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Endocrine Function

by John E. Morley, MB, BCh

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.

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

Major Hormones

Where Hormone Is Produced



Pituitary gland

Antidiuretic hormone ( vasopressin )

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

Corticotropin ( 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)


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


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

Parathyroid glands

Parathyroid hormone

Controls bone formation and the excretion of calcium and phosphorus

Thyroid gland

Thyroid hormone

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


In people, has unclear function, but in other species, regulates calcium balance

Adrenal glands


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


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



Raises the blood sugar level


Lowers the blood sugar level

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



Stimulates red blood cell production


Controls blood pressure



Controls the development of female sex characteristics and the reproductive system


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



Controls the development of male sex characteristics and the reproductive system

Digestive tract


Controls gallbladder contractions that cause bile to enter the intestine

Stimulates release of digestive enzymes from the pancreas

Glucagon-like peptide

Increases insulin release from the pancreas


Controls growth hormone release from the pituitary gland

Causes sensation of hunger

Adipose (fat) tissue


Blocks the effects of insulin on muscle


Controls appetite


Chorionic gonadotropin

Stimulates ovaries to continue to release progesterone during early pregnancy

Estrogen and progesterone

Keep uterus receptive to fetus and placenta during pregnancy


Thyrotropin-releasing hormone

Stimulates the release of thyroid-stimulating hormone and prolactin

Gonadotropin-releasing hormone

Stimulates release of luteinizing hormone and follicle-stimulating hormone

Corticotropin-releasing hormone

Stimulates release of adrenocorticotropic hormone

Growth hormone–releasing hormone

Stimulates release of growth hormone


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

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