Responsivity of adrenocorticotropin to corticotropin-releasing hormone and lack of suppressibility by dexamethasone are related phenomena in Cushing's disease.

The ACTH and cortisol responses to 100 micrograms ovine corticotropin-releasing hormone (CRH) in 19 consecutive patients with Cushing's disease were compared with those in 13 normal subjects. In 2 patients with Cushing's disease, plasma ACTH and cortisol did not increase after CRH administration. Compared to the normal subjects, maximal ACTH increments [135 +/- 25 (+/- SEM) vs. 42 +/- 6 pg/ml; P less than 0.001, by Wilcoxon's two-sample test] and maximal cortisol increments (17.7 +/- 1.8 vs. 9.4 +/- 1.1 micrograms/100 ml; P less than 0.01 by Wilcoxon's test) after CRH were significantly higher in the 17 CRH-responsive patients with Cushing's disease. In the normal subjects, there was a significant negative correlation between the basal cortisol level and the maximal ACTH (r = -0.65; P less than 0.05, by Spearman's rank correlation test) and cortisol (r = -0.95; P less than 0.001, by Spearman's test) responses to CRH. In contrast, in the CRH-responsive Cushing patients, there was no significant correlation between the basal cortisol level and the maximal ACTH (r = 0.10; P greater than 0.10, by Spearman's test) and cortisol (r = 0.14; P greater than 0.10, by Spearman's test) increments after CRH treatment. In the normal subjects, there was no significant correlation between the basal ACTH level and the maximal ACTH increments after CRH administration (r = -0.24; P greater than 0.10, by Spearman's test). Again in contrast, in the CRH-responsive Cushing patients, maximal ACTH increments after CRH treatment correlated positively with basal ACTH levels (r = 0.69; P less than 0.005, by Spearman's test). Moreover, in these patients, the maximal ACTH increments after CRH were positively correlated with the ACTH levels after suppression with higher and lower doses of dexamethasone. We conclude that in Cushing's disease, unlike in normal subjects, circulating cortisol is not the major modulator of ACTH and cortisol responses to CRH, and that in these patients, responsivity of ACTH to CRH and lack of suppressibility by dexamethasone are related phenomena.