Low Thyroid Levels in PPID Horses May Not Warrant Treatment

Measuring low thyroid hormone levels in horses with pituitary pars intermedia dysfunction (PPID, previously known as equine Cushing’s syndrome) does not necessarily indicate the horse is truly hypothyroid and requires treatment. Instead, dynamic testing of the thyroid gland should be performed, according to Purdue University veterinary researchers.^*

Horses with low levels of the thyroid hormones T₄ and T₃ do not necessarily have a problem with their thyroid gland. Instead, other factors may affect the levels of those hormones in the bloodstream, such as age, natural hormone rhythms, diet, climate, reproductive status, and physical training.

“In addition, chronic medical conditions such as PPID may affect thyroid hormone levels, causing a condition called ‘nonthyroidal illness syndrome’ or ‘euthyroid sick.’ In the case of PPID, the thyroid hormone levels are low, but the thyroid function is normal,” explained Ashley Fowler, Ph.D., a Kentucky Equine Research nutritionist.

To complicate matters, medications that bind to proteins in the bloodstream may displace the thyroid hormones from their normal protein-bound position. This means that thyroid hormones can appear artificially lower than actual levels when measured.

According to the researchers, horses with PPID generally have lower resting thyroid hormone concentrations compared to age-matched healthy control horses. Thyroid function testing, specifically the thyrotropin-releasing hormone (TRH) test, is normal. Thus, the lower thyroid hormone concentrations could be either a result of the disease itself or pergolide, the medication of choice to treat PPID.

“Pergolide binds to proteins in the bloodstream. These researchers hypothesized that pergolide may be displacing protein-bound thyroid hormones, resulting in lowered measured thyroid hormone concentrations with no effect on thyroid function,” Fowler said.

To evaluate the effect of pergolide on thyroid hormone concentrations and thyroid function, six healthy horses free from PPID were administered 1 mg pergolide for five days. On days 0 and 6 a TRH test was performed to assess thyroid gland function, and thyroid levels were measured.

“Pergolide did not result in a decrease in thyroid hormones either during or following treatment, and thyroid gland function was normal based on the TRH test,” relayed Fowler.

Thus, the thyroid gland retains its function in healthy horses treated with pergolide. Further, pergolide does not adversely affect thyroid hormone concentrations, suggesting that displacing the thyroid hormones from proteins in circulation does not explain why some horses with PPID have low thyroid levels.

Understanding the underlying mechanisms of thyroid hormone regulation in the context of PPID is crucial for accurate diagnosis and appropriate treatment, preventing unnecessary thyroid hormone supplementation and its potential negative effects on the horse’s health.

“Not all horses with PPID have insulin dysregulation, but for those that do, the nonstructural carbohydrate concentration of the forage should be no more than 10-12%. A ration balancer containing high-quality protein provides appropriate vitamin and mineral supplementation and may be helpful for muscle maintenance,” advised Fowler.

Look for the ration balancer All-Phase. This is a highly digestible source of protein, vitamins, and minerals formulated to provide natural-source vitamin E, chelated trace minerals, B vitamins, biotin, yeast culture, and organic selenium in a low-intake format.

 ^*Jargielo, M.M., J.E. Kritchevsky, and F.-R. Bertin. 2024. Pergolide mesylate administration does not decrease blood thyroid hormone concentrations and thyroid function in horses. American Journal of Veterinary Research 27:1-5.