Babretta Breuhaus, DVM, PhD, associate professor of equine medicine in the department of clinical sciences at North Carolina State University, provided a review of thyroid function and dysfunction at the 50th annual American Association of Equine Practitioners (AAEP) Convention in Denver, Colo., Dec. 4-8, 2004. Hypothyroidism (underactivity of the thyroid gland) often is a mis-diagnosed malady, she said, and horses with clinical signs of hypothyroidism often have normal thyroid function. Thyroid dysfunction is not well understood, and the "prevalence of true hypothyroidism in horses is unknown," she added.

Quite often, she said, horses suffering from laminitis, obesity, and poor fertility are administered thyroid hormone supplementation. But in many of these cases proper documentation or accurate diagnosis of hypothyroidism is non-existent. As a result, owners often spend a good deal of money on therapy that does no good and can actually do harm to the horse, Breuhaus told the group.

She then discussed normal thyroid function as well as hyperthyroidism (overactivity of the thyroid gland) and hypothyroidism. Hyperthyroidism in horses, she said, is rare. Hypothyroidism might also be quite rare, but often becomes the diagnosis when a horse is overweight with a cresty neck, has reduced exercise tolerance, or is predisposed to suffer from mild, recurrent bouts of laminitis.

"Horses fitting this description have traditionally been supplemented with thyroid hormone medication, and some clinicians continue to do so at this time with or without evaluating the status of thyroid function first," she reported. "However, currently there is no good documentation that thyroid hormone supplementation in these horses results in weight loss, reduction of fat deposits, or decrease in the frequency of bouts with laminitis. Also, more recent information suggests that the clinical signs in these horses are more likely associated with equine Cushing's disease or a newly described condition that has been termed equine metabolic syndrome. Before being placed on thyroid hormone supplementation, overweight horses with fat, cresty necks should undergo diagnostic testing for hypothyroidism, equine Cushing's disease, and equine metabolic syndrome. Subsequent treatment should be based on results of these tests