The Endocrine System
The Endocrine System
The endocrine system is the body's network of glands that produce more than fifty different known hormones or chemical messengers to maintain and regulate basic bodily functions. It is second only to the nervous system as the great controlling system of the body. Whereas nerve impulses from the nervous system immediately prod the body into action, hormones from the endocrine system act more slowly to achieve their widespread and varied effects. The bodily processes regulated by the endocrine system go on for relatively long periods of time. Some go on continuously. These life processes include growth and development, reproduction, immunity (the body's ability to resist disease), and homeostasis (the body's ability to maintain the balance of its internal functions).
DESIGN: PARTS OF THE ENDOCRINE SYSTEM
Glands are any organs that either secrete substances for further use in the body or excrete substances for elimination. Those that excrete substances for elimination are called exocrine glands (exo means "outside"). Exocrine glands have ducts or tubes that carry their secretions to the surface of the skin or into body cavities. Sweat glands and the liver are examples of exocrine glands.
Endocrine glands (endo means "inside") secrete or release substances that are used in the body. These glands lack ducts, releasing their secretions directly into the surrounding tissues and blood. Those secretions—hormones—then travel in the cardiovascular system to various points throughout the body.
The word hormone comes from a Greek word meaning "to arouse" or "to set in motion." Hormones control or coordinate the activities of other tissues, organs, and organ systems in the body. Most hormones are composed of amino acids, the building blocks of proteins. The smaller class of hormones are steroids, which are built from molecules of cholesterol (fat-like substance produced by the liver).
The Endocrine System: Words to Know
- Adrenal cortex (ah-DREE-nul KOR-tex):
- Outer layer of the adrenal glands, which secretes cortisol and aldosterone.
- Adrenal glands (ah-DREE-nul):
- Glands located on top of each kidney consisting of an outer layer (adrenal cortex) and an inner layer (adrenal medulla).
- Adrenal medulla (ah-DREE-nul muh-DUH-luh):
- Inner layer of the adrenal glands, which secretes epinephrine and norepinephrine.
- Adrenocorticotropic hormone (ah-dree-no-kor-tikoh-TROH-pik):
- Hormone secreted by the anterior pituitary that stimulates the adrenal cortex to secrete cortisol.
- Aldosterone (al-DOS-te-rone):
- Hormone secreted by the adrenal cortex that controls the salt and water balance in the body.
- Androgens (AN-dro-jens):
- Hormones that control male secondary sex characteristics.
- Antidiuretic hormone (an-tee-die-yu-REH-tik HOR-mone):
- Hormone produced by the hypothalamus and stored in the posterior pituitary that increases the absorption of water by the kidneys.
- Calcitonin (kal-si-TOE-nin):
- Hormone secreted by the thyroid gland that decreases calcium levels in the blood.
- Cortisol (KOR-ti-sol):
- Hormone secreted by the adrenal cortex that promotes the body's efficient use of nutrients during stressful situations.
- Epinephrine (ep-i-NEFF-rin):
- Also called adrenaline, a hormone secreted by the adrenal medulla that stimulates the body to react to stressful situations.
- Estrogens (ES-tro-jenz):
- Female steroid hormones secreted by the ovaries that bring about the secondary sex characteristics and regulate the female reproductive cycle.
- Any organ that secretes or excretes substances for further use in the body or for elimination.
- Glucagon (GLUE-ka-gon):
- Hormone secreted by the islets of Langerhans that raises the level of sugar in the blood.
- Gonad (GO-nad):
- Sex organ in which reproductive cells develop.
- Gonadotropic hormones (gon-ah-do-TROP-ik):
- Hormones secreted by the anterior pituitary that affect or stimulate the growth or activity of the gonads.
- Homeostasis (hoe-me-o-STAY-sis):
- Ability of the body or a cell to maintain the internal balance of its functions, such as steady temperature, regardless of outside conditions.
- Hypothalamus (hi-po-THAL-ah-mus):
- Region of the brain containing many control centers for body functions and emotions; also regulates the pituitary gland's secretions.
- Insulin (IN-suh-lin):
- Hormone secreted by the islets of Langerhans that regulates the amount of sugar in the blood.
- Islets of Langerhans (EYE-lets of LAHNG-er-hanz):
- Endocrine cells of the pancreas that secrete insulin and glucagon.
- Luteinizing hormone (loo-tee-in-EYE-zing):
- Gonadotropic hormone secreted by the anterior pituitary that stimulates, in women, ovulation and the release of estrogens and progesterone by the ovaries and, in men, the secretion of testosterone by the testes.
- Melatonin (mel-a-TOE-nin):
- Hormone secreted by the pineal gland that helps set the body's twenty-four-hour clock and plays a role in the timing of puberty and sexual development.
- Metabolism (muh-TAB-uh-lizm):
- Sum of all the physiological processes by which an organism maintains life.
- Negative feedback:
- Control system in which a stimulus initiates a response that reduces the stimulus, thereby stopping the response.
- Norepinephrine (nor-ep-i-NEFF-rin):
- Also called noradrenaline, a hormone secreted by the adrenal medulla that raises blood pressure during stressful situations.
- Ovaries (O-var-eez):
- Female gonads in which ova (eggs) are produced and that secrete estrogens and progesterone.
- Oxytocin (ahk-si-TOE-sin):
- Hormone produced by the hypothalamus and stored in the posterior pituitary that stimulates contraction of the uterus during childbirth and secretion of milk during nursing.
- Parathyroid glands (pair-ah-THIGH-roid):
- Four small glands located on the posterior surface of the thyroid gland that regulate calcium levels in the blood.
- Pineal gland (PIN-ee-al):
- Gland located deep in the rear portion of the brain that helps establish the body's day-night cycle.
- Pituitary gland (pi-TOO-i-tair-ee):
- Gland located below the hypothalamus that controls and coordinates the secretions of other endocrine glands.
- Progesterone (pro-JESS-te-rone):
- Female steroid hormone secreted by the ovaries that makes the uterus more ready to receive a fertilized ovum or egg.
- Prolactin (pro-LAK-tin):
- Gonadotropic hormone secreted by the anterior pituitary that stimulates the mammary glands to produce milk.
- Testes (TESS-teez):
- Male gonads that produce sperm cells and secrete testosterone.
- Testosterone (tess-TAHS-ter-ohn):
- Hormone secreted by the testes that spurs the growth of the male reproductive organs and secondary sex characteristics.
- Thymosin (thigh-MOE-sin):
- Hormone secreted by the thymus that changes a certain group of lymphocytes into germ-fighting T cells.
