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Endocrine Function

By

John E. Morley

, MB, BCh, Saint Louis University School of Medicine

Last full review/revision Apr 2019| Content last modified Apr 2019
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NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
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Topic Resources

The main function of endocrine glands is to secrete hormones directly into the bloodstream. Hormones are chemical substances that affect the activity of another part of the body (target site). In essence, hormones serve as messengers, controlling and coordinating activities throughout the body. (See also Endocrine Glands.)

Upon reaching a target site, a hormone binds to a receptor, much like a key fits into a lock. Once the hormone locks into its receptor, it transmits a message that causes the target site to take a specific action. Hormone receptors may be within the nucleus or on the surface of the cell.

Ultimately, hormones control the function of entire organs, affecting such diverse processes as growth and development, reproduction, and sexual characteristics. Hormones also influence the way the body uses and stores energy and control the volume of fluid and the levels of salts and sugar (glucose) in the blood. Very small amounts of hormones can trigger very large responses in the body.

Although hormones circulate throughout the body, each type of hormone influences only certain organs and tissues. Some hormones affect only one or two organs, whereas others have influence throughout the body. For example, thyroid-stimulating hormone, produced in the pituitary gland, affects only the thyroid gland. In contrast, thyroid hormone, produced in the thyroid gland, affects cells throughout the body and is involved in such important functions as regulating growth of cells, controlling the heart rate, and affecting the speed at which calories are burned. Insulin, secreted by the islet cells of the pancreas, affects the processing (metabolism) of glucose, protein, and fat throughout the body.

Most hormones are derived from proteins. Others are steroids, which are fatty substances derived from cholesterol.

Table
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Major Hormones

Where Hormone Is Produced

Hormone

Function

Adipose (fat) tissue

Leptin

Controls appetite

Resistin

Blocks the effects of insulin on muscle

Adrenal glands

Aldosterone

Helps regulate salt and water balance by causing the kidneys to retain salt and water and excrete potassium

Cortisol

Has widespread effects throughout the body

Especially has anti-inflammatory action

Maintains blood sugar level, blood pressure, and muscle strength

Helps control salt and water balance

Dehydroepiandrosterone (DHEA)

Has effects on bone, mood, and the immune system

Epinephrine and norepinephrine

Stimulate the heart, lungs, blood vessels, and nervous system

Digestive tract

Cholecystokinin

Controls gallbladder contractions that cause bile to enter the intestine

Stimulates release of digestive enzymes from the pancreas

Ghrelin

Controls growth hormone release from the pituitary gland

Causes sensation of hunger

Glucagon-like peptide

Increases insulin release from the pancreas

Hypothalamus

Corticotropin-releasing hormone

Stimulates release of adrenocorticotropic hormone

Gonadotropin-releasing hormone

Stimulates release of luteinizing hormone and follicle-stimulating hormone

Growth hormone–releasing hormone

Stimulates release of growth hormone

Somatostatin

Inhibits release of growth hormone, thyroid-stimulating hormone, and insulin

Thyrotropin-releasing hormone

Stimulates the release of thyroid-stimulating hormone and prolactin

Kidneys

Erythropoietin

Stimulates red blood cell production

Renin

Controls blood pressure

Ovaries

Estrogen

Controls the development of female sex characteristics and the reproductive system

Progesterone

Prepares the lining of the uterus for implantation of a fertilized egg and readies the mammary glands to secrete milk

Pancreas

Glucagon

Raises the blood sugar level

Insulin

Lowers the blood sugar level

Affects the processing (metabolism) of sugar, protein, and fat throughout the body

Parathyroid glands

Parathyroid hormone

Controls bone formation and the excretion of calcium and phosphorus

Pituitary gland

Corticotropin (also called adrenocorticotropic hormone [ ACTH])

Controls the production and secretion of hormones by the adrenal glands

Growth hormone

Controls growth and development

Promotes protein production

Luteinizing hormone and follicle-stimulating hormone

Control reproductive functions, including the production of sperm and semen in men and egg maturation and menstrual cycles in women

Control male and female sexual characteristics (including hair distribution, muscle formation, skin texture and thickness, voice, and perhaps even personality traits)

Oxytocin

Causes muscles of the uterus to contract during childbirth and after delivery and stimulates contractions of milk ducts in the breast, which move milk to the nipple

Prolactin

Starts and maintains milk production in the ductal glands of the breast (mammary glands)

Thyroid-stimulating hormone

Stimulates the production and secretion of hormones by the thyroid gland

Vasopressin (antidiuretic hormone)

Causes kidneys to retain water and, along with aldosterone, helps control blood pressure

Placenta

Chorionic gonadotropin

Stimulates ovaries to continue to release progesteroneduring early pregnancy

Estrogen and progesterone

Keep uterus receptive to fetus and placenta during pregnancy

Testes

Testosterone

Controls the development of male sex characteristics and the reproductive system

Thyroid gland

Calcitonin

Tends to decrease blood calcium levels and helps regulate calcium balance

Thyroid hormones

Regulate the rate at which the body functions (metabolic rate)

Endocrine Controls

To control endocrine functions, the secretion of each hormone must be regulated within precise limits. The body is normally able to sense whether more or less of a given hormone is needed.

Many endocrine glands are controlled by the interplay of hormonal signals between the hypothalamus, located in the brain, and the pituitary gland, which sits at the base of the brain. This interplay is referred to as the hypothalamic-pituitary axis. The hypothalamus secretes several hormones that control the pituitary gland.

The pituitary gland, sometimes called the master gland, in turn controls the functions of many other endocrine glands. The pituitary controls the rate at which it secretes hormones through a feedback loop in which the blood levels of other endocrine hormones signal the pituitary to slow down or speed up. So, for example, the pituitary gland senses when blood levels of thyroid hormone are low and releases thyroid stimulating hormone, which tells the thyroid gland to make more hormones. If the level gets too high, the pituitary senses that and decreases the amount of thyroid stimulating hormone, which then decreases the amount of thyroid hormone produced. This back-and-forth adjustment (feedback) keeps hormone levels in proper balance.

Many other factors can control endocrine function. For example, a baby sucking on its mother's nipple stimulates her pituitary gland to secrete prolactin and oxytocin, hormones that stimulate breast milk production and flow. Rising blood sugar levels stimulate the islet cells of the pancreas to produce insulin. Part of the nervous system stimulates the adrenal gland to produce epinephrine.

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