Single intravenous bolus of dexamethasone for the differential diagnosis of Cushing's syndrome.

We developed a 24 hour intravenous dexamethasone suppression test for the differential diagnosis of Cushing's syndrome. Basal ACTH and cortisol levels were measured at 8 and 9 AM; a bolus of 8 mg dexamethasone phosphate (in children 5 mg/m2) was administered intravenously, and cortisol levels were measured hourly until 3 PM, then every 2 hours until midnight, and the next morning at 8 and 9 AM. We studied 13 patients with an ACTH-secreting pituitary adenoma, four with an autonomous adrenal adenoma, a 10 year-old girl with primary adrenocortical nodular dysplasia, one male with an ACTH-secreting medullary carcinoma of the thyroid, and one male with an ACTH-secreting non-small cell carcinoma of the lung, and compared their results to those obtained in 8 lean and 12 obese normal individuals (controls). The clinical diagnosis was first ascertained by the response to the oral administration of dexamethasone in low and high doses (standard Liddle test), then by the intravenous dexamethasone suppression test, and finally confirmed surgically. Although both controls and patients with an ACTH-secreting pituitary adenoma significantly suppressed their cortisol levels within hours after the injection (50% reduction of basal value at 2 hours, and 75% at 4 hours, p < 0.0001), levels remained suppressed the next morning only in the controls, while in the patients they returned to basal values. No suppression was observed in any of the patients with an adrenal adenoma and the child with primary adrenocortical nodular dysplasia (whose ACTH levels were low), or in the patients with ectopic ACTH secretion tumors (whose ACTH levels were high).(ABSTRACT TRUNCATED AT 250 WORDS)