- Thymus (THIGH-mus):
- Glandular organ consisting of lymphoid tissue located behind the top of the breastbone that produces specialized lymphocytes; reaches maximum development in early childhood and is almost absent in adults.
- Thyroid gland (THIGH-roid):
- Gland wrapped around the front and sides of the trachea at the base of the throat just below the larynx that affects growth and metabolism.
- Thyroxine (thigh-ROK-seen):
- Hormone secreted by the thyroid gland that regulates the rate of metabolism and, in children, affects growth.
Each type of hormone affects only specific tissue cells or organs, called target cells or target organs. Each target cell has receptors on its membrane or inside of it to which a particular hormone can attach or bind. Only once this binding has occurred does the hormone bring about a change in the workings of a cell. Some hormones affect nearly every cell in the body; others affect only a single organ. Some cells have numerous receptors, acting as a target cell for many different hormones.
Unlike the organs or parts of other body systems, the principal endocrine glands are not physically connected together, but are scattered throughout the body. Located in the skull are the hypothalamus, pituitary gland, and pineal gland; in the throat are the thyroid gland and parathyroid glands; in the upper part of the chest is the thymus; in the abdominal region are the
pancreas and adrenal glands; in the pelvis of females are the ovaries; and in the scrotum of males are the testes.
Most of the endocrine glands serve only the endocrine system, but a few serve other systems as well. For this reason, they are called mixed glands. The pancreas is also a part of the digestive system because it secretes pancreatic juice into the small intestine (see chapter 2). The ovaries and testes are also part of the reproductive system because they produce gametes or the male and female reproductive cells—sperm cells and ova (see chapter 8). The thymus is also part of the lymphatic system because it helps a certain class of white blood cells to develop in order to fight germs and other foreign invaders (see chapter 5).
The hypothalamus is not a gland, but a small region of the brain containing many control centers for body functions and emotions. Composed of gray matter (brain tissue consisting of nerve cells that lack inner protective coverings), it is about the size of an almond and weighs only about 1/300 of the total mass of the brain.
The hypothalamus is often considered a part of the endocrine system for a number of reasons. It sends signals to the adrenal glands to release the hormones epinephrine and norepinephrine. It also produces its own hormones: antidiuretic hormone (ADH), oxytocin, and regulatory hormones. Both ADH and oxytocin are stored in the posterior pituitary gland until the hypothalamus sends nerve signals to the pituitary to release them. Regulatory hormones are divided into two classes: releasing hormones (RH) and inhibiting hormones (IH). Both types control the release of hormones by the pituitary gland. RH stimulate the production of pituitary hormones; IH inhibit or prevent the release of pituitary hormones.
The pituitary gland
Located at the base of the brain behind the nose, the pituitary gland is a small, oval gland approximately the size of a grape. It hangs by a thin piece of tissue from the interior surface of the hypothalamus. The pituitary is divided into two distinct lobes or regions: the anterior pituitary (the front lobe) and the posterior pituitary (the rear lobe). The anterior pituitary produces and secretes six hormones. The posterior pituitary secretes two hormones, but does not produce them. Those hormones are made by the hypothalamus, which uses the posterior pituitary as a storage area for the hormones until they are needed.
Six of the eight hormones released by the pituitary stimulate or "turn on" other endocrine glands. For this reason, they are referred to as tropic hormones (tropic comes from the Greek word tropos, meaning "to turn" or "change"). The remaining two hormones control other bodily functions. Because the pituitary's secretions control and regulate the secretions of other endocrine glands, it is often called the "master gland" of the endocrine system.
THE DISCOVERY OF HYPOTHALAMIC HORMONES
For some time, medical researchers have known that the hypothalamus controls the actions of the pituitary gland. Up until the late 1960s, however, they could not explain exactly how. In 1968, French-born American endocrinologist Roger Guillemin (1924– ) and others finally answered that unresolved question: hormones.
Prior to this discovery, English anatomist Geoffrey W. Harris had hypothesized that the hypothalamus releases hormones that regulate the pituitary gland. Harris and others, though, could not isolate and identify any hormones coming from the hypothalamus.
In the 1950s, Guillemin began an investigation to find the missing evidence. Working with fellow endocrinologist Andrew V. Schally, Guillemin used a tool developed by physicist Rosalyn Sussman Yalow to isolate and identify the chemical structure of hormones. Soon, Guillemin and Schally ended their scientific cooperation, pursuing their investigations separately.
Finally in 1968, while working with hypothalamic fragments from sheep brains, Guillemin and his coworkers isolated the hypothalamic hormone that causes the pituitary to release thyroid-stimulating hormone (TSH). The following year, both Guillemin and Schally identified the chemical structure of TSH. Guillemin then went on to isolate and determine the chemical structure of other hypothalamic hormones.
For their discoveries, which led to an understanding of the hormone produced by the hypothalamus, Guillemin, Schally, and Yalow shared the 1977 Nobel Prize for physiology or medicine.
The pineal gland
The pineal gland or body is a small cone-shaped gland located deep in the rear portion of the brain (pineal comes from the Latin word "pinea," meaning "pine cone"). Scientists are still somewhat mystified as to the endocrine function of this gland, which secretes the hormone melatonin.
The thyroid gland
One of the largest endocrine glands in the body, the thyroid gland is a butterfly-shaped gland that wraps around the front and sides of the trachea (windpipe) at the base of the throat just below the larynx (upper part of the trachea containing the vocal cords). It is divided into two lobes connected by a band of tissue called the isthmus. Because the thyroid contains a large number of blood vessels, it is deep red in color.
The thyroid gland is composed of hollow, spherical structures called thyroid follicles. The follicles release two main hormones, thyroxine and calcitonin. Thyroxine regulates the rate of metabolism and, in children, affects growth. Calcitonin decreases calcium levels in the blood.
The parathyroid glands
The parathyroid glands are four tiny masses of glandular tissue, each about the size of a pea, located on the posterior or rear surface of the thyroid gland (two on the back of each lobe). The parathyroids secrete parathyroid hormone (PTH) or parathormone, which controls the level of calcium in the blood.
The thymus is a soft, flattened, pinkish-gray mass of lymphoid tissue located in the upper chest under the breastbone. In a fetus and newborn infant, the thymus is relatively large (about the size of an infant's fist). Up until about the age of puberty, the thymus continues to grow. After this point
in life, it shrinks and gradually blends in with the surrounding tissue. Very little thymus tissue is found in adults.
