The most common hormonal disorders in horses affect the adrenal glands (Addison's disease and Cushing's disease), thyroid gland (hypothyroidism and goiter), and the pancreas (diabetes mellitus).

Cushing's Disease (Hypertrichosis)

Cushing's disease, also called hyperadrenocorticism, is the most common endocrine disease in horses. The signs are due primarily to chronic excess of the hormone cortisol. Increased cortisol levels may result from one of several mechanisms, such as destruction of a portion of the pituitary gland and overproduction of certain other hormones. Unlike Cushing's disease in dogs or people, the cause is not usually related to a pituitary tumor. However, pituitary tumors do occur, particularly later in the disease.

The disorder is seen in older horses (over 15 years of age) of any breed. Mares and geldings are most often affected. The most striking sign is development of an abnormally long or heavy hair coat (called hypertrichosis), which can grow up to 4 to 5 inches (10 to 12 centimeters) long, and is thick, wavy, and often matted. Other signs include excessive thirst and urination, increased appetite and weight, an enlarged abdomen, and bulging eyes. Horses with Cushing's disease tend to have a weakened immune system and may be prone to infections (such as dental disease or respiratory infections) or parasites. A diagnosis is based on a history and signs, physical examination, and appropriate blood tests.

Once a horse has been diagnosed with Cushing's disease, there are treatments available that may improve its condition and, in some cases, even return it to normal health. Pergolide and cyproheptadine are the 2 most commonly used drugs for treatment of this disorder in horses. However, treatment requires that the owner administer the drug daily and schedule regular veterinary checkups, including blood tests to monitor the horse's response. If treatment is stopped, the signs can reappear within a few weeks.

Even without medical treatment, many horses respond to careful management. This includes regular dental care, dewormings, hoof care, and an appropriate diet.

Addison's Disease

Addison's disease (hypoadrenocorticism) is caused by a deficiency of adrenal gland hormones. It is seen occasionally in horses. The cause is usually not known, but an autoimmune condition in which the body destroys some of its own tissue is likely. The adrenal gland can also be destroyed by other conditions, including cancer in other parts of the body. Secretion of aldosterone, the main mineralocorticoid hormone, is reduced, which affects the levels of potassium, sodium, and chloride in the blood. Potassium gradually builds up in the blood and, in severe cases, may cause the heart to slow down or beat irregularly (see Hormonal Disorders of Dogs: Disorders of the Adrenal Glands in Dogs).

Signs of Addison's disease include repeated episodes of vomiting and diarrhea, loss of appetite, dehydration, and a gradual loss of body condition. Weight loss is often severe. Although signs can be hard to identify while Addison's disease is developing, severe consequences, such as shock and evidence of kidney failure, can develop suddenly.

A tentative diagnosis is based on the history, signs, and certain laboratory abnormalities, such as very low levels of sodium and very high levels of potassium in the blood. The diagnosis is confirmed by specific evaluation of adrenal function. This is done by measuring the level of cortisol in the blood, treating the animal with adrenocorticotropin (a hormone that stimulates the adrenal gland in healthy animals), and then measuring the level of cortisol in the blood a second time. Affected horses have low baseline cortisol levels, and there is little response to administration of adrenocorticotropin.

An adrenal crisis is a medical emergency and requires treatment with intravenous fluids to restore levels of body fluids, salt, and sugar to normal. Hormone replacement therapy can often be started while the animal is being stabilized. Laboratory values are monitored regularly to assess the response to treatment and adjust doses if needed. For longterm treatment, replacement hormones can be given by mouth or injection. Supportive treatment and rest are indicated for horses with Addison's disease.

Diabetes Mellitus

Diabetes mellitus (often called simply diabetes) is a chronic disorder of carbohydrate metabolism caused by either a deficiency of insulin or a resistance to insulin. Diabetes caused by a deficiency of insulin (also called primary diabetes mellitus) is rare in horses; however, resistance to insulin (also called secondary diabetes mellitus) is more common and tends to develop in horses with Cushing's disease (see Hormonal Disorders of Horses: Cushing's Disease (Hypertrichosis)). A diagnosis of diabetes mellitus is based on finding high levels of sugar in the blood and urine after a period of fasting. Treatment with insulin cannot reverse the insulin resistance seen in secondary diabetes mellitus.

