The endocrine system is a widespread group of glands and organs that acts as the body's control system for producing, storing, and secreting chemical substances called hormones.
The primary glands that compose the endocrine system are the hypothalamus, pituitary, thyroid, parathyroid, adrenal, pineal, ovary, and testes. The pancreas, considered both an organ and a gland, is also part of the system. The thymus is sometimes considered an endocrine-system organ. Although not part of the endocrine system, other organs that secrete hormones are the heart, brain, lungs, kidneys, liver, skin, and placenta. The word "endocrine" means that in response to specific stimuli, the hormones produced by the glands are released into the bloodstream.
Hormones are compounds produced by the endocrine glands. They generally control the growth, development, and metabolism of the body; the electrolyte composition of body fluids; and reproduction. The specific functions of the endocrine glands and pancreas are unique.
The pituitary is the master gland of the endocrine system. Located at the base of the brain, the gland, which is about the size of a marble, consists of two parts: anterior and posterior. The anterior pituitary produces hormones that either stimulate other glands (such as adrenal, testis, ovary, and thyroid) to produce target-gland hormones, or directly affect the target organs.
Three of these hormones—adrenocorticotropic hormone (ACTH), gonadotropins, and thyroid-stimulating hormone (TSH)—act on other glands. ACTH stimulates the adrenal cortex to produce corticosteroid hormones and small amounts of male and female sex hormones. Gonadotropins are two hormones that regulate the production of male and female sex hormones and the egg (ova) and sperm (spermatozoa) cells. TSH stimulates the thyroid gland to produce and release thyroid hormone.
Another pituitary hormone, growth hormone (GH), has a central role in controlling the growth and development of the body and its components, including organs, tissue, and muscle. It also affects the metabolism of carbohydrates,
protein, and fat. For example, GH increases glucose levels in the blood by reducing the amount of glucose used by muscle cells and adipose tissue and by promoting glucose production from certain liver molecules. Other functions of GH include increasing the amount of amino acids that cells take from the blood and stimulating the breakdown of lipids (fats) in adipose tissue. The pituitary hormone prolactin acts with other hormones in female breast development and helps regulate breast-milk production (lactation).
Two hormones, vasopressin and oxytocin, are stored but not produced in the posterior pituitary. Vasopressin, also called arginine vasopressin (AVP), helps the body to conserve water by increasing reabsorption of water from the kidneys. Oxytocin stimulates contractions in the uterus during childbirth and activates milk injection caused by an infant sucking on the breast.
The adrenals are small glands on top of the kidneys. The adrenals have two parts: an outer layer called the cortex, and an inner layer called the medulla. The adrenal cortex produces a variety of hormones called corticosteroids, including hydrocortisone (cortisol), which helps increase blood glucose levels. It also reduces the amount of glucose absorbed by muscles and adipose tissue. Another function of cortisol is to protect the body from the adverse affects of stress, including emotional and physical trauma.
The adrenal medulla produces adrenaline and noradrenaline, substances that increase the heart rate and blood pressure during times of stress. Their action is referred to as the "fight-or-flight" response.
The thyroid gland is composed of two sections in front of the windpipe and below the voice box. It produces two hormones, thyroxine (T4) and tri-iodothyronine (T3), which together are called the thyroid hormones. They help regulate growth and development and help in childhood brain development. The thyroid also contains cells that produce the hormone calcitonin, which helps to maintain normal calcium levels in the blood.
The parathyroid complex is composed of four small glands, each the size of a pea, and each located on the four corners of the thyroid gland. They secrete parathyroid hormone, which regulates calcium levels in the blood.
The pancreas is located in the upper abdomen, just behind and below the stomach. It has two functions: to produce various enzymes that aid in digestion; and to produce insulin and glucagon, hormones that are key to the body's management of glucose (sugar) in the blood.