The thymus secretes several hormones that are known collectively as thymosins. Thymosins help change a certain group of white blood cells called lymphocytes into T cells, which are programmed to attack any foreign substance in the body.
The pancreas is a soft, pink, triangular-shaped gland that measures about 6 inches (15 centimeters) in length. It lies behind the stomach, extending from the curve of the duodenum (first part of the small intestine) to the
spleen. Primarily a digestive organ, the pancreas secretes pancreatic juice into the duodenum through the pancreatic duct. The digestive enzymes in this juice helps break down carbohydrates, fats, and proteins in the small intestine. While a part of the digestive system, the pancreas is also a part of the endocrine system, producing hormones that maintain blood glucose (sugar) levels.
Scattered like small islands among the cells that produce pancreatic juice are small groups of endocrine cells called the islets of Langerhans (or pancreatic islets). They are named after Paul Langerhans (1847–1888), the German physician and anatomist who discovered them. The pancreas contains between one and two million islets, which account for about 2 percent of the total mass of the pancreas. Each islet contains different cells that produce different hormones, the most important of which are insulin and glucagon. Both regulate the amount of glucose in the blood, but in opposite ways.
The adrenals are two glands, each sitting like a cap on top of each kidney. The adrenals are divided into two distinct layers: the adrenal cortex (outer layer) and the adrenal medulla (inner layer).
The adrenal cortex makes up about 80 percent of each adrenal gland. It is grayish yellow in color due to the presence of stored fats, especially cholesterol and various fatty acids. The cortex is extremely important to bodily processes; if it stops functioning, death occurs in just a few days. The cortex secretes about thirty steroid hormones, the most important of which are cortisol (also called hydrocortisone) and aldosterone. Cortisol, released during any stressful situation (physical injury, disease, fear, anger, hunger), regulates the body's metabolism of carbohydrates, proteins, and fats. Aldosterone regulates the body's water and salt balance.
The adrenal medulla is reddish brown in color partly because it contains many blood vessels. It secretes two hormones: epinephrine (also called adrenaline) and norepinephrine (noradrenaline). Both hormones are secreted during dangerous or stressful situations. They prepare the body for emergencies—"flight-or-fight" situations—by increasing heart rate, blood pressure, blood flow to the muscles, and other such processes.
The ovaries are the gonads or sex organs in females. The two almond-sized ovaries are located on each side of the pelvis, one at the end of each fallopian tube. The ovaries are attached to the uterus or womb by an ovarian ligament.
The ovaries secrete two groups of steroid hormones, estrogens and progesterone. Estrogens spur the development of the secondary sex characteristics: enlargement of the breasts, appearance of hair under the arms and in the genital area, and the accumulation of fat in the hips and thighs. Estrogens also act with progesterone to stimulate the growth of the lining of the uterus, preparing it to receive a fertilized egg.
The testes are the gonads in males. They are two small, egg-shaped structures suspended in the scrotum, a loose sac of skin that hangs outside the pelvic cavity between the upper thighs.
In addition to producing sperm cells, the testes produce male sex hormones called androgens (from the Greek word andros, meaning "man"). The
most important of these is testosterone. This hormone spurs the growth of the male reproductive organs and the production of sperm. In addition, testosterone brings about the male secondary sex characteristics: deepening of the voice; appearance of hair under the arms, on the face, and in the genital area; and increased growth of muscles and heavy bones.
WORKINGS: HOW THE ENDOCRINE SYSTEM FUNCTIONS
The main functions of the endocrine system and its hormone messengers are to maintain homeostasis (a stable internal environment in the body) and to promote permanent structural changes. Maintaining homeostasis is a response to a change in the body, such as low sugar or calcium levels in the blood. Permanent structural changes, occurring over a period of time, are those associated with growth and development.
Hormones bring about their effect on the body's cells mainly by altering the cells' metabolic activity—increasing or decreasing the rate at which they work. The effect is often rapid, such as increased or decreased heart rate. A few hormones, after binding to their target cells, cause those cells to produce proteins, which lead to long-term effects such as growth or sexual maturity.
ATHLETES AND ANABOLIC STEROIDS
Many professional and amateur athletes around the world take anabolic steroids in the hopes of enhancing their performance. Anabolic steroids are synthetic (man-made) drugs derived from the male hormone testosterone. The full name of the drug is androgenic (promoting masculine characteristics) anabolic (building) steroid (class of drug). Common names for the drug include 'roids, sauce, and juice.
It is estimated that 10 to 20 percent of male high school athletes, up to 30 percent of college and professional athletes, and up to 80 percent of bodybuilders use anabolic steroids to increase skeletal muscle and lean body mass. The drugs are taken either orally or injected.
Anabolic steroids do increase body weight and muscle mass. They also may improve muscular strength and endurance. These are the few benefits.
The drawbacks are many and serious. The major side effects include liver tumors, jaundice (yellowing of the skin), fluid retention, high blood pressure, severe acne, and trembling. In men, steroids can additionally cause shrunken testes, reduced sperm production, sterility, baldness, and the development of breasts. In women, they can also cause the growth of facial hair, menstrual irregularity, smaller breasts, and a deeper voice. In adolescents, the drugs can permanently stop bones from growing, resulting in shortened height for life.
Anabolic steroids not only affect the body but the mind as well. Users suffer from aggression, irritability, delusions, paranoid jealousy, and impaired judgment.
Taking anabolic steroids for nonmedical reasons is illegal under federal law. Hard training is still the most effective and safe way to improve muscle strength and overall athletic performance.
Hormones travel in the bloodstream or in the interstitial fluid (fluid between cells). Some hormones are long-distance travelers, floating throughout the body in search of their target cells. Others travel shorter distances, having been secreted near theirs.
Hormones are secreted by endocrine glands in response to a stimulus. That stimulus may be either changing blood levels of certain nutrients or other hormones. When a gland senses a change in the composition of blood or tissue fluid (low blood sugar, for example) and releases its hormones, that action is known as a direct response. When a gland releases its hormones because it has been stimulated by other hormones released by other glands, that action is known as an indirect response.
A feedback system tightly controls the on/off workings of endocrine glands. This system can be compared to a furnace thermostat on a wall in a house. When the temperature in a house falls below the temperature set on the thermostat, the thermostat is triggered and signals the furnace to turn on and begin heating. After the furnace has heated the air in the house to a temperature higher than that set on the thermostat, the thermostat signals the furnace to turn off.
