Associate Professor in Equine Internal Medicine The University of Queensland Gatton, Queensland, Australia
Abstract: Background The thyrotropin releasing hormone (TRH) stimulation test is used to diagnose pituitary pars intermedia dysfunction (PPID) by measuring ACTH concentration either 10 or 30 minutes post-TRH administration. Imprecise sampling time leads to significant ACTH variability in the 10-minute protocol.
Hypothesis/Objectives Determine if imprecise sampling time yields significant differences in ACTH concentrations using the 30-minute protocol of the TRH stimulation test.
Animals Twenty-seven horses > 12 years, including 12 horses with PPID.
Methods Blood was collected from each horse 0, 9, 10, 11, 29, 30 and 31 minutes after intravenous injection of 1 mg of TRH. ACTH concentrations were determined by chemiluminescent immunoassay. Differences in ACTH concentrations and variability in ACTH concentrations between protocols were analyzed with a one-way ANOVA and visualized with Bland-Altman plots. Significance was set at p < 0.05.
Results Early or late sampling yielded a significant difference in ACTH concentrations using the 10-minute protocol among all horses, the control horses but not those with PPID (p = 0.04, 0.01 and 0.37 respectively). There was no significant difference with the 30-minute protocol in any group (p = 0.45, 0.21 and 0.63). No significant differences in the variability in ACTH concentrations between early or late sampling in either protocol detected in any group (p = 0.35, 0.24, 0.34, respectively).
Conclusions and clinical importance The 30-minute protocol of the TRH stimulation test is less affected by imprecise sampling and might be more useful in clinical settings. Precise timing during TRH stimulation tests remains fundamental for the diagnosis of PPID.
