brain, testes, lungs, and spleen are not affected by thyroid hormones, however. T3 and T4 indirectly increase blood glucose levels as well as the insulin-promoted uptake of glucose by fat cells. Their release is modulated by TRH-RH from the hypothalamus. When temperature drops, a metabolic increase is triggered by TSH. Chronic stress seems to reduce TSH secretion which, in turn, decreases T3 and T4 output.
Depressed T3 and T4 production is the trademark of hypothyroidism. If it occurs in young children, this decreased activity can cause physical and mental retardation. In adults, it creates sluggishness—mentally and physically—and is characterized further by weight gain, poor hair growth, and a swollen neck. Excessive T3 and T4 cause sweating, nervousness, weight loss, and fatigue. The thyroid also secretes calcitonin, which serves to reduce blood calcium levels. Calcitonin's role is particularly significant in children whose bones are still forming.
The parathyroid glands are attached to the bottom of the thyroid gland. They secrete the polypeptide parathyroid hormone (PTH), which plays a crucial role in monitoring blood calcium and phosphate levels. Calcium is a critical element for the human body. Even though the majority of calcium is in bone, it is also used by muscles, including cardiac muscle, for contractions, and by nerves in the release of neurotransmitters. Calcium is a powerful messenger in the immune response of inflammation and blood clotting. Both PTH and calcitonin regulate calcium levels in the kidneys, the gut, bone, and blood.
PTH deficiency can be due to autoimmune diseases or to inherited parathyroid gland problems. Low PTH capabilities cause depressed blood calcium levels and neuromuscular problems. Very low PTH can lead to tetany or muscle spasms. Excess PTH can lead to weakened bones because it causes too much calcium to be drawn from the bones and to be excreted in the urine. Abnormalities of bone mineral deposits can lead to a number of conditions, including osteoporosis and rickets. Osteoporosis can be due to dietary insufficiencies of calcium, phosphate, or vitamin C. The end result is a loss of bone mass. Rickets is usually caused by a vitamin D deficiency and results in lower rates of bone formation in children. These examples show the importance of a balanced, nutritious diet for healthy development.
The two adrenal glands sit one on top of each kidney. Both adrenals have two distinct regions. The outer region (the medulla) produces adrenaline and noradrenaline and is under the control of the sympathetic nervous system. The inner region (the cortex) produces a number of steroid hormones. The cortical steroid hormones are derived from cholesterol and include mineralocorticoids (mainly aldosterone), glucocorticoids (mainly cortisol), and gonadocorticoids. Aldosterone and cortisol are the major human steroids in the cortex. However, testosterone and estrogen are secreted by adults (both male and female) at very low levels.
Aldosterone plays an important role in regulating body fluids. It increases blood levels of sodium and water and lowers blood potassium levels. Cortisol secretion is stimulated by physical trauma, exposure to cold temperatures, burns, heavy exercise, and anxiety. Cortisol targets the liver, skeletal muscle, and adipose tissue, and its overall effect is to provide amino acids and glucose to meet synthesis and energy requirements for metabolism and during periods of stress. Because of its anti-inflammatory action, cortisol is used clinically to reduce swelling. Excessive cortisol secretion leads to Cushing's syndrome, which is characterized by weak bones, obesity, and a tendency to bruise. Cortisol deficiency can lead to Addison's disease, which has the symptoms of fatigue, low blood sodium levels, low blood pressure, and excess skin pigmentation.
The adrenal medullary hormones are epinephrine (adrenaline) and nor-epinephrine (nor-adrenaline). Both of these hormones serve to supplement and prolong the "fight or flight" response initiated in the nervous system. This response includes increased heart rate, peripheral blood vessel constriction, sweating, spleen contraction, glycogen conversion to glucose, dilation of bronchial tubes, decreased digestive activity, and low urine output.
The pancreas secretes the hormones insulin, glucagon, and somatostatin, also known as growth hormone inhibiting hormone (GHIH). Insulin and glucagon have reciprocal roles. Insulin promotes the storage of glucose, fatty acids, and amino acids, while glucagon stimulates mobilization of these constituents from storage into the blood. Insulin release is triggered by high blood glucose levels. It lowers blood sugar levels and inhibits the release of glucose by the liver in order to keep blood levels down. Insulin excess can cause hypoglycemia leading to convulsions or coma, and insufficient levels of insulin can cause diabetes mellitus, which can be fatal if left untreated. Diabetes mellitus is the most common endocrine disorder.
Glucagon secretion is stimulated by decreased blood glucose levels, infection, cortisol, exercise, and large protein meals. Among other activities, it facilitates glucose release into the blood. Excess glucagon can result from tumors of the pancreatic alpha cells, and a mild diabetes seems to result. Some cases of uncontrolled diabetes are also characterized by high glucagon levels, suggesting that low blood insulin levels are not necessarily the only cause in diabetes cases.
The female reproductive hormones arise from the hypothalamus, the anterior pituitary, and the ovaries. Although detectable amounts of the steroid hormone estrogen are present during fetal development, at puberty estrogen levels rise to initiate secondary sexual characteristics. Gonadotropin releasing hormone (GRH) is released by the hypothalamus to stimulate pituitary release of LH and FSH, which propagate egg development in the ovaries. Eggs (ova) exist at various stages of development, with the maturation of one ovum taking about 28 days. The ova are contained within follicles that are support organs for ova maturation. About 450 of a female's 150,000 germ cells mature to leave the ovary. The hormones secreted by the ovary include estrogen, progesterone, and small amounts of testosterone.
As an ovum matures, rising estrogen levels stimulate additional LH and FSH release from the pituitary. Prior to ovulation, estrogen levels drop, and LH and FSH surge to cause the ovum to be released into the fallopian tube. The cells of the burst follicle begin to secrete progesterone and some estrogen. These hormones trigger thickening of the uterine lining, the endometrium, to prepare it for implantation should fertilization occur. The high progesterone and estrogen levels prevent LH and FSH from further secretion—thus hindering another ovum from developing. If fertilization does not occur, eight days after ovulation the endometrium deteriorates, resulting in menstruation. The falling estrogen and progesterone levels that follow trigger LH and FSH, starting the cycle all over again.
In addition to its major roles in the menstrual cycle, estrogen has a protective effect on bone loss, which can lead to osteoporosis.
Hormones related to pregnancy include human chorionic gonadotrophin (HCG), estrogen, human chorionic somatomammotrophin (HCS), and relaxin. HCG is released by the early embryo to signal implantation. Estrogen and HCS are secreted by the placenta. As birth nears, relaxin is secreted by the ovaries to relax the pelvic area in preparation for labor.
Male reproductive hormones come from the hypothalamus, the anterior pituitary, and the testes. As in females, GRH is released from the hypothalamus, which stimulates LH and FSH release from the pituitary. Testosterone levels are quite low until puberty. At puberty, rising levels of testosterone stimulate male reproductive development including secondary characteristics. LH stimulates testosterone release from the testes. FSH promotes early spermatogenesis. The male also secretes prostaglandins. These substances promote uterine contractions which help propel sperm towards an egg during sexual intercourse. Prostaglandins are produced in the seminal vesicles, and are not classified as hormones by all authorities.
Little, M. The Endocrine System. New York: Chelsea House Publishers, 1990.
Parker, M., ed. Steroid Hormone Action. New York: IRL Press, 1993.