The primary purpose of insulin is to lower blood-glucose levels in the body. It helps form glycogen, proteins, and lipids, which are stored in the body (usually in the liver, muscles, and adipose tissue) to be used for energy. Glucagon increases blood-glucose levels, an action opposite to that of insulin. A strict balance between the glucagon and insulin is required to maintain proper blood-sugar levels.
Located deep inside the brain, the hypothalamus maintains direct control of the pituitary gland. It acts as the central "control room" of the endocrine system, directing the activities of the other parts of the system. These activities include regulating eating and drinking, sexual behavior, blood pressure, heart rate, body temperature, emotions, and the sleeping/waking cycle. When the brain receives information indicating that hormonal changes are needed somewhere in the body, the hypothalamus secretes chemicals that stimulate or suppress hormone production in the pituitary gland.
Pineal and thymus
The pineal is located in the center of the brain. This gland secretes melatonin, a hormone that helps regulate the sleeping/waking cycle. Disturbances in the production of melatonin causes jet lag, experienced by many long-distance travelers. Melatonin also influences development of the male and female sex glands. The thymus processes lymphocytes in infants and is partly responsible for immune-system development.
Ovaries and testes
The ovaries and testes, also called the sex glands, produce cells and hormones essential to reproduction and development of the body, including male and female sex characteristics. The three types of sex hormones are estrogens, progestogens, and androgens (including testosterone).
The main role of estrogens is to coordinate development and function of the female genitalia and breasts. Estrogens are also associated with the start of the menstrual bleeding cycle. Estrogen production in the ovary ceases during menopause. Estrogen is also produced in men (by the testes), though at lower levels than occur in women.
Progestogens are produced in the ovaries during part of the menstrual cycle, and in the placenta during pregnancy. They cause changes in the lining of the uterus to prepare it for pregnancy, and they act with estrogens to stimulate mammary-gland development in the breasts to prepare for lactation. Progesterone is the main progestogen hormone.
The primary androgen produced in the testes is the steroid testosterone. While mainly associated with male development, testosterone is produced in small amounts in women by the ovaries. During pregnancy, testosterone helps to develop the internal and external male sex organs. In males, testosterone promotes the growth of the sex organs and develops or stimulates male characteristics, such as deepening voice; growth of facial, pubic, and other body hair; and muscle growth and strength. In adult males, testosterone maintains the masculine characteristics and sexual potency and regulates sperm production.
Role in human health
A wide variety and dozens of symptoms can indicate a hormonal imbalance in the body. However, a specific group of symptoms give an initial indication of a problem in the endocrine system. For example, excessive thirst, frequent urination, and unexplained weight loss are classic signs of diabetes mellitus, the most common endocrine disorder. Many primary-care physicians still treat endocrine problems, especially diabetes, themselves. However, the primary care doctor often makes a preliminary diagnosis and then refers the patient to an endocrine-system specialist, called an endocrinologist.
Disorders of the endocrine system often, but not always, result from an over-or underproduction of a particular
Common diseases and disorders
There are two basic classes of endocrine disorders: problems associated with hormone-production levels, and problems caused by tissues that are unable to respond to hormones. Hormone-production disorders are broken into two groups: insufficient hormone production, called hypofunction; and too much hormone production, called hyperproduction. Endocrine-system disorders include the following:
- Diabetes mellitus is a disease that includes type 1 and type 2 diabetes. Type 1 is an autoimmune disease caused when the immune system destroys certain insulin-producing cells in the pancreas. This causes the pancreas to produce little or no insulin. Type 1 diabetes usually develops in children and young adults, but it can appear at any age. Symptoms include increased thirst and urination, unexplained weight loss, blurred vision, and extreme fatigue. There is no cure; insulin, first used in 1921, remains the only treatment. Type 2 diabetes accounts for 90% to 95% of diabetes cases. It usually develops in adults over age 40 and is usually associated with obesity. In type 2, the pancreas produces insulin, but the hormone is not used effectively by the body, a condition called insulin resistance. Several years after onset, insulin production decreases below the level needed to maintain glucose homeostasis. The result is the same as for type 1 diabetes: glucose builds up in the blood because the body cannot use it efficiently. Symptoms develop gradually and include increased thirst and urination, weight loss, fatigue, nausea, blurred vision, frequent infections, and wounds or sores that heal slowly. Insulin resistance is treated with drugs such as thiazolidinedions (rosiglitazone and pioglitazone) and biguanides (metformen). When insufficient insulin is produced, type 2 diabetes appears. However, research indicates progression of insulin resistance to type 2 diabetes can usually be halted or slowed with the insulin-resistance medications, or by lifestyle changes that result in weight loss.
