Can Horses Get Grave's Disease/Bilateral Goiter?

Q:Have you ever heard of a bilateral goiter (both the left and right sides of the thyroid gland are enlarged) in a horse? I have a 7-year-old Tennessee Walking Horse gelding. He has had a goiter all his life that has not caused any problems. After coming home from being away for a week, I noticed a second lump, and possible goiter, on his neck. My veterinarian checked the lump and said he has never seen or heard of a bilateral goiter. He is looking into it, and I thought I would research it also.

My boy is showing classic "human Grave's disease" signs, not that it has ever been seen in a horse.

Alycia Partei, Reno, Nev.

A: You indicated that this gelding is exhibiting clinical signs that are typical for Grave's disease in people. Grave's disease is an autoimmune condition in which the patient's thyroid glands are overactive and enlarged. Thyroid overactivity is referred to as hyperthyroidism and has been rarely reported in old horses (older than 20 years) affected with thyroid cancer. Grave's disease has never been reported in horses and hyperthyroidism has not been reported in such a young horse. Although you did not describe the observed symptoms for this horse, hyperthyroidism in old horses tends to be associated with excitability, sweating, weight loss, increased appetite, and hair coat abnormalities.

As you noted, thyroid gland enlargement in adult horses is most commonly asymmetrical and benign (without clinical signs). Enlargement of either the left or the right thyroid is a very common finding in mature horses and is usually diagnosed as a benign thyroid adenoma (tumor). Less commonly, one-sided thyroid enlargement (bilateral in some cases) might be diagnosed as a developmental cyst (even in mature horses) or, rarely, cancer. Dietary factors might cause enlargement of both the left and right sides of the thyroid. Dietary factors that might be considered include abnormal levels of dietary iodine (is the horse's ration being supplemented with a kelp-based product?); excessive dietary calcium; consumption of Brassica plants (such as broccoli, cabbage, cauliflower, and mustard); white clover; rapeseed; linseed; or food contaminated by sewage.

In this case it is logical to evaluate the suspected goiters (left and right) to ensure that both are indeed thyroid gland enlargements (other local enlargements, such as lymph nodes, might masquerade as goiter). Veterinary examination by ultrasonography could be helpful to characterize the nature of the abnormal structures and to help differentiate between different causes of thyroid enlargement.

If your veterinarian suspects the clinical signs (similar to Grave's disease) could be resulting from thyroid disease, some readily available thyroid hormone blood tests could be considered in this case. Certainly, it should be straightforward to rule in or rule out whether this patient is affected with hyperthyroidism. If necessary, biopsy of one or both enlargements might be needed to characterize the reason for thyroid enlargement in this horse.

About the Author

Phillip Johnson, BVSc (Hon), MS, Dipl. ACVIM

Philip J. Johnson, BVSc (Hons), MS, Dipl. ACVIM, Dipl. ECEIM, MRCVS is Professor of Equine Medicine and Surgery at the University of Missouri College of Veterinary Medicine

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