Hypothyroidism

In hypothyroidism, decreased levels of thyroid hormones result in a slower metabolic rate. Adult horses rarely develop hypothyroidism. However, foals may be born with hypothyroidism if the pregnant mare grazed plants that contained goiter-producing substances (see Hormonal Disorders of Horses: Goiter), or if she was fed a diet with either not enough or too much iodine. Most commonly, these foals are affected by a specific syndrome in which they have both thyroid abnormalities and multiple congenital musculoskeletal abnormalities. This syndrome has been reported most commonly in western Canada, and it may be related to feeding a high nitrate diet (for example, greenfeed) to pregnant mares.

Because a deficiency of thyroid hormone affects the function of all organ systems, signs vary. Many signs could also be caused by other diseases. Most signs are directly related to slowing of metabolism, which results in lethargy, unwillingness or inability to exercise, and weight gain without an increase in appetite.

Diagnosis requires evaluation of signs and various laboratory tests, including demonstration of low serum concentrations of thyroid hormones that do not respond to administration of thyroid-stimulating hormone. Identifying hypothyroidism can be difficult at times because other conditions in the adult horse can make it appear that thyroid hormone levels are low when they are actually normal.

Hypothyroidism is treated using replacement with synthetic thyroid hormone. The success of treatment can be measured by the amount of improvement in signs. Serum thyroid hormone concentrations are also monitored to determine whether the dosage of thyroid hormone needs adjustment. Once the dose has been stabilized, thyroid hormone levels are usually checked once or twice a year. Treatment is generally lifelong.

Goiter

A goiter is a noncancerous enlargement of the thyroid gland that develops most often when the diet does not contain enough iodine. Goiters develop in people, all domestic mammals, and birds. In addition to iodine deficiency, other major causes of goiter include goiter-causing substances, too much iodine in the diet, and inherited defects in the body's production of thyroid hormones. Many animals with goiter appear to have normal thyroid hormone levels, but signs of hypothyroidism may develop in some, especially in newborns (see Hormonal Disorders of Horses: Hypothyroidism).

Iodine Deficiency

Goiter due to iodine deficiency was common in many areas of the world before iodized salt was routinely added to animal diets. Outbreaks of goiter caused by iodine deficiency are now sporadic and affect fewer animals, but iodine deficiency is still responsible for most goiters seen in horses.

A lack of iodine reduces the ability of the thyroid to make thyroid hormone. Because thyroid hormone levels drop, the pituitary gland secretes more thyroid-stimulating hormone, which results in the thyroid gland enlarging in an effort to make more thyroid hormone. Often, the enlarged thyroid gland can make enough thyroid hormone to bring the levels into the normal range. However, foals born of mares on iodine-deficient diets are more likely to develop severe thyroid enlargement and have signs of hypothyroidism. The neck is usually enlarged, and the skin and other tissue may be thickened, flabby, and swollen. Many foals die before or soon after birth, but treatment with iodized salt may resolve the goiter and associated signs in mildly affected foals. Prevention is much more effective than treatment, and adding stabilized, iodized salt to the diet is recommended in areas known or suspected to be deficient in iodine.

Iodine Toxicity

Foals born of mares fed too much iodine may develop extreme thyroid enlargement and die before birth or soon after. Signs include general weakness, long hair, and limb abnormalities.

Goiter-causing Substances

Certain plants can produce goiter if eaten in sufficient amounts, especially when the diet does not contain enough iodine. Soybeans, cabbage, rape, kale, and turnips all contain goitrogens. Cooking or heating (and the usual processing of soybean meal) destroys the goiter-causing substance in these plants. Goiter-causing substances act by interfering with production of thyroid hormone. As in iodine deficiency, the pituitary gland responds to the lower levels of thyroid hormone by increasing its secretion of thyroid-stimulating hormone. In turn, the thyroid responds by enlarging in an attempt to produce more thyroid hormone. In adult animals the disease is usually not significant, but newborn foals can develop severe thyroid enlargement and signs of hypothyroidism.

Last full review/revision July 2011 by Deborah S. Greco, DVM, PhD, DACVIM; Robert J. Kemppainen, DVM, PhD; Mark E. Peterson, DVM, DACVIM; David Bruyette, DVM, DACVIM