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Introduction to Hormonal Disorders of Horses

By Deborah S. Greco, DVM, PhD, DACVIM, Nestle Purina PetCare ; Robert J. Kemppainen, DVM, PhD, Department of Anatomy, Physiology & Pharmacology, College of Veterinary Medicine, Auburn University

Hormones are chemical messengers that have many different functions. The effects of hormones in the body are wide-ranging and varied. Some familiar examples of hormones include insulin, which is important in the development of diabetes, and estrogen and progesterone, which are involved in the female reproductive cycle.

The Endocrine System

The endocrine system consists of a group of tissues that release hormones into the bloodstream for travel to other parts of the body. Most endocrine tissues are glands (such as the thyroid gland) that release hormones directly into small blood vessels within and around the tissue. Several important hormones are released from tissues other than glands, such as the heart, kidney, and liver. Some hormones act only on a single tissue, while others have effects on virtually every cell in the body. Hormones are present in the blood in very small quantities, so laboratory tests done to measure hormone levels must be very sensitive (See table: Major Hormones).

The major endocrine glands in the horse

The pituitary gland is located near the center and bottom of the brain. It produces a number of critical hormones that control many parts of the body, including several other endocrine glands. For this reason, it is sometimes called a “master gland.” Because large numbers of hormones are produced by the pituitary, a variety of different conditions can be caused by pituitary disease or tumors. The specific illness and signs depend on the cause and the area of the pituitary gland that is affected.

The adrenal glands are located just in front of the kidneys. The adrenal gland has 2 parts—the cortex and the medulla. The adrenal cortex consists of 3 layers, each of which produces a different set of steroid hormones. The outer layer produces the mineralocorticoids, which help to control the body’s balance of sodium and potassium salts. The middle layer produces glucocorticoids, which are involved in metabolizing nutrients as well as in reducing inflammation. The inner layer produces sex hormones such as estrogen and progesterone. The adrenal medulla plays an important role in response to stress or low blood sugar (glucose). It releases epinephrine (sometimes called adrenaline) and norepinephrine, both of which increase heart output, blood pressure, and blood glucose, and slow digestion.

The pancreas is composed of several types of cells that have distinct functions involved in the production of hormones and digestive enzymes. The islets of Langerhans in the pancreas consist of 3 types of cells, each of which produces a different hormone. Most of the cells, which are called beta cells, produce insulin. Insulin affects, either directly or indirectly, the function of every organ in the body, particularly the liver, fat cells, and muscle. In general, insulin increases the transfer of glucose and other compounds into body cells. It also decreases the rate of fat, protein, and carbohydrate breakdown.

The other 2 cell types in the islets of Langerhans produce the hormones glucagon and somatostatin. Insulin and glucagon work together to keep the concentration of glucose in the blood and other body fluids within a relatively narrow range. Glucagon controls glucose release from the liver, and insulin controls glucose transport into numerous body tissues.

The thyroid gland is a 2-lobed gland in the neck. It produces the iodine-containing hormones, T3 and T4, which affect many processes in the body. In general, the thyroid hormones regulate metabolic rate, or the speed at which body processes “run.” Thyroid hormones act on many different cellular processes. Some of their actions occur within minutes to hours, while others take several hours or longer. Thyroid hormones in normal quantities work along with other hormones, such as growth hormone and insulin, to build tissues. However, when they are secreted in excess, they can contribute to breakdown of proteins and tissues.

The parathyroid glands, which extend down the sides of the neck, help to regulate the body’s levels of calcium and phosphorus in the blood. The way the body processes calcium and phosphate, the function of vitamin D (which acts more like a hormone than a vitamin), and the formation of bone are all tied together into a system that involves 2 other hormones—parathyroid hormone and calcitonin—that are secreted by the parathyroid glands.

Development of Endocrine System Disease

The body monitors and adjusts the level of each hormone by using a feedback system specifically for that hormone. Hormones function to keep factors such as temperature and blood sugar (glucose) levels within certain ranges. Sometimes, pairs of hormones with opposite functions work together to keep body functions in balance.

Endocrine system diseases can develop when too much or not enough hormone is produced, or when normal pathways for hormones to be used and removed are disrupted. Signs can develop because of a problem in the tissues that are the source of the hormone, or because of a problem in another part of the body that is affecting the secretion or action of a particular hormone.

A tumor or other abnormal tissue in an endocrine gland often causes it to produce too much hormone. When an endocrine gland is destroyed, not enough hormone is produced. Diseases caused by overproduction or excess of a hormone often begin with the prefix hyper-. For example, in hyperthyroidism, the thyroid gland produces too much thyroid hormone. Diseases caused by a lack or deficiency of a hormone often begin with the prefix hypo-. For example, in hypothyroidism, the thyroid gland does not produce enough thyroid hormone.

In many cases, the abnormal gland not only overproduces hormone, it also does not respond normally to feedback signals. This causes hormone to be released in situations in which its levels would normally be reduced. Sometimes, the over-production is caused by stimulation from another part of the body. Occasionally, a tumor outside the endocrine system can produce a substance similar to a hormone, causing the body to respond as though that hormone were being produced.

Diseases caused by not enough hormone secretion can also have multiple causes. Endocrine tissue can be destroyed by an autoimmune process, in which the body incorrectly identifies some of its own tissue as foreign and destroys the tissue cells. In early stages of tissue loss, the body may compensate by producing additional hormone from the remaining tissue. In these cases, signs of disease may be delayed until the tissue has been destroyed completely.

Disorders resulting in signs of reduced endocrine activity may also develop because tissues distant from the hormone source are disrupted. This can occur when the function of one hormone is to stimulate the production of a second hormone. For example, the pituitary gland secretes a hormone that stimulates the thyroid gland to secrete thyroid hormones. If the levels of the thyroid-stimulating hormone from the pituitary gland are abnormally low, the levels of thyroid hormones will also be low even if the thyroid gland is healthy. Another potential cause for reduced endocrine function is tissue loss caused by tumors that do not produce hormones themselves but compress or destroy the nearby endocrine gland.

Endocrine diseases and related conditions also result from changes in the response of tissues targeted by a hormone. An important example is type 2 diabetes mellitus, in which the body produces insulin but the cells no longer respond to it. This condition is often associated with obesity.

Treatment of Endocrine System Disease

Endocrine diseases caused by the presence of too much hormone may be treated surgically (tumor removal), by radiotherapy (such as the use of radioactive iodine to destroy an overactive thyroid gland), or with medication. Syndromes of hormone deficiency are often successfully treated by replacing the missing hormone, such as insulin injections to treat diabetes mellitus. Steroid and thyroid hormone replacements can usually be given by mouth.

Animals taking hormone replacement treatment must be monitored for adverse effects and periodically retested to make sure the dosage is correct. In some cases, such as after surgical removal of an endocrine tumor, the diseased gland will recover and hormone replacement will no longer be needed. However, most of the time, lifelong treatment is required.

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