Anhidrosis: Altered Thyroid Function a Potential Contributor

Despite the fact that 20% of racehorses in the southern United States are affected by anhidrosis--the inability or decreased ability to sweat--the underlying causes of this medical condition remain unknown.

Sweating is the primary means by which exercising horses dissipate heat. Anhidrotic horses are typically exercise intolerant, have an increased body temperature, and recover from exercise more slowly than their sweaty counterparts.

Some suggest hypothyroidism, the decreased production of thyroid hormones, might be a mechanism for anhidrosis. This theory came about after one study reported an improvement in clinical signs in anhidrotic horses supplemented with iodine (iodinated casein).

To determine if thyroid function was indeed altered in ahidrotic horses, Babetta Breuhaus, DVM, PhD, Dipl. ACVIM, from the department of clinical sciences at North Carolina State University's College of Veterinary Medicine, evaluated the thyroid function in 10 client-owned horses with clinical signs consistent with anhidrosis and 10 normal horses (controls).

Breuhaus hypothesized that all horses included in this study would have normal thyroid function. But she found anhidrotic horses had a significantly different thyroid stimulating hormone (TSH) response to thyroid releasing hormone (TRH) than did control horses, particularly in the winter.

It should be noted, however, that resting concentrations of thyroid hormones and TSH as well as the thyroid hormone responses to TRH were not different between the two groups of horses.

In the published report, the author cautioned readers that the biological relevance of these findings is not clear at this point and further research is necessary.

Breuhaus suggested that the take-home messages are:
  • Thyroid function in anhidrotic horses was mostly normal;
  • The abnormal TSH response to TRH should be not be overanalyzed; and
  • If horses are supplemented with iodinated casein or thyroid hormones and subsequently show an improvement in their ability to sweat, it is not proof that hypothyroidism causes anhidrosis.

The study, "Thyroid function in anhidrotic horses," was published in early 2009 (Volume 23) in the Journal of Veterinary Internal Medicine.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.