Repeatability of a thyrotropin‐releasing hormone stimulation test for diagnosis of pituitary pars intermedia dysfunction in mature horses

BACKGROUND: Pituitary pars intermedia dysfunction (PPID) is a common endocrinopathy of horses diagnosed with a thyrotropin‐releasing hormone (TRH) stimulation test. HYPOTHESIS/OBJECTIVES: Describe the repeatability of TRH stimulation in horses with and without PPID in winter and autumn. ANIMALS: Twenty adult horses; 6 controls and 6 with PPID tested in autumn, 8 controls and 6 with PPID tested in winter with 3 controls and 3 with PPID tested in both seasons. METHODS: Thyrotropin‐releasing hormone stimulation was performed on 2 consecutive occasions, 1 week before and 1 week after the winter solstice and the autumn equinox. Blood was collected before and 30 minutes after IV injection of 1 mg of TRH. ACTH concentration was determined by a chemiluminescent assay. Repeatability and test‐retest reliability were assessed by repeated measures analysis of variance, intraclass correlation coefficient and within‐horse coefficients of variation (CV). Bland‐Altman plots were generated to visualize agreement between repetitions. RESULTS: In winter, no week effect was detected on the results of the TRH simulation and the test had an excellent test‐retest reliability. In autumn, after‐TRH ACTH concentrations were significantly lower on week 2 (P = .02) and the test only had a good test‐retest reliability. There were significantly larger within‐horse CV during autumn (P = .04) and after TRH stimulation (P = .04). There were 2 misclassifications in winter and 4 in autumn. CONCLUSIONS AND CLINICAL IMPORTANCE: The TRH stimulation test was repeatable when performed 2 weeks apart in winter; however, in autumn, more variability in after‐TRH ACTH concentrations resulted in decreased repeatability.