Diagnosis of Cushing's syndrome

No single test has a 100% diagnostic accuracy, and combinations of tests are required. We routinely obtain 3 baseline IRMA-ACTH levels, followed by CRF stimulation and overnight high-dose (8 mg) dexamethasone suppression (HDS). If ACTH-independent adrenal disease is suspected, thin-section CT (with or without isotope scanning) of the adrenals is performed. In ACTH-dependent Cushing's syndrome, the clinical manifestations in conjunction with the CRF and 8 mg overnight HDS tests are used to differentiate pituitary from ectopic ACTH production. If plasma ACTH increases > 34% following CRF, and plasma cortisol decreases > 60% after HDS, pituitary MRI is the next step; if not, the ectopic ACTH syndrome is more likely and IPSS and attempts to localise the source of the ACTH production are required (see algorithmn).