10 Endocrine System Fun Facts

This system regulates metabolism, growth, and sexual development

The endocrine system acts as a chemical messaging system between body parts.
The endocrine system acts as a chemical messaging system between body parts. MedicalRF.com / Getty Images

The endocrine system, like the nervous system, is a communication network. While the nervous system uses electrical impulses to transmit signals between the brain and body, the endocrine system uses chemical messengers called hormones that travel through the circulatory system to affect target organs. So, one messenger molecule might affect many different types of cells, all over the body.

The word endocrine comes from the Greek words endon, meaning "inside" or "within" and "exocrine," from the Greek word krīnō, meaning "to separate or distinguish." The body has both an endocrine system and an exocrine system to secrete hormones. The difference between them is that the exocrine system secretes hormones through ducts that diffuse a short distance to their target, while the endocrine system is ductless, secreting hormones into the circulatory system for distribution throughout the entire organism.

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There Are More Glands Than You Think

Textbooks cite variable numbers of endocrine glands, largely because many cells groups can secrete hormones. The primary endocrine system glands are:

However, other groups of cells may secrete hormones, including the placenta (estrogen and progesterone) and stomach (ghrelin). Older sources may cite the thymus as a member of the endocrine system, but it is excluded from modern texts because it doesn't actually secrete any hormones.

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Endocrinology Has Been Practiced Over 2,000 Years

The medical and scientific study of the endocrine system is called endocrinology. Even though ancient healers had no way to understand the function of endocrine glands, Chinese healers in 200 B.C. used the compound saponin from seeds and the mineral gypsum to extract pituitary and sex hormones from human urine to make medicine. Endocrinology wasn't recognized as a science in its modern form until the nineteenth century.

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Hormones Weren't Discovered Until the 20th Century

While Chinese healers extracted and used hormones for centuries, the chemical nature of those hormones remained elusive. In the 1800s, scientists knew that some form of chemical messaging occurred between organs. Finally, in 1902, English physiologists Ernest Starling and William Bayliss coined the word "hormones" to describe pancreatic secretions.

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Osteoporosis Is an Endocrine Disorder

Osteoporosis produces degeneration of the skeleton.
Osteoporosis produces degeneration of the skeleton. PASIEKA / Getty Images

Osteoporosis is a disease in which bone becomes less dense and more susceptible to fractures. According to the Centers for Disease Control and Prevention, osteoporosis affects nearly one in 10 adults over the age of 50. Although osteoporosis affects the bones, it's actually an endocrine disease. In women, low estrogen levels are the most common underlying cause. Hyperthyroidism can also cause secondary osteoporosis.

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Ancient Physicians Tasted Urine to Diagnose Diabetes

Urine test strips are more sensitive test for diabetes than tasting urine (and less gross).
Urine test strips are more sensitive test for diabetes than tasting urine (and less gross). Peter Dazeley / Getty Images

The most common endocrine disorder is diabetes, which affects about 8 percent of the U.S. population, according to the National Institute of Diabetes and Digestive and Kidney Diseases. Diabetes occurs when the pancreas doesn't produce enough insulin.

In conventional medicine, diabetes is diagnosed using urine and blood tests, but doctors have been able to identify it for centuries. The Greek physician Hippocrates (c. 460 to 377 B.C.), however, diagnosed diabetes by tasting his patient's urine. Because insulin controls blood sugar, a person with uncontrolled diabetes leaks sugar into urine, causing it to taste sweet.

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A Gland Can Have Both Endocrine and Exocrine Functions

The pancreas contains both endocrine and exocrine glands.
The pancreas contains both endocrine and exocrine glands. PIXOLOGICSTUDIO / Getty Images

Endocrine glands are clusters of cells, rather than entire organs. The pancreas is an organ that contains both endocrine and exocrine tissue. Insulin and glucagon are two endocrine hormones released by the pancreas. Pancreatic juice, secreted by a duct into the small intestine, is an exocrine product.

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The Endocrine System Responds to Stress

Physical and emotional stress cause the endocrine system to produce more hormones. For example, more adrenaline and growth hormone are released, to aid in physical exertion and speed the metabolism. However, the system is designed to improve short term survival. Prolonged stress causes endocrine disorders, including obesity and the autoimmune thyroid disorder Graves' disease.

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A Scientist Tested Hormone Replacement Therapy on Himself

Rooster combs and crowing result from sex hormones.
Rooster combs and crowing result from sex hormones. Raatzie / Getty Images

In 1849, German physiologist Arnold Adolph Berthold demonstrated that removing and reimplanting rooster testes affected the bird's secondary sex characteristics, including comb growth, crowing, and fighting.

Endocrinologist Charles-Édouard Brown-Séquard took this idea to the next level, injecting himself with dog and guinea pig testes extracts. The 72-year-old published his results in The Lancet, saying the treatment restored his strength and vitality. While hormone replacement therapy does work, Brown-Séquard's results may have been a result of the placebo effect.

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Other Animals Have Endocrine Systems

In frogs, the thyroid gland controls development from egg to tadpole to adult.
In frogs, the thyroid gland controls development from egg to tadpole to adult. Reimar Gaertner/UIG / Getty Images

Humans and other vertebrates (e.g., cats, dogs, frogs, fish, birds, lizards) all have a hypothalamus-pituitary axis that serves as the basis for the endocrine system. Other vertebrates also have a thyroid, although it may serve a slightly different function. For example, in frogs, the thyroid regulates the transformation from a tadpole into an adult. All vertebrates have an adrenal gland, too.

Endocrine signaling isn't limited to vertebrates. All animals with a nervous system have an endocrine system.

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Plants Produce Hormones Without an Endocrine System

Hormone rooting powder tells plant tissue to grow roots.
Hormone rooting powder tells plant tissue to grow roots. Andy Crawford / Getty Images

Plants don't have an endocrine or exocrine system, but they still produce hormones to control growth, fruit ripening, repair, and metabolism. Some hormones diffuse to local tissue, like exocrine hormones. Others are transported through plant vascular tissue, much like endocrine hormones.

Endocrine System Key Takeaways

  • The endocrine system is a chemical messaging network.
  • Endocrine glands secrete hormones, which are carried by the circulatory system throughout the body.
  • The primary endocrine glands are the pituitary, hypothalamus, pineal gland, thyroid, parathyroid, adrenal, pancreas, ovary, and testis.
  • Hormones maintain homeostasis in the body. Improper function is associated with diseases, including osteoporosis, obesity, diabetes mellitus, and thyroid disease.


  • Hartenstein V (September 2006). "The neuroendocrine system of invertebrates: a developmental and evolutionary perspective". The Journal of Endocrinology. 190 (3): 555–70. doi:10.1677/joe.1.06964.
  • Marieb, Elaine (2014). Anatomy & physiology. Glenview, IL: Pearson Education, Inc. ISBN 978-0321861580.
  • Temple, Robert G (1986) The Genius of China: 3000 Years of Science, Discovery, and Invention. Simon and Schuster. ISBN-13: 978-0671620288
  • Vander, Arthur (2008). Vander's Human Physiology: the mechanisms of body function. Boston: McGraw-Hill Higher Education. pp. 345–347. ISBN 007304962X.