- Hypothyroidism is caused by the thyroid gland producing too little thyroid hormone. It can lead to severe hypothyroidism, a disorder that usually develops after age 40. Symptoms include intolerance to cold, lethargy, fatigue, weight gain, and mental sluggishness. Congenital hypothyroidism is present at birth and has the same symptoms. If left untreated, it can lead to mental retardation. The standard treatment for both hypothyroid disorders is thyroid hormone-replacement medications such as levothyroxine (Synthroid, Unithroid, Levoxyl, and Levothroid) and triiodothyrine.
- Hyperthyroidism is due to an excess of thyroid hormones and affects women more frequently than men. Symptoms include nervousness, weight loss, intolerance to heat, diarrhea, heart palpitations, and insomnia. Some patients experience protruding eyes and trembling. Treatments include medications to inhibit thyroid-hormone production, and removal or destruction of the thyroid gland with radioactive iodine. The most common cause of the excessive thyroid production is Graves' disease, an autoimmune disorder of the thyroid gland.
- Addison's disease is caused by underactivity or immune-system destruction of the adrenal gland. It can be life-threatening if left untreated. Symptoms include weakness, fatigue, nausea, dehydration, fever, and hyperpigmentation (darkening of the skin without sun exposure.) The standard treatment is with corticosteroid hormones and adequate dietary salt.
- Cushing's syndrome and Cushing's disease are different disorders with similar symptoms: obesity, weakness, easily bruised skin, acne, and hypertension (high blood pressure.) Cushing's syndrome is usually caused by excessive production of glucocorticoid hormones in the adrenal gland. However, it can sometimes be caused by benign or cancerous tumors of the adrenal gland. Cushing's disease usually results from the overproduction of the adrenocoticotropic hormone in the pituitary gland, due to a benign tumor. Treatment for both disorders can include surgery, radiation therapy, chemotherapy, and blocking production of the glucorticoid hormones with drugs.
- Less common endocrine disorders include acromegaly, gigantism, and hypogonadism. Acromegaly occurs in adults and gigantism in children. Both are caused by a pituitary tumor that spurs overproduction of growth hormone. Hypogonadism causes delayed sexual maturity in children and infertility in adults. It is caused by underproduction of follicle-stimulating hormone (FSH) in the pituitary gland.
Adipose tissue—Connective tissue in which fat is stored and that has the cells distended by droplets of fat.
Autoimmune—A term that refers to a condition in which antibodies or T cells attack the molecules, cells, or tissue of the body organ or system producing them.
Electrolyte—A nonmetallic electric conductor in which current is carried by ion movement.
Lactation—The secretion of milk by the mammary gland in the breasts.
Lymphocytes—Weak cells produced in the lymphoid tissue.
Menstruation—The discharge of blood from the uterus that occurs in approximately monthly intervals in females, starting at puberty.
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American Association of Clinical Endocrinologists. 1000 Riverside Avenue, Suite 205, Jacksonville, FL 32204.(904) 353-7878. <http://www.aace.com>.
American Diabetes Association. 1701 North Beauregard Street, Alexandria, VA 22311. (800) 342-2383. <http://www.diabetes.org>.
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Ken R. Wells