The Endocrine System

Approximately 13% of the questions on all three tests for certification—CBET, CRES, and CLES—relate to anatomy and physiology. Most of us are knowledgeable about the respiratory and circulatory systems and have a basic understanding of the nervous system, but not too many have much knowledge of the gastrointestinal and musculoskeletal systems, and few know much about the endocrine system.

In this brief review of the endocrine system, the various glands and problems associated with their malfunction are covered, as are the devices used in testing for abnormalities of the system.

The pituitary gland— “the master gland"— makes hormones that control other endocrine glands. It is located at the base of the brain near the nasal cavity. Nine hormones are released by the pituitary gland. Among them is growth hormone, which stimulates the growth of bones. Too much will cause gigantism, while too little causes dwarfism. The pituitary also produces prolactin, which activates milk production in women who breast feed; thyrotropin (also known as thyroid-stimulating hormone or TSH), which stimulates the thyroid gland to produce thyroid hormones; and corticotropin, which stimulates the adrenal gland to produce cortisol and other hormones. The pituitary also secretes endorphins, chemicals that act on the nervous system to reduce sensitivity to pain, and hormones that signal the ovaries and testes to make sex hormones. The skin can also be affected with darkening or lightening related to hormone levels. Even the kidneys can be induced to retain water with too much antidiuretic hormone.

The thyroid gland is located in the neck just below the Adam’s apple. The hormones released by the thyroid help maintain normal growth and metabolism. The gland also stores iodine, a critical element whose lack can cause fatigue due to hypothyroidism, brain damage, and even cretinism. A hyperactive thyroid can cause high heart and respiratory rates and increased temperature. Too little of thyroid hormone can cause physical and mental slowness, obesity, and hair loss, in addition to other less common problems.

The four parathyroid glands are to the side of the thyroid. The hormone they release controls the calcium serum levels of the body. Too much can cause kidney stones, bone fractures, and general malaise. Too little can cause tingling in the hands and feet, muscle spasms of the hands, and, rarely, seizures. Hypoparathyroidism is extremely rare.

A physician will feel the thyroid during most physical examinations. If abnormalities are felt, a nuclear scan may be ordered. (As a side note, people that had radiation treatment—x-rays of their tonsils when they were young—have higher incidents of thyroid problems than do people working in the nuclear industry. The iodine tablets that are distributed to people living near nuclear power plants help protect them from thyroid cancer in the event of a radioactive release.)

The adrenal glands are located on the kidneys. They secrete various steroid hormones and other hormones that control salt and water metabolism. They also secrete adrenaline and noradrenalin—the “fight or flight" hormones—which increase metabolic, heart, and respiratory rates for short periods of time. The adrenal glands also release more than 30 natural steroids.

Other hormones increase protein breakdown, impair the utilization of glucose, promote sodium retention and potassium loss through the kidneys, and increase blood lipids. They also stimulate the ovaries and testes.

While reading up on the endocrine system, I was surprised to learn that the placenta is considered part of the system. It stimulates the release of progesterone and acts in combination with prolactin to induce lactation.

The pancreas is located transversely behind the stomach, between the spleen and duodenum. In some texts it is listed as an organ and in others a gland. Secretions of the pancreas include enzymes that break down proteins, fats, and carbohydrates in the duodenum. Another secretion helps reduce the acidic chyme that is passed from the stomach to the duodenum. In addition, three types of cells—alpha, beta, and delta—work to regulate glucose. The alpha cell secretion, glucagon, releases glucose into the blood stream. The beta cell secretion, insulin, promotes the uptake, storage, and use of glucose by the liver, muscles, and fat tissues. The delta cells secretion, somatostatin, regulates the alpha and beta secretions. Problems with the pancreas can lead to diabetes mellitus and digestive disorders.

The thymus is located under the upper sternum. Its function is not well understood other than it is a major part of the immune system. During the last stages of fetal life and the early neonatal period the thymus “preprocesses" stem cells, sensitizing them to mature into lymphocytes that travel through the blood into the lymphoid tissue where they become an important part of a persons immune system. The thymus reaches its maximum development during puberty and slowly decreases its influence over a person’s life span. It is thought to be responsible for rejections of transplanted organs and plays a role in the natural resistance to cancer.

All components of the endocrine system, except the thymus, interact. This feedback system is not well understood, but a breakdown in one part of the endocrine system can lead to problems in other parts of the system and in the organs of the body.

Keeping a safe hormonal balance using artificial means is often difficult. The use of steroids or recreational drugs will also impact the endocrine system. Some newer research also indicates that various food supplements used to enhance performance in sports or weight loss impact the endocrine system.

Hormone levels can be confirmed with blood tests on most automated blood chemistry systems now in use. Urine testing is also used. Blood glucose levels do not show the hormone but rather the results of an imbalance or absence of certain hormones. Because of the high number of hormones and natural steroids that the endocrine system releases, tests are not always easy or accurate. Repeat testing is very common.

The use of CT images for pituitary abnormalities is common. It is often used as a “rule out" for suspected tumors on the pituitary. MRI scans are becoming more common for suspected endocrine problems.

The endocrine system may be the key to longer and better quality of life but it is a specialty that is slow to develop.

David Harrington is director of staff development and training at Technology in Medicine Inc in Holliston, Mass.