Diagnosing Hypothyroidism in Horses

The horse’s endocrine system produces hormones that are distributed throughout the body by the blood. Complex cycles regulate hormone activity, and many hormones affect the actions of others.

Veterinarians are often called upon to perform endocrine testing. The most important warning that a veterinarian can give the horse owner is to be prepared for some abnormal results, even in normal horses, when performing endocrine testing. Very often, the concentration of one or more hormones will be outside the normal range. The difficulty in interpreting endocrine tests lies in determining whether abnormalities are significant and if some other factor (insulin resistance, drug administration, etc.) is influencing the hormone. The “big picture” of the horse’s laboratory results, clinical signs, history, and other co-existing conditions determines which, if any, endocrine abnormalities need to be addressed.

Hypothyroidism is often diagnosed but uncommonly confirmed with definitive testing. The condition has been associated with reduced fertility, obesity, cold intolerance, and poor coat. In contrast to many of the clinical signs regularly attributed to hypothyroidism, experimental horses that have had their thyroid glands removed had decreased feed intake and decreased weight gain.

Several commonly administered drugs, such as glucocorticoid steroids, phenylbutazone, and trimethoprim-sulfonamide antibiotics, can decrease thyroid hormone concentrations. Horses that are ill for other reasons can also have decreased thyroid hormones (nonthyroidal illness or sick euthyroid syndrome). A study of 329 broodmares in central Kentucky reported that thyroid hormone concentrations and thyroid hormone supplementation were not significantly associated with the mare becoming pregnant.

Diagnostic tests for thyroid hormones include some or all of the following:

T₄ (thyroxine or tetraiodothyronine) and T₃ (triiodothyronine) are the most commonly measured thyroid hormones. All T₄ circulating in the blood has been released from the thyroid gland. Most T₃ is produced when other tissues in the body remove iodine from T₄. Both T₃ and T₄ are very highly bound to proteins in the blood, and only the free (unbound) hormone has any active effects. The biologic activity of T₃ is much higher than T₄.

Free T₃ (fT₃) is the more potent and active thyroid hormone in circulation. Measurement of fT₃, total T₃, and T₄ can give a clearer picture of true thyroid status if other factors, such as drug administration, are affecting total T₃ and T₄.

The thyroid releasing hormone (TRH) stimulation test is the only test that can evaluate function of the entire thyroid hormone axis from the hypothalamus in the brain, pituitary gland, and to the thyroid gland. Unfortunately, the test is infrequently performed and TRH produced specifically for equine diagnostic testing is unavailable. However, TRH can be purchased from chemical suppliers. The advantage of the TRH stimulation test is that it shows how well the pituitary and thyroid glands respond to the hormonal signal to actually produce thyroid hormones. In this test, thyroid hormones are measured before and 2 and 4 hours after injection of TRH. Normal horses will respond with at least double T₃ and T₄ 2-4 hours after TRH injection.

Younger horses, especially foals, have higher thyroid hormone concentrations than adult horses. Male horses and pregnant mares may have higher thyroid hormones, but some studies found no difference. Thyroid hormone concentrations are elevated during cold weather and decreased during hot. Thyroid hormones tend to be highest during the day and lowest at night and in the early morning. Strenuous exercise and dietary levels of energy, protein, zinc, and copper also affect thyroid hormone concentrations.

Thyroid hormones contain iodine, and either iodine deficiency or excessive supplementation during pregnancy can result in foals born with goiter, characterized by an enlarged thyroid gland. Mares should have access to adequate trace mineral salt that contains iodine during pregnancy. Supplements containing kelp or seaweeds are often implicated in cases of iodine excess. Seaweed-derived supplements should be fed only as directed by the manufacturer.