Abstract

Low-dose dexamethasone suppression testing has been recommended for biochemical screening when Cushing's syndrome is suspected. The criterion for normal suppression of cortisol after dexamethasone is controversial. To assess diagnostic utility (sensitivity), we report the results of low-dose dexamethasone suppression testing in 103 patients with spontaneous Cushing's syndrome. There were 80 patients with Cushing's disease (78%), 13 with the ectopic ACTH syndrome (13%), and 10 with cortisol-producing adrenocortical adenomas (10%). Fourteen (18%) of 80 patients with Cushing's disease suppressed serum cortisol to less than 5 micro g/dl (<135 nmol/liter) after the overnight 1-mg test, whereas six patients (8%) actually showed suppression of serum cortisol to less than 2 micro g/dl (<54 nmol/liter). In addition, the 2-d, low-dose dexamethasone suppression test yielded false-negative results in 38% of patients when urine cortisol was used and 28% when urinary 17-hydroxycorticosteroids were used. Serum cortisol after the 1-mg test correlated with baseline urinary free cortisol (r = 0.705, P < 0.001), plasma ACTH level (r = 0.322, P = 0.001), and urinary free cortisol after the 2-d test (r = 0.709, P = 0.001). This study provides evidence that low-dose dexamethasone may suppress either plasma cortisol or urinary steroids to levels previously thought to exclude Cushing's syndrome and that these tests should not be used as the sole criterion to exclude the diagnosis of endogenous hypercortisolism.