Endocrine glands react to changes in the blood and body in much the same way. When nutrients or chemicals in body fluids are abnormal (either high or low), endocrine glands secrete their hormones. After those levels return to normal (reaching a state of homeostasis), the glands stop secreting their hormones. This control of hormone secretion, where information is fed back to the gland to stop its hormone production, is called negative feedback.
Actions of the hypothalamus
Receiving nerve signals from other parts of the brain, the hypothalamus functions as a monitoring and control station for many body activities. It thus plays an important role in the actions of other endocrine glands, especially the pituitary. Therefore, its role is best considered under the discussions of the actions of those various other glands.
Actions of the pituitary gland
The "master" pituitary gland is small in size, but large in its actions. The eight hormones secreted by its two lobes have a direct effect on the actions of other endocrine glands, controlling growth and fluid balance in the body. The anterior pituitary secretes six hormones: growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, and three gonadotropic hormones. The posterior pituitary secretes antidiuretic hormone and oxytocin.
GROWTH HORMONE. Growth hormone (GH) or human growth hormone stimulates overall body growth by spurring target cells to grow in size and divide. GH increases the rate at which those cells take in and utilize proteins (cell structure is made up largely of proteins). GH also causes fats to be broken down and used by the cells for energy. Its greatest effects are on the development of muscles and bones, especially in children. The release of GH is controlled by two regulatory hormones from the hypothalamus: growth hormone releasing hormone (GHRH) and growth hormone inhibiting hormone (GHIH). GHRH stimulates the pituitary to release GH during exercise, when blood sugar levels are low, when amino acid levels in the blood are high, or when the body in under stress. When the body is returned to a state of homeostasis or when blood sugar levels are high, the hypothalamus secretes GHIH, and the pituitary stops releasing GH.
THYROID-STIMULATING HORMONE. Thyroid-stimulating hormone (TSH), as its name implies, influences the growth and activity of the thyroid. TSH prompts the thyroid to release thyroxine, which stimulates the cells in the body to increase their metabolism (energy production) and intake of oxygen. A releasing hormone from the hypothalamus signals the pituitary to secrete TSH. This occurs when the body's metabolic rate decreases.
ADRENOCORTICOTROPIC HORMONE. Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to secrete cortisol and other hormones. During any stressful situation such as injury, low blood sugar levels, and exercise, the hypothalamus secretes a releasing hormone that triggers the pituitary to release ACTH.
THE BODY'S NATURAL ALARM CLOCK
It is known that some people can automatically awake in the morning without an alarm clock. German researchers sought an explanation for this phenomenon, and in early 1999 they announced their results. The researchers discovered that the actions of two hormones, adrenocorticotropic hormone (ACTH) and cortisol, were the reason.
In stressful situations, the hypothalamus secretes a releasing hormone that triggers the anterior pituitary to release ACTH. ACTH then travels to the adrenal cortex, stimulating it to release cortisol. In short, cortisol stimulates most body cells to increase their energy production, which heightens the body's ability to react quickly to a stressful or emergency situation.
The researchers found that during the latter stages of sleep, these hormones were released, causing the body to awaken. Most scientists agree that sleep is a state of unconsciousness. From their findings, however, the researchers concluded that even during sleep, the mind maintains some voluntary control. When an individual goes to bed knowing he or she has to get up earlier than normal because of a stressful event (an exam at school or a big presentation at work), the mind "remembers" and so awakens the body in anticipation of that event.
GONADOTROPIC HORMONES. As their name suggests, the gonadotropic hormones affect the gonads or reproductive organs. Releasing hormones from the hypothalamus regulate the secretion of all three gonadotropic hormones: prolactin, follicle-stimulating hormone (FSH), and luteinizing hormone (LH). In females, prolactin stimulates the development of mammary glands in breasts and their secretion of milk. FSH stimulates the development of follicles in the ovaries of females. Ovarian follicles are tiny, saclike structures within which ova or eggs develop. FSH also stimulates the secretion of estrogen by the follicle cells. In males, FSH begins the productions of sperm in the testes. LH stimulates ovulation (the release of an egg from an ovary) and the release of estrogens and progesterone from the ovaries in females. In males, LH stimulates the testes to produce testosterone.
ANTIDIURETIC HORMONE. Antidiuretic hormone (ADH) is produced by the hypothalamus and stored in the posterior pituitary. ADH causes the kidneys to reabsorb water from the urine that is being formed. That water is then transported into the bloodstream, maintaining blood pressure. When too much water is lost from the body, such as through sweating, diarrhea, or any type of dehydration, the hypothalamus detects an increased amount of "salt" in the blood. It then triggers the posterior pituitary to release ADH. The kidneys decrease the production of urine and blood pressure increases. Alcohol and certain drugs, however, inhibit the secretion of ADH. Large amounts of urine are consequently excreted from the body and blood pressure decreases. If that fluid is not replaced, an individual may feel dizzy due to low blood pressure.
OXYTOCIN. Oxytocin is also produced by the hypothalamus and stored in the posterior pituitary. The hormone plays an important role in childbirth. When a woman goes into labor, the uterus begins to stretch and nerve impulses are sent to the hypothalamus. The hypothalamus then stimulates the posterior pituitary to release oxytocin, which travels to the uterus. Once there, it triggers strong contractions of the uterine muscles, helping to bring about delivery of the baby. After birth, oxytocin promotes the release of milk from the mammary glands. When a baby suckles a mother's nipple, nerve impulses are sent to the mother's hypothalamus, which then signals the release of oxytocin. The hormone stimulates the contraction of the muscle cells around the mammary ducts, causing the ejection of milk through the nipple.
Actions of the pineal gland
Scientists believe that melatonin, secreted by the pineal gland, establishes the body's sleep and waking patterns. Keeping the body in sync with the cycles of day (light) and night (dark), the pineal gland functions as the body's biological clock. Scientists know that, in general, the secretion of melatonin is spurred by darkness. Known as the sleep "trigger," melatonin is secreted cyclically in response to the fall of darkness at the end of each day. In the morning, when light enters the eyes, that visual information is relayed from the eyes to the hypothalamus. The pineal gland is then stimulated to decrease melatonin production during daylight hours. Scientists also theorize that the hormone plays a role in the timing of puberty and sexual development, preventing it from occurring during childhood before adult body size has been reached.
