David Byrne

The objectives of this study were 1) to evaluate the adrenocorticotropic hormone (ACTH) response to thyrotropin releasing hormone (TRH) administration at different times of the year and in different locations, 2) to determine the effects of insulin dysregulation testing on ACTH concentrations and 3) to determine the reproducibility of two tests of insulin dysregulation. For the first objective, 13 healthy horses in Perth and 29 in Townsville were identified by clinical parameters and normal monthly endogenous plasma ACTH. TRH-stimulation tests were performed at two time points during the circannual pituitary cycle at both sites. Perth horses were sampled at 10 minutes post-TRH (T10) and Townsville horses were sampled at 30 minutes post-TRH (T30).For the second and third objectives, a group of 15 healthy Thoroughbred and Standardbred horses underwent oral sugar tests (OST) and two-step insulin response tests (IRT) three times, at weekly intervals. For one week, endogenous ACTH was measured at the basal and testing time points in this group and in an additional control group of 15 horses. For the TRH-stimulation tests, the T10 or T30 time point ACTH concentrations were significantly higher during the dynamic phase of the circannual pituitary cycle at each site, compared to during the dynamic phase (Perth T10 p=0.001; Townsville T30 p<0.0001). The mean T10 ACTH concentration in Perth during the dynamic phase was 248.5 pg/mL (range 80.7 - 511 pg/mL and 95% CI 170.2 - 326.9 pg/mL). The mean T30 ACTH concentration in Townsville during the dynamic phase was 112.3 pg/mL (range 53.6 – 227.0 pg/mL and 95% CI 93.4 - 131.2 pg/mL). For the OST, no substantial increase in insulin concentrations was observed. There was insufficient agreement of the test results between weeks. There was no statistical difference in ACTH concentrations before and at the 60 minutes post-sugar time point (p = 0.27). For the IRT, there was insufficient agreement between glucose concentrations at 25, 30 and 35 minutes post-insulin. There was insufficient agreement of the test results between weeks. There was no statistical difference in ACTH concentrations before and at 30 minutes post-insulin (p = 0.5).