Also called: Endocrine Glands
The endocrine system is a network of glands that produce and secrete hormones involved in controlling many functions of the body. This includes growth and development, stability of internal bodily systems (homeostasis), metabolism and sexual function.
Glands and hormones are the primary features of the endocrine system. Glands are collections of cells that produce and secrete hormones. Hormones act as chemical messengers in the body, moving information and instructions from one group of cells to another by traveling through the blood.
The major glands of the endocrine system include:
- Pituitary gland. Located at the base of the brain, this “master gland” controls and regulates most of the functions of the other endocrine system glands.
- Hypothalamus. Located in the lower central part of the brain just above the pituitary gland. It regulates bodily systems such as breathing, hunger, thirst and temperature.
- Adrenal glands. Two triangular-shaped glands located above each kidney that are responsible for controlling a host of functions, including regulation of salt and water balance in the body. They also are partly responsible for controlling blood pressure.
- Thyroid gland. A small gland located in the lower front part of the throat, that produces thyroid hormones that regulate the body’s metabolism, weight, bone growth and development of the brain and nervous system.
- Parathyroid glands. Two pairs of tiny glands located behind the thyroid gland that are essential to regulate the balance of calcium in the body.
- Pancreas. Located behind the stomach, the pancreas serves two functions: digestive (exocrine) and hormonal (endocrine). The hormones it makes include insulin and glucagon.
- Pineal body. Located in the base of the brain, it is involved in regulating the body’s wake-sleep cycle.
- Thymus. Located in children in the chest above and in front of the heart, this gland is present in infancy and early childhood and provides immune system functioning for the growing child.
- Gonads. Located in the lower portion of the trunk of the body, the reproductive organs produce steroid hormones specific to the sexual characteristics and reproductive functions of males and females beginning at puberty.
Each of the endocrine glands produces certain types of hormones used by the body. The release of these hormones causes specific reactions to occur in the body. For example, the hypothalamus produces hormones such as growth hormone-releasing hormone (GHRH) and the pituitary gland produces hormones including prolactin and endorphins.
About the endocrine system
The endocrine system is a network of glands that plays a vital role in a wide variety of bodily functions. The glands of this system produce and secrete hormones that control the following processes:
- Growth and development
- Internal balance of bodily systems (homeostasis)
- Regulation of mood and energy levels
- Responses to surroundings, stress and injury
- Sexual function and reproduction
- Tissue function
The endocrine system specializes in processes of the body that occur gradually, whereas the nervous system generally directs faster processes, such as breathing and body movements. However, in many cases the endocrine system and nervous system work together to guide some body processes.
Glands and hormones are the primary features of the endocrine system. Glands are collections of cells that produce and secrete substances called hormones that act as chemical messengers in the body. Glands that do not make hormones, such as sweat glands, salivary glands and the prostate gland, are not part of the endocrine system. Hormones move information and instructions from one group of cells to another.
Imbalances in the endocrine system can cause problems. Too much or too little of a hormone, trouble with the receptor sites, trouble with the feedback system or regulating system, glandular problems, or problems with the blood supply can throw the whole system out of balance. This creates problems in the body on a glandular or functional level. For this reason, the chemicals of the endocrine system must be kept in precise balance. Imbalances can lead to systemic dysfunctions and disorders that affect body functions necessary for sustaining life.
Some endocrine diseases are autoimmune conditions, meaning the body’s immune system mistakenly identifies healthy tissues as a threat and attacks them. Examples include type 1 diabetes, Hashimoto’s thyroiditis, Graves’ disease and Addison disease. Some endocrine disorders involve more than one gland, such as autoimmune polyglandular syndrome.
Some endocrine conditions involve benign (noncancerous) tumors, such as thyroid nodules and insulinoma. In rare cases, cancer can develop in the endocrine glands.
Physicians have many ways of assessing endocrine function and diagnosing endocrine disorders, including glucose tests, thyroid blood tests, other blood tests, a physical examination, urine tests, ultrasound and thyroid imaging tests. A primary care physician will typically refer a patient with an endocrine condition to an endocrinologist.
