ORAL AND INTRAVENOUS SUPPRESSION TESTS IN THE DIAGNOSIS OF CUSHING'S SYNDROME

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Abstract
SUMMARY: Adrenocortical suppression tests, based on the fall in urinary 17-hydroxy-corticosteroid excretion during the oral administration of dexamethasone, were found to be of value in the diagnosis of Cushing's syndrome, but less useful in differentiating bilateral adrenal hyperplasia from adrenal tumour. Such tests have the disadvantage of requiring accurate urine collections and of taking several days to perform. A test is described, based on the decrease in plasma cortisol concentration during i.v. infusion of dexamethasone at a rate of 1 mg./hr. The results obtained in 12 patients with Cushing's syndrome and bilateral adrenal hyperplasia differed from those found in control subjects in that there was a delay between the start of the infusion and the fall of plasma cortisol, and the rate of fall was less rapid. The values found after 180 min., expressed either as μg./100 ml. or as a percentage of the resting level, differed significantly (P < 0·001) in the two groups. The test proved valuable as an aid to the diagnosis of Cushing's syndrome, was easy to perform, and could be completed in 3 hr. In some patients with Cushing's syndrome, the administration of synthetic glucocorticoids appeared to result in an increased urinary steroid excretion. A transient increase in plasma cortisol levels was also observed in some of these patients during the early period of dexamethasone infusion. It is thought that this finding reflects an alteration in steroid metabolism induced by dexamethasone and fluorocortisol.