Endocrine System: Facts, Functions and Diseases

The endocrine system is the collection of glands, each of which secretes different types of hormones that regulate metabolism, growth and development, tissue function, sexual function, reproduction, sleep and mood, among other things.

The word endocrine derives from the Greek words "endo," meaning within, and "crinis," meaning secrete.

Description of the endocrine system

The endocrine system is made of eight major glands, which are groups of cells that produce and secrete chemicals. A gland selects and removes materials from the blood, processes them, and secretes the finished chemical product for use somewhere in the body. Almost every organ and cell in the body is affected by the endocrine system.

A group of glands that signal each other in sequence are usually referred to as an axis. One example is the hypothalamic-pituitary-adrenal axis, which coordinates interactions among the hypothalamus, the pituitary gland and the adrenal, also called "suprarenal" glands, which are small, conical organs on top of the kidneys.

The endocrine system sends signals throughout the body, much like the nervous system, but unlike the immediate responses triggered by the nervous system, the effects can take a few hours or weeks.

Hormones released from endocrine tissue into the bloodstream where they travel to target tissue to elicit a response.

Endocrine glands are vascular and generally do not have ducts, using intracellular vacuoles, or granules, to store hormones. They differ from, exocrine glands — salivary glands, sweat glands and glands within the gastrointestinal tract — which have ducts or a hollow lumen.

The endocrine system gets some help from organs such as the kidney, liver, heart and gonads, which have secondary endocrine functions. The kidney, for example, secretes hormones such as erythropoietin and renin.

Diseases of the endocrine system

Hormone levels that are too high or too low are an indication of a problem with the endocrine system. Hormone diseases also occur if your body does not respond to hormones in the appropriate ways. Stress, infection and changes the blood's fluid and electrolyte balance can also influence hormone levels.

The most common endocrine disease in the United States is diabetes, a condition in which the body does not properly process glucose. This is due to the lack of insulin or, if the body is producing insulin, it is not working effectively.

Hormone imbalances can have a significant impact on the reproductive systems, particularly in women. Endocrinologists treat patients with fertility issues and also assess and treat patients with health concerns surrounding menstruation and menopause.


Hypothyroidism occurs when the thyroid gland does not produce enough thyroid hormone to meet the body’s needs. Insufficient thyroid hormone can cause many of the body’s functions to slow or shut down completely.

Thyroid cancer begins in the thyroid gland and starts when the cells in the thyroid begin to change, grow uncontrollably and eventually form a tumor.

Hypoglycemia, also called low blood glucose or low blood sugar, occurs when blood glucose drops below normal levels. This typically happens as a result of treatment for diabetes when too much insulin is taken. While it can occur in people not undergoing treatment for diabetes, it is fairly rare.

A metabolic disorder occurs when there is an imbalance of substances needed to keep the body functioning — hormone levels may be too high or low. Metabolic disorder when some organs, such as your liver or pancreas, become diseased or do not function normally. Diabetes is an example.

The bones can be impacted by hormones. Osteoporosis and osteomalacia (rickets), which causes bones to soften, come under the guise of endocrinologists.

Infographic: The human body's endocrine system.

What is an endocrinologist?

After completing four years of medical school, endocrinologists finish four years of medical school and then spend three or four years in an internship and residency program. These specialty programs cover internal medicine, pediatrics, or obstetrics and gynecology.

They spend two or three more years learning how to diagnose and treat hormone conditions. Overall, an endocrinologist's training will take more than 10 years. They are certified by the American Board of Internal Medicine. Endocrinologists typically specialize in one or two areas of endocrinology, such as diabetes or infertility.

Some milestones in the study of the endocrine system:

200 B.C.: The Chinese began isolating sex and pituitary hormones from human urine and using them for medicinal purposes

1025: In medieval Persia, Avicenna (980-1037) provided a detailed account on diabetes mellitus in "The Canon of Medicine" (c. 1025), described the abnormal appetite, the collapse of sexual functions and the sweet taste of diabetic urine.

1835: Irish doctor Robert James Graves described a case of goiter with exophthalmos. The thyroid condition Graves' disease was later named after him.

1902: William Bayliss and Ernest Starling performed an experiment in which they observed that acid instilled into the duodenum caused the pancreas to begin secretion, even after they had removed all nervous connections between the two.

1889: Joseph von Mering and Oskar Minkowski observed that surgically removing the pancreas resulted in an increase of blood sugar, followed by a coma and eventual death.

1921: Otto Loewi in 1921 discovered neurohormones by incubating a frog's heart in a saline bath.

1922: Leonard Thompson, a 14-year-old boy, was the first person with diabetes to receive insulin. Drug maker Eli Lilly soon started mass production of insulin.


Facts, Functions & Diseases:

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