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 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 Hormone Function 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, egg maturation, and menstrual cycles; 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 and milk ducts in the breast to contract   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 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)   Calcitonin In people, function is unclear; in other species, regulates calcium balance Adrenal glands Aldosterone Helps regulate salt and water balance by retaining salt and water and excreting 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 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 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 Testes Testosterone Controls the development of male sex characteristics and the reproductive system Digestive tract Cholecystokinin 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 pancreas   Ghrelin Controls growth hormone release from the pituitary gland; causes sensation of hunger Adipose (fat) tissue Resistin Blocks the effects of insulin on muscle   Leptin Controls appetite Placenta Chorionic gonadotropin Stimulates ovaries to continue to release progesterone during early pregnancy   Estrogen and progesterone Keep uterus receptive to fetus and placenta during pregnancy

Last full review/revision May 2006 by John E. Morley, MB, BCh