Major glands of the endocrine system
Glands are units of cells that secrete substances used in other parts of the body. There are two types of glands in the body:
- Exocrine. Glands that release their secretions through a duct or directly to epithelial tissue (skin and linings of body cavities and passageways). Examples include the sweat glands, sebaceous glands, salivary glands and prostate gland. Glands in the digestive system that secrete enzymes through ducts also are considered to be exocrine glands.
- Endocrine. A variety of ductless glands that secrete more than 20 hormones into the bloodstream or lymph nodes. These substances are then transported to other parts of the body and carry messages for how those body parts should act.
Endocrine glands use material in the blood, and synthesize and secrete the chemical or hormone that supports the specified function elsewhere. The major glands in the endocrine system are:
- Pituitary gland. Located at the base of the brain, this “master gland” plays a crucial role in regulating the workings of the endocrine system. Though only the size of a pea, the pituitary gland controls and regulates most of the functions of the other endocrine glands.
The pituitary gland is made up of two parts: the anterior and posterior lobes. Each lobe produces its own hormones. Various factors can influence the production and secretion of pituitary hormones. These range from shifts in emotion to seasonal changes. Pituitary problems include growth disorders such as acromegaly.
- Hypothalamus. Located in the lower central part of the brain just above the pituitary gland, the hypothalamus regulates homeostasis in the body, including breathing, metabolism, hunger, thirst and temperature. It is considered the “switchboard” of the endocrine system because it is the part of the brain that exerts control over the endocrine system.
The hypothalamus and the pituitary gland are the regulators of the endocrine system. The hypothalamus secretes hormones that direct the pituitary to either stimulate or suppress the secretion of pituitary hormones. Diabetes insipidus (“water diabetes”) usually involves a problem with the hypothalamus or pituitary gland. This condition is unrelated to the far more common diabetes mellitus (“sugar diabetes”).
- Adrenal glands. Two triangular-shaped glands located above each kidney. There are two distinct parts of the adrenal glands (adrenal cortex and the adrenal medulla) that secrete two distinct sets of hormones. The adrenal glands are responsible for controlling a host of functions, including regulation of salt and water balance, immune system regulation, body reaction to stress, and sexual development. Adrenal gland disorders include hypoadrenal (reduced function)conditions such as Addison disease and hyperadrenal (excessive function) conditions such as Cushing’s syndrome, a disease that may be caused by excessive levels of cortisol (hormone produced by the adrenal glands).
- Thyroid gland. A small gland located in the lower front part of the throat, the thyroid gland produces thyroid hormones that regulate the body’s metabolism, bone growth and development of the brain and nervous system. Thyroid hormones maintain blood pressure, heart rate, digestion, muscle tone, body temperature, weight and reproductive functions. Thyroid disorders include hypothyroidism and hyperthyroidism.
- Parathyroid glands. Two pairs of tiny glands located behind the thyroid gland, one set on each side of the gland. They are essential to regulate the balance of calcium in the body. Hyperparathyroidism may cause hypercalcemia, and hypoparathyroidism may cause hypocalcemia.
- Pancreas. Located behind the stomach, the pancreas serves two functions: digestive (exocrine) and hormonal (endocrine). The portion of the pancreas that secretes digestive enzymes is considered exocrine in its function. The other part consists of cells called islets of Langerhans. They secrete hormones including insulin and glucagon, which work together to regulate glucose (blood sugar).
The various forms of diabetes mellitus, including type 1 diabetes and type 2 diabetes, are pancreatic endocrine diseases. An example of a pancreatic exocrine disease is pancreatitis.
- Pineal body. Located in the base of the brain, it is involved in regulating the body’s wake-sleep cycle.
- Thymus. Located in children in the chest above and in front of the heart, this gland is present in infancy and early childhood and provides immune system functioning for the growing child. The gland grows throughout childhood until puberty, when its function begins to decline. Afterward, the tissue is replaced by fat cells and the body’s immune system replaces the immune function formerly provided by the thymus.
- Gonads. Located in the lower trunk, the reproductive organs produce steroid hormones specific to the sexual characteristics and reproductive functions of males and females beginning at puberty. The reproductive organs in males are the testes (testicles) and in females are the ovaries.
Other parts of the body that release hormones include the gastrointestinal tract and the kidneys.
Major hormones of the endocrine system
Each of the endocrine glands produces certain types of hormones used by the body. The release of these hormones causes specific reactions to occur in the body.
