Equine Endocrine Disorders

Endocrine disorders are diseases that affect the endocrine glands, the glands that produce hormones. Hormones are chemicals that, once released into the bloodstream, have profound effects on many bodily processes. Horses are subject to many of the same hormonal imbalances as humans. The most common equine hormone disorders affect the adrenal glands, the thyroid gland, and the pancreas. Although serious and sometimes life-threatening, most endocrine disorders, once diagnosed through blood screening, can be successfully treated with hormone replacements or other therapy.

Diseases of the Adrenal Glands

There are two major diseases of the adrenal glands in horses, one resulting from an excess of hormone secretion and one resulting from insufficient hormone production.

Cushing's Disease

Cushing's disease, also called hyperadrenocorticism, is the most prevalent equine hormone imbalance. In this disorder, the adrenal glands overproduce the hormone cortisol, usually because of damage to the pituitary gland, rather than due to tumors.

Horses most at risk for Cushing's disease are mares or geldings over 15 years of age. Symptoms of Cushing's disease include the following:

  • Abnormal growth of long or heavy hair (hypertrichosis)
  • Excessive thirst and urination
  • Increased appetite and weight gain
  • Enlarged abdominal girth
  • Bulging eyes

A systemic symptom of equine Cushing's disease is a weakened immune system that makes the animals prone to dental, respiratory, and parasitic infections. Fortunately, with proper medication and medical management, most horses with Cushing's disease respond well to treatment.

Addison's Disease

Addison's disease, also known as hypoadrenocorticism, is much less common in horses than Cushing's disease. Addison's results from a deficiency of adrenal hormones. While the underlying cause is unknown, Addison's disease is believed to be an autoimmune disorder. The adrenal gland may also be damaged because of a cancer elsewhere in the body.

In Addison's disease, the decrease in hormone secretion leads to disturbed levels of vital minerals in the blood, including potassium, sodium and chloride. The buildup of potassium in the blood is especially serious and may cause an abnormal heart rate.

Symptoms of equine Addison's disease include vomiting, diarrhea, loss of appetite, and dehydration. Horses suffering with this condition may suffer severe weight loss and become deconditioned. In extreme cases, shock and kidney failure may occur suddenly.

If diagnosed early, Addison's disease may improve with administration of replacement hormones. An adrenal crisis, however, requires urgent medical care and the immediate administration of body fluids and supplemental minerals.

Diseases of the Pancreas

The one major endocrine disorder affecting the pancreas is diabetes mellitus. It is relatively uncommon in horses.

Diabetes Mellitus

Diabetes mellitus, known often simply as diabetes, results from a deficiency of insulin production (type 1 diabetes) or a resistance to insulin (type 2 diabetes). Type 1 diabetes is rarely found in horses, but type 2 is more common, frequently developing in horses that already have Cushing's disease. Both types of diabetes are treated with the administration of insulin and are manageable, though chronic, illnesses.

Diseases of the Thyroid Gland

There are several disorders of the thyroid gland that horses may develop. Depending on how hormone secretion is affected by the particular disorder, the symptoms are variable.


While mature horses rarely develop hypothyroidism, foals may be born with the condition if the broodmare ate a diet with an inappropriate amount of iodine during gestation. Most often, foals born to mothers who ingested too much or too little iodine during their pregnancies have congenital abnormalities of the thyroid gland and the musculoskeletal system.

Symptoms of hypothyroidism occur because of insufficient production of thyroid hormone that slows the metabolism. These symptoms may include:

  • Lethargy
  • Lack of interest in, or ability to, exercise
  • Weight gain without overeating

Once hypothyroidism is diagnosed, synthetic thyroid hormone is administered. Although the treatment is usually necessary throughout the horse's lifetime, horses can regain complete health with proper medication.


A goiter is a benign enlargement of the thyroid gland, observed most frequently in horses with diets deficient in iodine. In most of the world, iodized salt is routinely added to equine feed so this deficiency is no longer common.

Strangely, too much iodine in the diet can also be a reason for goiter development, though this is rare. It is also possible for a horse to inherit a propensity for deficient production of thyroid hormone that may result in a goiter.

In most horses with detectable goiters, the thyroid hormone levels test as normal, but in foals there may be evidence of hypothyroidism at birth, including swollen, flabby or thickened tissue. Similarly, foals born of mares fed too much iodine during gestation, may develop large goiters and other abnormalities. Most foals born with either condition die just before, during, or after birth. In order to prevent such tragedies, it is important to monitor the thyroid hormone levels in brooding mares.

In addition to goiter resulting from too much or too little iodine, certain plants, particularly if consumed without sufficient iodine reserves in the body, can interfere with the production of thyroid hormone production. These plants include: soybeans, cabbage, kale and turnips.

Additional Resources