A study completed by Dr. Janice E. Kritchevsky at Purdue University College of Veterinary Medicine shows that the use of thyroxine as a supplement in racehorses can be detrimental to performance and cause cardiac arrhythmias.
Kritchevsky noted that thyroid issues in horses are rare; bloodwork that shows thyroid hormone concentrations above the norm don't necessarily indicate a thyroid issue, but can occur after training stress, ingestion of a high-grain diet, trailering or illness. Supplementing thyroid medication to a horse that is fighting a cold can compromise the horse's natural response to infection, Kritchevsky said.
Horses with insulin resistance or Equine Metabolic Syndrome can benefit from thyroid hormone supplementation. It's recommended that any horse suspected to have thyroid issues should have bloodwork taken more than once to determine if a thyroid condition is truly the cause.
Owners may see a dull horse become more alert once thyroxine is administered, making them believe that the thyroid supplement is working. Kritchevsky used racing Standardbreds in a study and found that the horses became more alert and difficult to handle after the administration of the thyroid medications, but the horses also fatigued more easily.
According to the study, the drug is not performance-enhancing. In fact, four of the six horses developed cardiac arrhythmias while on the medication and one horse developed atrial fibrillation, which limits performance and can be career ending.
The study was primarily funded by Equine Guelph and was completed after horses on Ontario tracks were routinely testing positive for elevated blood concentrations of thyroxine post-race. Some officials believe thyroxine should be regulated; additional research may include developing a test that can indicate a true high thyroid.
Read more at Equine Guelph.
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