The hypothalamus produces several hormones that regulate the pituitary gland. They include:
- Growth hormone-releasing hormone (GHRH). Sent to the pituitary gland to release growth hormone (GH).
- Thyrotropin-releasing hormone (TRH). Sent to the pituitary gland to release thyroid-stimulating hormone (TSH).
- Corticotropin-releasing hormone (CRH). Sent to the pituitary gland to release adrenocorticotropin hormone (ACTH).
- Somatostatin. Suppresses production of GH and TSH in the pituitary gland.
- Gonadotropin-releasing hormone (GRH). Stimulates the pituitary gland to produce and release luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which aid puberty and reproductive functions in males and females.
The pituitary gland produces different hormones from its two regions, the anterior and posterior lobes. The hormones produced by the anterior (front) lobe include:
- Growth hormone(GH). Stimulates bone and muscle growth and healthy body composition. In children, too much GH can cause gigantism, a condition in which excessive growth of the long bones results in excessive tallness. Too little GH in children can stunt growth. Deficiencies in adults lead to fat, muscle and bone mass imbalances and deficiencies. Excessive growth hormone in adulthood can lead to acromegaly, a condition in which the bones increase in size.
- Thyroid-stimulating hormone (also called TSH or thyrotropin). Stimulates the release of thyroid hormones to control metabolism, energy, growth and the nervous system.
- Adrenocorticotropin hormone (ACTH). Stimulates the adrenal glands to produce aldosterone (which controls sodium and water retention by the kidneys) and cortisol(a steroid hormone that helps maintain blood pressure and glucose levels in times of stress).
- Luteinizing hormone (LH). Stimulates the cells of the testes to produce and regulate testosterone. Stimulates ovulation (egg release) in women and release of estrogen from a gland known as the corpus luteum. LH also stimulates the secretion of sex hormones in the gonads.
- Follicle-stimulating hormone (FSH). Stimulates maturation of ovarian follicles (eggs) in females and promotes the process of spermatogenesis (sperm production) in males. FSH also stimulates the secretion of sex hormones in the gonads.
- Prolactin (PRL). Stimulates milk production in females after childbirth. It plays a role in the metabolism of carbohydrates and fats. PRL also affects the levels of sex hormones in the ovaries and testes.
- Corticotropin. Stimulates the adrenal glands to produce hormones.
- Endorphins. Chemical messengers sent to the nervous system to reduce pain sensitivity by deadening pain receptors. Endorphins are chemically related to morphine and are naturally produced by the body in response to pain.
- Melanocyte-stimulating hormone (MSH). Stimulates the melanocytes in the skin to produce melanin (skin pigment).
The posterior (back) lobe of the pituitary gland stores the following hormones that are secreted by the hypothalamus and transported to the pituitary gland:
- Antidiuretic hormone (ADH or vasopressin). Inhibits the amount of water and sodium excreted from the body. ADH causes the kidneys to produce concentrated urine in lower volumes, and problems with this hormone can cause kidney problems. ADH also stimulates constriction of blood vessels and smooth muscles.
- Oxytocin. Stimulates milk production and contracts the uterus during childbirth.
The adrenal glands have two distinct parts (adrenal cortex and the adrenal medulla) that secrete two distinct sets of hormones.
The adrenal cortex is the outer portion of the adrenal gland, and it produces corticosteroids such as cortisone. Cortisone is an anti-inflammatory hormone that suppresses immune system response. Corticosteroids raise blood pressure and can produce problems for people with hypertension. The following are corticosteroids secreted by the adrenal cortex:
- Glucocorticoids, such as cortisol. Helps regulate metabolism of carbohydrates (including glucose), fats and proteins. Cortisol also causes the release of glycogen to elevate levels of glucose in the bloodstream and help the body cope with stress.
- Mineralocorticoids (such as aldosterone). Secreted to stimulate sodium and water retention and the excretion of potassium from the kidneys. These functions help to maintain blood volume and pressure.
The adrenal medulla is the inner portion of the adrenal gland associated with the sympathetic nervous system. It produces the following hormones:
- Epinephrine (also known as adrenaline). Produced in response to fright, stress, caffeine, anger and hypoglycemia (low glucose). Adrenaline increases heart rate, as well as the rate of metabolism and blood flow to muscles. This hormone is secreted as an emergency response to stress. Adrenaline inhibits insulin and raises glucose by stimulating the liver to break down stored glycogen into glucose and release it into the bloodstream.
