Comparison of plasma cortisol and corticosterone in the dexamethasone suppression test for melancholia

The suppression of plasma corticosterone (B), measured by radioimmunoassay (RIA), was compared to simultaneous suppression of plasma cortisol (F), measured as total corticoids by a competitive protein binding (CPB) assay, in the overnight dexamethasone suppression test (DST). Baseline plasma B concentrations in 10 control subjects were 4.04 +/- 1.07 ng/ml (X +/- S.D.) at 0800 hr and 1.51 +/- 0.68 ng/ml at 1600 hr. Post-dexamethasone 1600 hr B levels in the controls were 0.46 +/- 0.29 ng/ml. An early escape of plasma B (greater than 1.2 ng/ml), like that of F (greater than 5 micrograms/dl), during the overnight 24 hr 1.0 mg dose DST was noted in patients with melancholia (endogenous depression). Half-hourly catheter samples in a normal subject stimulated to escape from dexamethasone suppression showed that in general, plasma B concentrations parallel plasma F concentrations over a 12 hr period. Repeated weekly DSTs on two patients with different psychiatric diagnoses resulted in B : F correlations of 0.74 and 0.60. Overall agreement between B- and F-DST outcomes in all categories tested at 1600 and 2300 hr was 93%; the agreement in the melancholic and non-endogenous depressed groups was 100%. Post-dexamethasone, both B and F were suppressed 55-60% below the criterion level in controls. In those patients who escaped from dexamethasone suppression, the percentage increase in plasma B above the criterion level was significantly greater (+55%) than the corresponding percentage change in plasma F. Most patients with borderline abnormal F-DSTs (3.5 - 4.9 micrograms/dl) exhibited clearly abnormal B-DSTs (greater than 1.2 ng/ml). We conclude that the use of dexamethasone suppression of plasma B (using 1.2 ng/ml as the abnormal criterion value) is an additional indicator of an abnormal DST in depressed patients.