The endocrine system is the collection of glands that produce 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 to secrete, according to Health Mentor Online. In general, a gland selects and removes materials from the blood, processes them and secretes the finished chemical product for use somewhere in the body. The endocrine system affects almost every organ and cell in the body, according to the Merck Manual.
Although the hormones circulate throughout the body, each type of hormone is targeted toward certain organs and tissues, the Merck Manual notes. 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.
The thyroid also secretes a range of hormones that affect the whole body. "Thyroid hormones impact a host of vital body functions, including heart rate, skin maintenance, growth, temperature regulation, fertility and digestion," said Dr. Jerome M. Hershman, a professor of medicine at the David Geffen School of Medicine at UCLA and author of the thyroid sections of the Merck Manual.
"In this way, the thyroid gland is the body's master metabolic control center," said Cindy Samet, a chemistry professor at Dickinson College in Carlisle, Pennsylvania. "Brain, heart and kidney function, as well as body temperature, growth and muscle strength — and much more — are at the mercy of thyroid function."
Diseases of the endocrine system
Hormone levels that are too high or too low indicate 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 in the blood's fluid and electrolyte balance can also influence hormone levels, according to the National Institutes of Health.
The most common endocrine disease in the United States is diabetes, a condition in which the body does not properly process glucose, a simple sugar. This is due to the lack of insulin or, if the body is producing insulin, because the body is not working effectively, according to Dr. Jennifer Loh, chief of the department of endocrinology for Kaiser Permanente in Hawaii.
Diabetes can be linked to obesity, diet and family history, according to Dr. Alyson Myers of North Shore-LIJ Health System. "To diagnose diabetes, we do an oral glucose tolerance test with fasting."
It is also important to understand the patient's health history as well as the family history, Myers noted. Infections and medications such as blood thinners can also cause adrenal deficiencies.
Diabetes is treated with pills or insulin injections. Managing other endocrine disorders typically involves stabilizing hormone levels with medication or, if a tumor is causing an overproduction of a hormone, by removing the tumor. Treating endocrine disorders takes a very careful and personalized approach, Myers said, as adjusting the levels of one hormone can impact the balance of other hormones.
Hormone imbalances can have a significant impact on the reproductive system, particularly in women, Loh explained.
Another disorder, hypothyroidism, a parathyroid disease, occurs when the thyroid gland does not produce enough thyroid hormone to meet the body's needs. Loh noted that insufficient thyroid hormone can cause many of the body's functions to slow or shut down completely. It has an easy treatment, though. "Parathyroid disease is a curable cause of kidney stones," said Dr. Melanie Goldfarb, an endocrine surgeon and director of the Endocrine Tumor Program at Providence Saint John's Health Center in Santa Monica, California, and an assistant professor of surgery at the John Wayne Cancer Institute in Santa Monica. The damaged part of the gland is removed surgically.
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, according to Loh. Tumors — both benign and cancerous — can also disrupt the functions of the endocrine system, Myers explained. Between the years of 1975 and 2013, the cases of thyroid cancer diagnosed yearly have more than tripled, according to a 2017 study published in the Journal of the American Medical Association (JAMA). "While overdiagnosis may be an important component to this observed epidemic, it clearly does not explain the whole story," said Dr. Julie Sosa, one of the authors of the new study and the chief of endocrine surgery at Duke University in North Carolina. The American Cancer Society predicts that there will be about 53,990 new cases of thyroid cancer in 2018 and around 2,060 deaths from thyroid cancer.
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 Loh noted that the condition can occur in people not undergoing treatment for diabetes, such an occurrence is fairly rare.
What is an endocrinologist?
After completing four years of medical school, people who want to be endocrinologists then spend three or four years in an internship and residency program. These specialty programs cover internal medicine, pediatrics, or obstetrics and gynecology, according to the American Board of Internal Medicine.
Endocrinologists-in-training then 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 after the undergraduate degree. 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. These specialists treat patients with fertility issues and also assess and treat patients with health concerns surrounding menstruation and menopause, Loh noted.
Milestones in the study of the endocrine system
200 B.C.: The Chinese begin isolating sex and pituitary hormones from human urine and using them for medicinal purposes
1025: In medieval Persia, the writer Avicenna (980-1037) provides a detailed account on diabetes mellitus in "The Canon of Medicine" (c. 1025), describing the abnormal appetite, the collapse of sexual functions and the sweet taste of diabetic urine.
1835: Irish doctor Robert James Graves describes a case of goiter with bulging eyes (exophthalmos). The thyroid condition Graves' disease was later named after the doctor.
1902: William Bayliss and Ernest Starling perform an experiment in which they observe that acid instilled into the duodenum (part of the small intestine) causes the pancreas to begin secretion, even after they had removed all nervous connections between the two organs.
1889: Joseph von Mering and Oskar Minkowski observe that surgically removing the pancreas results in an increase of blood sugar, followed by a coma and eventual death.
1921: Otto Loewi in 1921 discovers neurohormones by incubating a frog's heart in a saline bath.
1922: Leonard Thompson, at age 14, is the first person with diabetes to receive insulin. Drugmaker Eli Lilly soon starts mass production of insulin.
Additional reporting by Alina Bradford, Live Science contributor.
Editor’s Note: If you’d like more information on this topic, we recommend the following book:
Systems of the human body
- Circulatory System: Facts, Function & Diseases
- Digestive System: Facts, Function & Diseases
- Immune System: Diseases, Disorders & Function
- Lymphatic System: Facts, Functions & Diseases
- Muscular System: Facts, Functions & Diseases
- Nervous System: Facts, Function & Diseases
- Reproductive System: Facts, Functions and Diseases
- Respiratory System: Facts, Function & Diseases
- Skeletal System: Facts, Function & Diseases
- Skin: Facts, Diseases & Conditions
- Urinary System: Facts, Functions & Diseases
Parts of the human body
- Bladder: Facts, Function & Disease
- Human Brain: Facts, Anatomy & Mapping Project
- Colon (Large Intestine): Facts, Function & Diseases
- Ears: Facts, Function & Disease
- Esophagus: Facts, Function & Diseases
- How the Human Eye Works
- Gallbladder: Function, Problems & Healthy Diet
- Human Heart: Anatomy, Function & Facts
- Kidneys: Facts, Function & Diseases
- Liver: Function, Failure & Disease
- Lungs: Facts, Function & Diseases
- Nose: Facts, Function & Diseases
- Pancreas: Function, Location & Diseases
- Small Intestine: Function, Length & Problems
- Spleen: Function, Location & Problems
- Stomach: Facts, Function & Diseases
- The Tongue: Facts, Function & Diseases
- Learn about endocrine diseases from the National Institutes of Health.
- The Merck Manual provides comprehensive information about the endocrine system.
- Kim Ann Zimmermann, Live Science Contributor
Kim Ann Zimmermann is a contributor to Live Science. She holds a bachelor’s degree in communications from Glassboro State College.