- Norepinephrine. Allows the body to maintain normal functioning as opposed to emergency reactions. This hormone stimulates and maintains a normal heart beat.
The thyroid gland produces hormones including thyroxineandtriiodothyronine. These help regulate metabolism. Hyperthyroidism occurs when the thyroid gland is too active. Symptoms include unexplained weight loss and restlessness. Hypothyroidism, in which the thyroid gland is underactive, causes symptoms such as unexplained weight gain and fatigue.
The parathyroid glands produce parathyroid hormone (PTH). This hormone increases calcium and reduces phosphorus levels in the body and is produced in response to low levels of dietary calcium in the diet. Proper nerve, muscle and bone structure and functioning are supported by PTH.
The pancreas produces hormones such as:
- Glucagon. Produced by the alpha cells in the islets of Langerhans, glucagon is secreted by the pancreas to stimulate the liver to change stored glycogen to glucose for release into the bloodstream to raise glucose levels. This is an emergency response when the blood sugar levels are too low.
- Insulin. Produced by the beta cells in the islets of Langerhans, insulin enables sugar in the blood (glucose) to enter cells so they can produce heat and energy. Glucose is the body’s fuel, and insulin is the catalyst that enables the body to use it. If this system works properly, it lowers the blood sugar levels. If not, blood sugar levels may rise out of control, leading to diabetes. Insulin also stimulates glycogen production and storage.
- Amylin. Produced by the beta cells, it delays emptying of the stomach, promotes feelings of fullness (satiety) and inhibits glucagon. Synthetic amylin is an injected medication recently approved to help treat some cases of type 1 diabetes.
- Somatostatin. Produced by the islet delta cells, it suppresses insulin and glucagon.
The pineal body produces melatonin. This hormone regulates sleep cycles and is stimulated by the nerves of the eyes. Production increases sharply at night and decreases sharply in the daytime. It also helps the pituitary gland inhibit production of GRH by the hypothalamus.
The thymus releases thymosin. This hormone stimulates the production of T-cells, which attack foreign substances invading the body and are part of the body’s immune system. Thymosin stimulates an immune system response from the lymphoid tissues.
The gonads consist of the ovaries and testes (testicles). The ovaries are the female reproductive organs that produce eggs. They also produce the following female sex hormones:
- Estrogen. Hormones that affect female sexual characteristics such as breast development and widening of the hips that is evident in puberty. Estrogen also affects reproductive functions such as menstruation, pregnancy and egg production.
- Progesterone. Also crucial in reproductive functions such as menstruation, pregnancy and egg production. Progesterone prepares the uterus for the implantation of the embryo. Also, secretions of progesterone from the placenta help maintain pregnancy. Progesterone also plays a part in regulating menstruation.
- Inhibin. A protein that inhibits the release of FSH from the pituitary gland to regulate egg development.
Testes are the male reproductive organs that produce sperm and male sex hormones (androgens), including testosterone. These hormones affect the development of male sexual characteristics during puberty, including sexual growth and development, facial hair and voice changes. These hormones continue to affect aspects of an adult male’s sexual life, including sex drive and sperm production, and contribute to male hair patterns, bone mass and muscle mass.
Some gonadal disorders are linked to insulin resistance, including polycystic ovarian syndrome in females and hypogonadism (low levels of testosterone) in men. Deficits in sex hormones can contribute to conditions including sexual dysfunction and some musculoskeletal disorders.
Questions for your doctor
Preparing questions in advance can help patients have more meaningful discussions with healthcare providers regarding their condition. Patients may wish to ask their doctor the following questions about the endocrine system:
- What are the symptoms that might indicate a problem with my endocrine system?
- What problem do you suspect?
- Which of my endocrine glands may be involved?
- How will you diagnose a potential problem with my endocrine system?
- Do I need to prepare in any special way for diagnostic testing?
- What are the potential health threats posed by this disorder?
- What are my treatment options?
- Are there medications available to treat my condition?
- What are the risks and side effects of these treatments?
- What is my long-term prognosis?