Steroid laboratory tests in the diagnosis of Cushing's syndrome

Abstract

The best steroid tests currently available for identifying patients with Cushing's syndrome are the low dose dexamethasone suppression test with determination of urinary 17-hydroxycorticosteroid (17-OHCS) excretion and the single dose dexamethasone suppression test with determination of plasma 17-OHCS concentration. The results of steroid laboratory tests may be helpful in suggesting the lesion responsible for Cushing's syndrome but no single test can be used to identify the cause with great confidence. (1) The high dose dexamethasone suppression test is best for guiding the physician in selecting therapy. In 82 per cent of the patients with Cushing's syndrome due to adrenal hyperplasia from excessive pituitary secretion of ACTH unassociated with a pituitary tumor, urinary 17-OHCS excretion decreased to less than half the control value with this test. Suppression of this degree is very uncommon in patients with adrenal tumors. (2) Urinary 17-OHCS excretion increased with metyrapone administration in patients with Cushing's syndrome caused by adrenal hyperplasia not associated with tumors whereas this response was infrequent in patients with other causes of Cushing's syndrome. (3) In patients with Cushing's syndrome, urinary 17-ketosteroids (17-KS) were elevated to values at least four times the upper limit of normal in 52 per cent of the patients with adrenal carcinoma, in 15 per cent of the patients with extrapituitary ACTH-secreting tumors and in only 3 per cent of the patients with adrenal hyperplasia not associated with tumor. Urinary 17-KS levels were normal or low in 69 per cent of the patients with adrenal adenomas and in only 10 per cent of the patients with adrenal carcinomas.