Actions of the thyroid gland
Thyroxine, the major hormone secreted by the thyroid follicles, is often considered to be the body's major metabolic hormone. When the body's metabolic rate decreases, the anterior pituitary secretes thyroid-stimulating hormone, which triggers the thyroid to secrete thyroxine. Thyroxine then stimulates energy production in cells in the body, increasing the rate at which they consume oxygen and utilize carbohydrates, fats, and proteins. When cells increase their energy production, they generate more heat as a result. This is important when the body is trying to adapt to cold temperatures. In children, thyroxine is essential to the normal development of the muscular, nervous, and skeletal systems. In adults, it is important for continued tissue growth and development. Iodine is an important component of thyroxine. Without the proper amount of iodine in an individual's diet, thyroxine would not be produced, and physical and mental growth and abilities would then diminish.
SAD: MORE THAN JUST THE WINTER BLUES
It has been recorded since the time of the ancient Greeks that the varying seasons have had an effect on people's moods and behavior. Generally, the short, dark days of late autumn and winter dampen many people's spirits, and the longer and lighter days of spring and summer have the opposite effect.
While members of the medical community have noted this annual winter depression, they did not fully explore its reasons until the early 1980s. Since then, medical researchers have concluded that the lack of sunlight from November through March is indeed responsible for what is commonly referred to as the "winter blues."
Many people feel mildly "depressed" during the winter, but some suffer from severe symptoms. These include daytime drowsiness, fatigue and low energy level, diminished concentration, irritability, carbohydrate craving and increased appetite, weight gain, and social withdrawal. This mood disorder that affects people only during the autumn and winter seasons is called seasonal affective disorder or SAD.
SAD is a very real problem that affects approximately 10 million people each year in the United States (women suffering from SAD outnumber men four to one). Researchers believe that people with SAD have lost the natural rhythm that signals the body to fall asleep and to awake at the proper times. Melatonin, secreted by the pineal gland when light is low, helps bring the body to rest. Daylight signals the gland to stop producing the hormone to allow the body to come awake.
Researchers do not know why some people are affected more than others. They have discovered, however, that an effective treatment for SAD sufferers is light, particularly morning light. When exposed to a light box that emits bright, artificial sunlight (called phototherapy or light therapy) for thirty minutes a day, almost 80 percent of SAD patients showed marked improvement in their moods.
Calcitonin, the second important hormone secreted by the thyroid, helps maintain normal levels of calcium in the blood. It is secreted directly into the bloodstream when the thyroid detects high levels of calcium in the blood. Calcitonin travels to the bones, stimulating the bone-building cells to absorb calcium from the blood. It also targets the kidneys, stimulating them to absorb and excrete the excess calcium. When blood calcium levels return to normal, the thyroid stops secreting calcitonin.
Actions of the parathyroid glands
Like the thyroid's calcitonin, the parathyroid's parathyroid hormone (PTH) also regulates the levels of calcium in the blood. However, its stimulus and effect are just the opposite. Thus, calcitonin and PTH are antagonistic: they work against each other to maintain the normal levels of calcium in the bloodstream.
The parathyroids secrete PTH when blood calcium levels are low. Like calcitonin, PTH targets the bones and the kidneys. In the bones, PTH stimulates the bone-dissolving cells to break down bone, thus releasing calcium (a component of bone) into the bloodstream. In the kidneys, PTH decreases the amount of calcium that is excreted in the urine. Both of these actions raise the levels of calcium in the blood. When those levels have returned to normal, the parathyroids stop secreting PTH.
Actions of the thymus
The thymus and its collective hormones, thymosins, play an important role in helping the body develop immunity (the ability to resist disease). In a fetus and infant, immature or not fully developed lymphocytes (type of white blood cell) are produced in the bone marrow, the spongylike material that fills the cavities inside most bones. A certain group of these lymphocytes then travel to the thymus. There, thymosins changed them into T lymphocytes or T cells (the letter T refers to the thymus). While maturing, dividing, and multiplying in the thymus, T cells are "programmed" to recognize the difference between cells that belong to the body and those that are foreign or abnormal. Once they are fully mature, T cells leave the thymus and enter the bloodstream. They circulate to the spleen, lymph nodes, and other lymphatic tissue where they await the call to defend the body.
Actions of the pancreas
Although the islets of Langerhans make up but a small part of the pancreas, they work tirelessly, acting like an organ within an organ. The main hormones they secrete—glucagon and insulin—are vital to the normal functioning of the body. They regulate blood glucose (sugar) levels in the same way that parathyroid hormone and calcitonin regulate blood calcium levels.
GLUCAGON. Glucagon is secreted by the islets of Langerhans in response to low blood glucose levels. To raise those levels (and the body's energy), glucagon then travels to the liver. The liver performs a multitude of functions in the body. One of those is to store excess glucose that is not immediately required by the body's cells for energy. In order to store that glucose, the liver converts it to glycogen (a starch form of the sugar glucose made up of thousands of glucose units). Glucagon stimulates the liver to change glycogen back into glucose and secrete it into the bloodstream for use by the cells for energy production. When glucose levels rise to normal, the islets of Langerhans stop secreting glucagon.
INSULIN. Insulin has the opposite effect: it lowers blood glucose levels that are too high. When those high levels are detected, the islets of Langerhans secrete insulin, which then travels to almost all cells in the body. Insulin stimulates the cells to take in more glucose and use it to produce energy. Insulin also stimulates the liver to take in more glucose and store it as glycogen for later use by the body. After they break down glucose, the cells use the energy created to build proteins and enhance their energy reserves. Insulin is the only hormone that decreases blood glucose levels and is absolutely necessary in order for the cells to utilize glucose. Without it, the cells cannot take in glucose. After glucose levels return to normal, the islets of Langerhans stop secreting insulin.
Actions of the adrenal glands
The small adrenal glands, capping the kidneys, control numerous activities in the body. The hormones they secrete aid in cell metabolism, adjust the water balance, and increase cardiovascular and respiratory activity.
CORTISOL. In times of physical stress (injury, exercise, anger, fear), the hypothalamus secretes a releasing hormone that causes the anterior pituitary to release adrenocorticotropic hormone (ACTH). ACTH, in turn, targets the adrenal cortex, stimulating it to secrete cortisol. Like insulin, cortisol stimulates most body cells to increase their energy production. Unlike insulin, cortisol causes the cells to increase energy output by using fats and amino acids (proteins) instead of glucose. In stressful situations, this is extremely important because glucose is conserved for use by the brain (glucose is the sole source of energy for neurons or cells in nervous tissue).
Cortisol also has an anti-inflammatory effect, suppressing the activities of white blood cells and other components in the body's defense line. Inflammation is an important first step in tissue repair, but if left unchecked, will lead to excessive tissue destruction. Cortisol limits the inflammation process to what is necessary for immediate tissue repair by blocking the effects of histamine (a chemical released by damaged cells that brings more blood flow to the area).
