Pergolide mesylate administration does not decrease blood thyroid hormone concentrations and thyroid function in horses.

Objective: Pergolide, labeled for the treatment of pituitary pars intermedia dysfunction, is 90% protein bound and might cause a decrease in blood thyroid hormone concentrations by displacing them from binding proteins. This study investigated the effect of pergolide on the equine thyroid function.

Methods: This was an analytic, observational, cohort study. Six horses (17 to 24 years old and 530 to 599 kg) received 1 mg of pergolide mesylate orally once a day from days 1 to 6. Total T4 (tT4) was measured daily from day 0 to 11 (before, during, and after pergolide treatment). Thyrotropin-releasing hormone (TRH) stimulation tests were conducted on days 0 and 6. Total T3 (tT3), tT4, and free T4 by equilibrium dialysis (fT4) were measured at baseline and 2 hours (tT3) and 4 hours (tT3, T4, and fT4) after TRH administration. The effect of pergolide on thyroid hormone concentration was determined by ANOVA, with P < .05 considered significant.

Results: No effect of pergolide was detected on tT4 during and after treatment. Administration of TRH resulted in significant increases in tT3 (effect size: +165.8 ng/dL [95% CI, 109.4 to 222.2 ng/dL]), tT4 (+1.162 µg/dL [95% CI, 0.7135 to 1.610 µg/dL]), and fT4 (+1.195 µg/dL [95% CI, 0.7195 to 1.670 µg/dL]). There was, however, no significant effect of pergolide on any thyroid hormone concentration.

Conclusions: Pergolide has no detected effect on blood thyroid hormone concentrations and equine thyroid function.

Clinical relevance: Protein-bound agents do not necessarily affect thyroid hormone concentrations. In horses with pituitary pars intermedia dysfunction, low thyroid hormone concentrations are likely due to the condition itself, not pergolide treatment.