What additional tests may be useful in identifying the cause of cushing syndrome?
Classically, the high-dose dexamethasone suppression test (2 mg every 6 hours for eight doses) suppresses the production of cortisol by >90% (and its urinary metabolites) in patients with Cushing disease, because pituitary microadenomas remain sensitive to high levels or activity of circulating corticosteroids.
A more convenient “high-dose” test measures plasma cortisol at bedtime, followed by one 8 mg dose of dexamethasone and another cortisol level test 8 hours later; “suppression” is diagnosed if the cortisol level drops by 50% compared to baseline. This test does not distinguish between Cushing disease and the far less common ectopic corticotropin secretion, so a chest x-ray is usually scrutinized for evidence of a tumor that is secreting corticotropin.
Head and abdominal CT scans may identify a pituitary or adrenal tumor; MRI is slightly less sensitive for adrenal tumors than a CT with thin cuts (5 mm) through the adrenals. Sometimes a chest x-ray and/or a chest CT is done because the most common source of ectopic corticotropin secretion is lung cancer, usually small cell.