ALDOSTERONE. Aldosterone, another steroid hormone secreted by the adrenal cortex, targets the kidney cells that regulate the formation of urine. A decrease in blood pressure or volume, a decrease in the sodium (salt) level in blood, and an increase in the potassium level in blood all stimulate the secretion of aldosterone. Once released, aldosterone spurs the kidney cells to reabsorb sodium from the urine and to excrete potassium instead. Sodium is then returned to the bloodstream. When sodium is reabsorbed into the blood, water in the body follows it, thus increasing blood volume and pressure. Aldosterone also reduces the amount of sodium and water lost through the sweat and salivary glands. When normal blood, sodium, and potassium levels are all reached, the adrenal cortex stops releasing aldosterone.
EPINEPHRINE AND NOREPINEPHRINE. When an individual is (or feels) threatened physically or emotionally, the hypothalamus readies the body to "fight" or "take flight" by sending impulses to the adrenal medulla. In response, the medulla secretes norepinephrine (in small amounts) and epinephrine (in larger amounts). Norepinephrine causes blood vessels in the skin and skeletal muscles to constrict, raising blood pressure. Epinephrine causes an increase in heart rate and contraction, stimulates the liver to change glycogen to glucose for use as energy by the cells, and stimulates fatty tissue to break down and release stored fats for use as energy by the cells as well. The actions of both hormones bring about increased levels of oxygen and glucose in the blood and a faster circulation of blood to the body organs, especially the brain, muscles, and heart. Reflexes and body movements quicken and the body is better able to handle a short-term emergency situation.
SCHARRER AND HER SOUTH AMERICAN COCKROACHES
German-born American biologist Berta Scharrer (1906–1995) and her biologist husband Ernst Scharrer pioneered the field of neuroendocrinology, the study of the interaction between the nervous system and the endocrine glands and their secretions. Fighting against accepted scientific beliefs about cells—as well as against prejudice toward women in the sciences—Scharrer established the concept of neurosecretion, or the releasing of substances such as hormones by nerve cells.
Prior to the discoveries of Scharrer and her husband, scientists believed that neurons or nerve cells could not have a dual function. They either secreted hormones, in which case they were endocrine cells belonging to the endocrine system, or they conducted electrical impulses, making them nerve cells belonging to the nervous system.
In the 1930s, after having come to America, Scharrer and her husband set out to prove their theories with no real professional standing and therefore lacking a budget for lab animals. Scharrer reportedly collected cockroaches in the basement of the lab and used them for experiments. Soon she began experimenting on South American cockroaches she had discovered scurrying around in the bottom of a cage of lab monkeys that had arrived from South America. Scharrer found that they made better research subjects because they were slower than the American cockroach. From that point forward, she used the South American cockroaches, which traveled with her wherever she and her husband moved.
By 1950, Scharrer's research and theories on neurosecretion had become accepted as fact by the scientific community. For her pioneering scientific work, Scharrer received many honors. Included among these was the naming of a cockroach species, scharrerae, in her honor.
Actions of the ovaries
The ovaries do not begin to function until puberty, usually between the ages of eleven and fourteen in girls. At this time, the anterior pituitary gland secretes follicle-stimulating hormone, which causes follicles or tiny saclike structures to grow and mature in an ovary. Ova or eggs within these specialized structures also begin to mature. While an egg is developing in an ovarian follicle, the follicle cells surrounding the egg secrete estrogens. Increased levels of estrogens then signal the anterior pituitary gland to secrete luteinizing hormone, which causes the ovary to release a single mature egg—a process called ovulation.
After ovulation has occurred, a structure in the ovary secretes progesterone, which prevents another egg from beginning to develop and causes the lining of the uterus to grow thicker with blood vessels (estrogens also help in this latter action). The mature egg then travels through a fallopian tube to the uterus. If the egg has not fertilized by male sperm, it breaks down. About ten days later, the lining of the uterus begins to break apart and is shed outside the body during the monthly process called menstruation.
If the egg has been fertilized, it attaches to the wall of the uterus and pregnancy occurs. High levels of estrogens and progesterone are then produced to prevent another egg from maturing. In addition, progesterone prevents the muscles of the uterus from contracting so that the developing embryo will not be disturbed. Estrogens and progesterone both prepare the mammary glands to produce milk.
At puberty, the estrogens released by the follicle cells also bring about the female secondary sex characteristics. The breasts enlarge and their duct system to carry milk develops, the uterus enlarges, fat is deposited in the hips and thighs, and hair develops under the arms and in the genital area.
Actions of the testes
Puberty in boys usually occurs between the ages of twelve and sixteen. At this time, the anterior pituitary gland releases luteinizing hormone, which stimulates the testes to produce testosterone. This hormone produces many growth changes in an adolescent boy: growth of all the reproductive organs, growth of facial and body hair, growth of the larynx (resulting in a deeper voice), and growth of the skeletal muscles. Follicle-stimulating hormone, also secreted from the anterior pituitary, initiates the production of sperm in the testes. Testosterone then helps the sperm mature. This process, begun at puberty, continues throughout life.
AILMENTS: WHAT CAN GO WRONG WITH THE ENDOCRINE SYSTEM
As much as 10 percent of the population will experience some endocrine disorder in their lifetime. Most endocrine disorders are caused by an increased
or decreased level of particular hormones. Tumors (abnormal tissue growth) in endocrine glands are one of the major causes of hormone overproduction. Hormone underproduction is often due to defective receptors on cells. The result is that the cells fail to notify an endocrine gland when production of its particular hormone is too low. Injury to or disease of an endocrine gland can also result in low hormone levels.
The following are just a few of the many disorders that can result from an improperly functioning endocrine system.
Acromegaly and gigantism
Acromegaly is a disorder in which the anterior pituitary produces too much growth hormone (GH). This causes an increased growth in bone and soft tissue, especially in the extremities—nose, jaw, fingers, and toes. If the disorder occurs in children who have not yet fully developed, the increased levels of GH also result in the exceptional growth of the long bones. This condition, a variation of acromegaly, is known as gigantism.
ENDOCRINE SYSTEM DISORDERS
Acromegaly (ak–ro–MEG–ah–lee): Disorder in which the anterior pituitary overproduces growth hormone, resulting in abnormal enlargement of the extremities—nose, jaw, fingers, and toes; in children, the disorder produces gigantism.
Addison's disease (ADD–i–sonz): Disorder in which the adrenal cortex underproduces cortisol and aldosterone, resulting in the disruption of numerous bodily functions.
