What are endocrine glands?

Structure and Function

Endocrine glands produce chemical messenger molecules called hormones. Hormones bind to receptors on the cells of the target organ, which causes the cells to respond internally by modifying their biochemical pathways. Unlike exocrine glands, which secrete their products via ducts, endocrine glands secrete hormones directly into the bloodstream: they are ductless. Together, the endocrine glands form the endocrine system.

The glands of the endocrine system and their target organs work in concert; many target organs of endocrine glands are, themselves, endocrine glands. In this way, relatively small signals can be amplified until the desired response takes place in the target organs. This response often leads to a signal being sent to the initiating endocrine gland, which reduces secretion of the original hormone, dampening the entire pathway. This process is called negative feedback.

The hypothalamus and the pituitary gland are located next to each other at the base of the brain. They are the master regulators of the endocrine system. The hypothalamus receives input from nerves in the brain. This triggers the hypothalamus to secrete hormones that act directly on the pituitary, causing it to secrete its own hormones. Depending on the original stimulus, the output of the hypothalamus and pituitary regulates the thyroid gland, adrenal glands, or the gonads.

The thyroid gland is in the neck wrapped around the trachea. The thyroid maintains control of metabolism, the rate at which the body uses energy. Associated with the thyroid gland are four parathyroid glands. These secrete parathyroid hormone, which is important for maintaining correct calcium levels in the blood.

There are two adrenal glands, one on top of each kidney. They are responsible for the secretion of glucocorticoids, mineralocorticoids, and androgens. Adrenal glands are important in fluid and electrolyte homeostasis and in the stress response.

The gonads (ovaries and testes) are located in the pelvic region. In addition to producing the gametes (eggs and sperm), these glands secrete androgens and estrogens, hormones that are essential for the development and maintenance of sexual characteristics.

The pancreas is in the abdominal cavity, underneath the stomach, closely associated with the digestive tract. A majority of the pancreas acts as an exocrine gland secreting digestive enzymes. However, scattered throughout the pancreas are groups of endocrine cells called islets of Langerhans. The two major hormones secreted by these islets are insulin and glucagon; they are crucial for the maintenance of constant glucose levels in the body.

The pineal gland
is located in the center of the brain. It secretes melatonin, a hormone that regulates sleep patterns.

Disorders and Diseases

Endocrinology is the medical field in which endocrine glands are studied. The diagnosis of endocrine gland disorders involves testing the levels of hormones in the blood of the patient. This can be difficult, because most hormones are secreted in pulses. Testing must be performed over hours, days, or weeks, depending on the hormone.

Most diseases that involve the endocrine glands are a result of either the oversecretion or undersecretion of hormones. This upsets the optimal physiological homeostasis and forces negative feedback loops to work inappropriately. Three examples of endocrine disorders are diabetes mellitus, hyperthyroidism, and hypothyroidism.

Diabetes mellitus is the most common endocrine disorder in the United States. The pancreas of an individual suffering from diabetes mellitus produces either too little insulin or insulin that cannot be used by the target cells. The result is abnormally high levels of glucose in the blood. Symptoms of diabetes mellitus include increased thirst and urination, fatigue, and blurred vision. Long-term complications of high blood-glucose levels include blindness, numbness in the feet, kidney failure, and heart disease. Treatment for diabetes mellitus includes insulin injection, drugs that improve the ability of target cells to react to glucose, and drugs that lower glucose levels in the blood.

Thyroid disorders are the second most common endocrine gland problem. Hyperthyroidism is the oversecretion of thyroid hormones caused, for example, by an enlarged thyroid gland (Graves’ disease), or by nodules on the thyroid. Symptoms include a fast heart rate, weight loss, and intolerance of heat. Treatment includes drugs that reduce thyroid hormone production, surgery, or radiation therapy.

Hypothyroidism, insufficient thyroid hormone production, is more common. It is usually caused by inflammation of the thyroid (for example, Hashimoto’s thyroiditis), or by over-compensatory medical treatment (for example, too much thyroid being removed during surgery to treat thyroid nodules). Symptoms include weight gain, cold intolerance, and hair loss. Hypothyroidism is usually treated with levothyroxine, a synthetic form of thyroxine which is one of the major thyroid hormones.

Perspective and Prospects

Carvings from ancient Egyptian times have been identified that show people with enlarged thyroid glands (goiter) and acromegaly (a disorder in which the pituitary gland produces too much growth hormone). However, it was in the eighteenth century that scientists and clinicians really began to understand the concept of endocrine glands and identified them as ductless glands that secrete hormones.

The revolution in the treatment of endocrine gland dysfunction came with the ability to synthesize human hormones in the laboratory. A few examples of synthetic hormones now available are insulin, growth hormone, and thyroxine. Previously, these hormones were purified from animal, or even cadaver, organs. Synthetic hormones are cheaper and safer and the doses are more easily regulated than the hormones available previously.

New testing methods and treatments for endocrine disorders are being researched. One example is the possibility of an insulin drug that can be administered orally instead of by injection. An exciting prospect for the future is the use of stem cell therapy to provide replacement hormones within the affected individual’s own body.

Bibliography

Carmichael, Kim. "Hyperthyroidism." Health Library, November 26, 2012.

"Endocrine System." MedlinePlus, April 8, 2013.

Gardner, Dave, and Delores Shoback. Greenspan’s Basic and Clinical Endocrinology. 9th ed. New York: McGraw-Hill Medical, 2011.

Koeppen, Bruce, and Bruce Stanton. Berne and Levy Physiology. UPdated 6th ed. Philadelphia: Elsevier, 2010.

Neal, Matthew. How the Endocrine System Works. Malden, Mass.: Blackwell Science, 2001.

Wood, Debra, and Kim Carmichael. "Type 1 Diabetes." Health Library, November 26, 2012.

Wood, Debra, and Lawrence Frisch. "Type 2 Diabetes." Health Library, July 17, 2012.