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The Thyroid Gland: The Horse's PowerhouseBy Kentucky Equine Research Staff · June 27, 2017

The thyroid gland plays an important role in many body systems, including heat production, growth, and metabolism. When the gland functions poorly, virtually every system in the body suffers, making the thyroid gland a proverbial powerhouse. Consider these five points about the horse’s thyroid gland:

  • The main hormones produced by the thyroid gland, abbreviated T4 and T3, circulate through the bloodstream, affecting how those body systems function.
  • When the thyroid gland does not function optimally, only low levels of T4 circulate through the body. This condition is called hypothyroidism.
  • In human medicine, classic signs of hypothyroidism include lethargy, cold intolerance, and weight gain.
  • Based on those signs observed in hypothyroid humans, some horse owners believe their horses may have hypothyroidism if their horses have weight gain despite being fed little and accumulation of fat in the crest of the neck and over the rump.
  • Other problems blamed on the thyroid gland include infertility in mares, anhidrosis, and exertional rhabdomyolysis (tying-up).

After years of research, many signs of presumptive hypothyroidism in horses are now attributed to insulin dysregulation or equine metabolic syndrome (EMS). In fact, true equine hypothyroidism appears to be quite uncommon, yet supplementation with levothyroxine sodium, a synthetic form of T4, occurs commonly throughout the equine industry to help “fix” energy levels, poor coats, and even painful feet.

When hypothyroidism is suspected, your veterinarian can collect a blood sample and have it analyzed for not only T4 and T3 but also thyroid-releasing hormones and “free” hormone levels. One of the most common and simple tests measures T4 levels in the blood. Again, many tests are available to choose from, but one research group* warns that differences among those T4 tests affect the results. For example, some tests are adapted from those used to test human samples, which may not reflect the actual levels of thyroid hormone in horses.

“There are several morals to this story,” noted Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research (KER).

“First, hypothyroidism is rare and may appear similar to, or mistaken for, other hormonal abnormalities. Second, testing may not definitely diagnose the condition. And third, all horses deserve a full physical examination whenever an owner suspects something isn’t right. Consulting with your veterinarian will avoid unnecessary supplements and more quickly initiate appropriate treatment and management changes,” Crandell added.

If the primary issue is a lackluster hair coat or lethargy, consider a nutritional consultation before jumping on the thyroid train. KER offers Bio•Bloom PS to support hoof and coat health. (Choose Bio-Bloom in Australia.) Further, an antioxidant supplement such as Nano•E may help recovery following exercise.

“Be certain to select quality nutritional supplements and appreciate that certain supplements and medications, including levothyroxine, are controlled substances in some competitive sports,” Crandell reminded.

*Tangyuenyong, S., Y. Nambo, K. Nagaoka, et al. Sensitive radioimmunoassay of total thyroxine (T4) in horses using a simple extraction method. Journal of Veterinary Medical Science. In press.