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

Abstract

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.