Cushing's syndrome (KU–shingz SIN–drome): Disorder caused by an overproduction of steroids (mostly cortisol) by the adrenal cortex, resulting in obesity and muscular weakness.
Diabetes mellitus (die–ah–BEE–teez MUL–le–tus): Disorder in which the body's cells cannot absorb glucose, either because the pancreas does not produce enough insulin or the cells do not respond to the effects of insulin that is produced.
Gigantism (jie–GAN–tizm): Disorder in children in which the anterior pituitary overproduces growth hormone, resulting in abnormal enlargement of the extremities (nose, jaw, fingers, and toes) and the long bones, causing unusual height.
Hyperthyroidism (hi–per–THIGH–roy–dizm): Disorder in which an overactive thyroid produces too much thyroxine.
Hypothyroidism (hi–po–THIGH–roy–dizm): Disorder in which an underactive thyroid produces too little thyroxine.
Acromegaly is a rare disorder, occurring in approximately 50 out of every 1 million people. Both men and women are affected. Because the symptoms come on gradually, the disorder is often not identified until the patient is middle aged.
In 90 percent of the cases, acromegaly is caused by a noncancerous tumor that develops within the pituitary. The tumor causes the anterior pituitary to ignore growth hormone inhibiting hormone (GHIH), a regulating hormone secreted by the hypothalamus that stops the pituitary from producing GH. GH is thus secreted without a stopping mechanism.
The first step in treating acromegaly is the surgical removal of the tumor. Afterward, some patients often require medications that help to reduce the secretion of GH. With treatment, an individual suffering from acromegaly may be able to live a normal life span. Without treatment, an individual will most likely die early because of the disorder's adverse effects on the heart, lungs, and brain.
Addison's disease is a disorder in which the adrenal cortex produces too little cortisol and aldosterone, resulting in the disruption of numerous bodily functions. About 4 in every 100,000 people suffer from this disorder. It strikes men and women of all ages.
The most common cause of Addison's disease is the destruction or shrinking of the adrenal cortex. In about 70 percent of the cases, this is caused by an autoimmune disorder: a condition in which the body produces antibodies that attack and destroy the body's own tissues instead of foreign invaders such as viruses and bacteria. In the case of Addison's disease, antibodies attack and destroy cells of the adrenal cortex.
Addison's disease tends to be a gradual, slowly developing disease. By the time symptoms are noted, about 90 percent of the adrenal cortex has been destroyed. The most common symptoms include fatigue and loss of energy, decreased appetite, nausea, vomiting, diarrhea, abdominal pain, muscle weakness, dizziness when standing, and dehydration. Unusual areas of darkened skin and dark freckling also appear. Women suffering from the disease may stop having normal menstrual periods. As the disease progresses, the symptoms become more severe: abnormal heart rhythms, uncontrollable nausea and vomiting, a drastic drop in blood pressure, kidney failure, and unconsciousness.
Individuals suffering from Addison's disease are treated with steroid medications that replace cortisol and aldosterone in the body. Taking these medications for the rest of their lives, those individuals can expect to live a normal life span.
Cushing's syndrome is a disorder caused by an overproduction of steroids—mostly cortisol—by the adrenal cortex, resulting in obesity and muscular weakness. The disorder occurs in about 15 out of every 1 million people per year. It usually strikes adults (men and women) between the ages of twenty and fifty.
Cushing's syndrome can be caused by a tumor either in the pituitary gland or in one of the adrenal glands. The anterior pituitary secretes adrenocorticotropic hormone (ACTH), which stimulates the adrenal cortex to release cortisol. A tumor in the pituitary causes the overproduction of ACTH, which in turn causes the overproduction of cortisol. This is the most common cause. A tumor in the adrenal glands can also lead to the overproduction of cortisol by the adrenal cortex.
The following are some of the symptoms that appear when cortisol is produced in excess: abnormal weight gain (resulting especially in a round or "moon" face), purple and pink stretch marks across the abdomen and sides, high blood pressure, weak bones and muscles, low energy, mood swings and depression, and abnormal hair growth on the face in women.
Treatment for Cushing's syndrome includes the surgical removal of either the pituitary tumor or the adrenal tumor. After surgery, some patients are also given drugs that help decrease cortisol production. If a patient's entire adrenal gland is removed, the patient will have to take steroid medications for the rest of his or her life.
Diabetes mellitus (commonly referred to simply as diabetes) is a disorder in which the cells of the body cannot absorb glucose. This condition is brought about either because the pancreas no longer produces enough insulin or because the cells do not respond to the effects of the insulin that is produced. Approximately 14 million Americans—about 5 percent of the population-have diabetes, and almost 50 percent of them are unaware they have it. Some 300,000 people in the United States die each year from the disorder.
Common symptoms of diabetes include frequent urination, excessive thirst, tiredness, weight loss, hunger, and slow wound healing. The long-term effects of diabetes include loss of vision decreased blood supply to the hands and feet, and pain. If left untreated, the disorder can lead to kidney failure, heart disease, stroke, coma, and death. There are two types of diabetes mellitus: Type I diabetes and Type II diabetes.
Type I diabetes, sometimes called juvenile diabetes, begins most commonly in childhood or adolescence. In this form of diabetes, the pancreas produces little or no insulin. Scientists believe that Type I diabetes may be brought about by a virus or microorganism that trigger's an autoimmune disorder: antibodies that normally destroy foreign invaders end up destroying the islets of Langerhans, the pancreatic cells that produce insulin.
The disorder can be controlled with daily injections (using a small needle and a syringe) of insulin. A strict diet must also be followed. Too little food (or eating too late to coincide with the action of the injected insulin), alcohol, or increased exercise can all lead to low blood sugar levels. A diabetic (person suffering from diabetes) may then become cranky, confused, tired, sweaty, and shaky. If untreated, the diabetic can lose consciousness and have a seizure. Before the condition becomes too serious, the diabetic should have something sweet to eat or drink like candy, sugar cubes, juice, or some other high-sugar snack to balance his or her sugar levels.
Type II diabetes, sometimes called adult-onset diabetes, is the more common form of diabetes. More than 90 percent of the diabetics in the United States suffer from this form. It occurs most often in people who are overweight and who do not exercise. It is also more common in Native Americans, Hispanic Americans, and African Americans.
