Diurnal ACTH and plasma cortisol variations in healthy dogs and in those with pituitary-dependent Cushing's syndrome before and after treatment with retinoic acid.

Daytime variations in ACTH and plasma cortisol were studied in healthy dogs and in dogs with pituitary-dependent hypercortisolism (PDH), before and after treatment with retinoic acid. In control dogs ACTH showed a higher concentration at 8.00 AM and between 2.00 and 6.00 PM, with the lowest concentration registered at 10.00 AM (p<0.05 vs. 8.00 AM and 2.00 PM and p<0.01 vs. 4.00 PM). Cortisol did not show significant differences. In dogs with PDH, ACTH was lower at 8.00 AM (ACTH: p<0.01 vs. 2.00 and 4.00 PM; and p<0.05 vs. 6.00 PM). The lowest cortisol concentration was registered at 8.00 AM and 8.00 PM and the highest at 4.00 PM (p<0.05 vs. 8.00 AM and p<0.01 vs. 8.00 PM). After treatment, the lowest ACTH concentration was registered at 10.00 AM (p<0.01 vs. 2.00 and 4.00 PM). To conclude, the adrenal is desensitized in PDH possibly showing negative in diagnostic tests.