Introduction: Located next to the trachea and behind the larynx, the thyroid secretes regulatory hormones. These hormones are important for functions such as maintaining proper growth and metabolic rates. For such a small gland, the thyroid gland plays a major role in mammalian bodies. It is the only gland that produces iodine. The most common thyroid problem found in horses is called goiter (an abnormal enlargement of the thyroid gland). This most often occurs in neonates and young animals. A less common thyroid problem can occur in adult horses where low levels of thyroid hormones are produced or utilized. Both problems in the neonates and adults result in hypothyroidism (low levels of thyroid hormones being produced).
Causative Agents: Hypothyroidism in newborn horses is most often caused by the mother eating excessive amounts of iodine or plants that cause goiter (goitrogenic) while with foal. When excessive iodine or a goitrogenic substance is consumed, the level of circulating thyroid hormones decreases and the thyroid gland enlarges in an attempt to produce more thyroid hormone. Plants that are most notorious for containing iodine or goitrogenic substances include soybeans, cabbage, rape, kale, and turnips.
Hypothyroidism in adult horses is most often the result of a lack of thyroid production by the thyroid. Some adult horses with hypothyroidism, however, may actually have problems with the pituitary gland or hypothalamus. The pituitary gland is a master gland controlling other glands in the body and, along with the hypothalamus, is found at the base of the brain. A healthy pituitary and hypothalamus are necessary to stimulate the thyroid to produce its hormones. When any one of the glands fails to work as it should, the others are affected.
Furthermore, it is possible that a horse’s body will not properly utilize the hormones that are produced. Utilization of these hormones can be drastically affected by stress, high energy diets, high protein diets, and diets high in zinc and copper. Recent research has revealed that the administration of glucocorticoids, phenylbutazone, deprivation of food, and consumption of endophyte-infected fescue grass can also lower plasma levels of thyroid hormone in horses that have absolutely no affliction of the thyroid gland.
Clinical Signs: In foals and young horses, growth will be stunted and thyroid deficient goiter is present. Young animals can also suffer respiratory problems and gastric ulcers. Many die at birth or shortly thereafter. If they do not die, they often sustain irreversible problems such as collapsed tarsal or carpal bones. With goiter, an afflicted horse's thyroid lobes swell to at least twice their normal size and are soft and dark red. Hair loss, thickened skin and other tissue, edema, and an enlarged neck are additional signs of goiter.
In adult animals, the common signs associated with a problem thyroid gland include sensitivity to cold, dull, coarse hair that fails to shed in warmer temperatures, depression and lethargy, high blood cholesterol concentrations, anemia, decreased rectal temperatures, and odd placement of fat on the animal.
Diagnosis: To diagnose hypothyroid problems in horses, a thyroid stimulating hormone (TSH) response test of some type is usually performed. There are many different ways of running these tests, so only one method will be outlined. A pre-TSH blood sample is taken from the horse and an injection of TSH (5 IU) is given intramuscularly. Blood samples are again taken from the horse at 1 hour and at 4 hours after the TSH administration. Certain thyroid hormones (T
3 and T4) should be at least two times normal at these times in horses with hypothyroidism. A similar test can be used in foals. These tests are often used to help confirm a possible thyroid case that has been identified simply with clinical signs and history.Treatment: For foals that have problems, some of the damage may be permanent and non-reversible. In other cases, supplementing the foal with thyroid hormone can help correct the problems. Additionally, if the cause is the consumption of iodine or plants that promote goiter, remove the mare from that environment. For adult animals, the treatments may include thyroid hormone replacement therapy. One recommended dose is 20 micrograms/kg of thyroxine given orally once a day. Regular blood testing should be performed to monitor thyroid hormone and response to treatment.