In Type II diabetes, the pancreas may produce enough insulin, but the body's cells have become resistant to the effects of insulin. Age, obesity (more than 20 percent above ideal body weight), and a family history of diabetes all play a role in the cause. The symptoms of Type II diabetes can begin so gradually that a person may not know they suffer from the disorder. Sometimes the symptoms can develop over several years in overweight adults over the age of forty.
As is the case for Type I diabetes, there is no cure for Type II diabetes. Treatment focuses on keeping blood glucose levels within the normal range. For many Type II diabetics, weight loss is an important goal in helping to control their diabetes. Moderate exercise and a well-balanced, nutritious diet are key steps. To keep blood glucose levels from surging too high, food intake must be distributed over the course of an entire day. For some Type II diabetics, medications are available to help lower blood glucose levels.
Hyperthyroidism is a disorder in which an overactive thyroid gland produces too much thyroxine. It generally results from a tumor of the thyroid. It can also be brought about by an autoimmune disorder in which antibodies bind to the thyroid cells, forcing them to produce excessive amounts of thyroxine. This latter form of hyperthyroidism is known as Graves' disease.
Regardless of the form, hyperthyroidism has the same symptoms: weight loss with increased appetite, shortness of breath, nervousness and anxiety, rapid heart beat, weak muscles, intolerance of heat, and difficulty relaxing and sleeping. In addition, the eyes may bulge and the thyroid may be visibly enlarged (a condition known as a goiter).
Treatment for hyperthyroidism may include the surgical removal of the thyroid tumor (if one is present) or part of the thyroid. Since the thyroid is the only body part that absorbs iodine, radioactive iodine may be administered to destroy the hormone-producing cells and shrink the enlarged gland. Medications to decrease or block the production of thyroxine may also be given. With proper treatment, most individuals suffering from hyperthyroidism can lead normal lives.
Hypothyroidism is a disorder in which an underactive thyroid gland fails to produce or secrete as much thyroxine as the body needs. Since thyroxine is essential to physical growth and body metabolism, a low supply of this hormone can slow life-sustaining processes and damage organs and tissues in every part of the body.
The disorder is one of the most common chronic (long-term) diseases in the United States. As many as 11 million adults and children may be affected by hypothyroidism. Women are twice as likely as men to suffer from the disorder.
Hypothyroidism is most often the result of Hashimoto's disease. In this disease, the body's defense system fails to recognize that the thyroid gland is part of the body's own tissues and attacks it as if it were a foreign body. Sometimes the gland is destroyed in the process. Infections caused by viruses and bacteria and a diet lacking iodine can also bring about hypothyroidism.
Symptoms, which may not appear until years after the thyroid has stopped functioning, include fatigue, decreased heart rate, weight gain, depression, muscle pain or weakness, dry skin, extreme sensitivity to pain, and puffiness of the face.
If hypothyroidism occurs in early childhood, the condition is known as cretinism. This condition results in dwarfism: the head and trunk, which should be about the same length as the legs, grow about one-and-a-half times larger. Cretins (those suffering from cretinism) have scanty hair and very dry skin. They are often mentally retarded. However, if the condition is discovered early enough and medications to replace thyroxine are given, mental retardation and other symptoms can be prevented.
Synthetic or man-made thyroid hormone medications are also given to adults to treat hypothyroidism. This treatment generally maintains normal thyroid hormone levels, allowing an individual to lead a normal lifestyle.
TAKING CARE: KEEPING THE ENDOCRINE SYSTEM HEALTHY
The endocrine glands and the hormones they secrete are involved in almost all aspects of normal body functioning. Because it is so complex, the system operates on a delicate balance. If it malfunctions, a variety of problems, both great and small, will result.
It is therefore important to monitor the system's workings and seek appropriate treatment if a disorder begins to develop. Preventive measures to protect the endocrine system can be taken. The systems of the body respond well to a healthy diet and regular exercise, and the endocrine system is no exception.
Some endocrine disorders are related to diet. Obesity can lead to Type II diabetes, the most common endocrine disorder in the United States. A lack of iodine in the diet can lead to goiter, or enlargement of the thyroid (with the introduction of iodized table salt, however, goiter is uncommon in the United States). Eating a nutritious, healthy, balanced diet and keeping the body at a healthy weight will diminish the risk of possibly developing certain endocrine disorders.
Stress taxes all body systems. Any condition that threatens the body's homeostasis or steady state is a form of stress. Conditions that cause stress may be physical, emotional, or environmental. One of the main functions of certain endocrine glands is to secrete hormones that help the body respond to stressful situations. However, that function is only meant to be short-term. When stress lasts longer than a few hours, higher energy demands are placed on the body. More hormones are then secreted to meet those demands, but at a price. They tend to weaken the body's defenses, leaving the body open to infection.
Stress over an extended period of time can result in high blood pressure and a lack of cortisol and other steroid hormones released by the adrenal cortex. All this can lead to organ damage and failure. Combining exercise with proper amounts of sleep, relaxation techniques, and positive thinking will help reduce stress and keep hormone levels balanced.
FOR MORE INFORMATION
Landau, Elaine. Standing Tall: Unusually Tall People. New York: Franklin Watts, 1997.
Silverstein, Alvin, and Virginia B. Silverstein. Diabetes: The Sugar Disease. New York: HarperCollins Children's Books, 1980.
Young, John K. Hormones: Molecular Messengers. New York: Franklin Watts, 1994.
American Diabetes Association
Homepage of the American Diabetes Association, which seeks to prevent and cure diabetes. Site includes the latest news information relating to diabetes.
ECME: Environmental Estrogens (The endocrine system)
University site provides extensive information on the endocrine system. Includes links to other parts of the site focusing on endocrine glands, hormones, and recent research.
Claims to be the largest web site for thyroid, parathyroid, adrenal, and pancreas disorders. Presents information on endocrine diseases, conditions, hormone problems, and treatment options.
The Endocrine Society
Homepage of the Endocrine Society, the world's largest and most active organization devoted to the research, study, and clinical practice of endocrinology.
Site includes a large image of the human endocrine system with each part linked to a paragraph explanation of its structure and function. Also includes an endocrine system overview.
The Endocrine System
Site presents a detailed chapter on the endocrine system—including hormone action, endocrine organs, and problems—from the On-Line Biology textbook.
The Endocrine System
Site provides a picture of the location of all the endocrine glands in the body; also provides information on each.
"The Endocrine System." UXL Complete Health Resource. 2000. Encyclopedia.com. (May 25, 2016). http://www.encyclopedia.com/doc/1G2-3437000014.html
"The Endocrine System." UXL Complete Health Resource. 2000. Retrieved May 25, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3437000014.html