Investigation of cushing’s syndrome

Abstract

In the biochemical investigation of suspected Cushing’s syndrome, screening tests are used to identify potential cases and should have high sensitivity, be inexpensive and easy to perform. Appropriate screening tests are a midnight salivary cortisol and/or the 1 mg dexamethasone suppression test (DST). Normal screening tests indicate the patient is unlikely to have Cushing’s syndrome. An abnormal screening test should be followed up by diagnostic testing using 24 hour urinary free cortisol levels and either a low dose oral or iv DST. After the diagnosis of Cushing’s syndrome is confirmed, differential diagnostic testing should be undertaken. A suppressed plasma ACTH suggests adrenal disease. Those with ACTH-dependent Cushing’s require further localising investigation to distinguish between pituitary and ectopic sources of ACTH. The current gold standard is bilateral inferior petrosal sinus sampling (IPSS). The relative sensitivity and specificity of other biochemical tests in the differential diagnosis of ACTH-dependent Cushing’s will be discussed. The majority of cases will have Cushing’s disease, where the treatment of choice is trans-sphenoidal pituitary surgery. There is no clear consensus of what constitutes biochemical remission post-operatively; the strictest definition requires a plasma cortisol of