Plasma acth and cortisol levels in depressed patients: Relation to dexamethasone suppression test

Blood samples collected from normal subjects and newly hospitalized depressed patients at 8 AM on the day before and at 8 AM and 4 PM the day after receiving dexamethasone, 1 mg orally at 11 PM, were analyzed for ACTH and cortisol. The mean plasma ACTH values of these two groups were not significantly different at any of the times, while the cortisol levels of the depressed patients were significantly higher than those of the normal subjects at 8 AM pre-dexamethasone (P<0.001). There was no correlation between plasma ACTH and cortisol values in either group. The cortisol responses to dexamethasone in depressed patients revealed two subgroups. In one subgroup, the cortisol was suppressed as much as in normal subjects, but in the other, cortisol levels were not suppressed. The post-dexamethasone ACTH rebounded at 4 PM in the latter subgroup to higher values than in the subgroup with suppressed cortisol levels and in the normal subjects. After dexamethasone, the ACTH values were negatively correlated with plasma cortisol only in the normal subjects (P<0.01), not in the depressed patients. These results indicate that ACTH levels do not account for the elevated cortisol and the failure of dexamethasone to suppress cortisol levels in some depressed patients.