Evaluation of Baseline Cortisol, Endogenous ACTH , and Cortisol/ ACTH Ratio to Monitor Trilostane Treatment in Dogs with Pituitary‐Dependent Hypercortisolism

Background The effectiveness of trilostane treatment is currently monitored by regular ACTH stimulation tests, which are time‐consuming and expensive. Therefore, a monitoring system without a stimulation protocol and with less client expense would be preferable. Hypothesis/Objectives The aim of our study was to evaluate if baseline cortisol, endogenous ACTH ( ACTH ) concentration or the baseline cortisol to ACTH ratio (cortisol/ ACTH ratio) could replace the ACTH stimulation test. Animals Forty trilostane‐treated dogs with pituitary‐dependent hypercortisolism ( PDH ) were included in this prospective study. Methods A total of 148 ACTH stimulation tests and 77 ACTH concentrations and cortisol/ ACTH ratios were analyzed. Control of cortisol release was classified according to cortisol concentration after ACTH administration as excessive (<1.5 μg/dL; group 1), adequate (1.5–5.4 μg/dL; group 2), or inadequate (>5.4 μg/dL; group 3). Results Baseline cortisol concentrations had considerable overlap between excessively, adequately, and inadequately controlled dogs. Only baseline cortisol >4.4 μg/dL (in 12% of tests) was a reliable diagnosis of inadequate control. Endogenous ACTH concentrations did not differ between groups. The overlap of the cortisol/ ACTH ratio between groups was large. Correct classification was only possible if the cortisol/ ACTH ratio was >15, which occurred in 4% of tests. Conclusions and Clinical Importance To monitor trilostane treatment the ACTH stimulation test cannot be replaced by baseline cortisol, ACTH concentration, or the cortisol/ ACTH ratio.