The ACTH ratio validates the thyrotropin-releasing hormone stimulation test to diagnose pituitary pars intermedia...

To evaluate the diagnostic performance of a post-/pre-thyrotropin-releasing hormone (TRH) plasma ACTH ratio for the diagnosis of pituitary pars intermedia dysfunction (PPID) in horses.

This was a prospective, case-controlled study conducted monthly from January through December 2018 on 21 horses with PPID and 63 control horses. The ratios were calculated by dividing the plasma ACTH concentration obtained 30 minutes after IV injection of TRH by the pre-TRH plasma ACTH concentration. The effect of PPID and month were assessed using a linear mixed-effect model, and the diagnostic performance of the ratio, including accuracy, sensitivity, and specificity, was assessed via receiver operating characteristic curves. Diagnostic cutoff values were calculated via the Youden index when relevant.

Both PPID status and month had a significant effect on post-/pre-TRH plasma ACTH ratio, with higher ratios in horses with PPID in 7 of 12 months. The diagnostic performance of the post-/pre-TRH plasma ACTH ratio varied by month, with accuracies ranging from fair to good but consistently low positive likelihood ratios.

The post-/pre-TRH ACTH ratio allows evaluation of the pituitary pars intermedia response, validating the use of the TRH stimulation test in the diagnosis of PPID. However, the post-/pre-TRH plasma ACTH ratio does not improve the diagnostic characteristics of a TRH stimulation test.

The TRH stimulation test is recommended for evaluation of the pars intermedia of the equine pituitary gland in the context of diagnosing PPID, but the use of the post-/pre-TRH plasma ACTH ratio does not provide superior performance over the post-TRH ACTH concentration alone.