Differential Diagnosis and Imaging in Cushing's Syndrome

Establishing the cause of Cushing’s syndrome (CS) should be undertaken only after the clinical and biochemical diagnosis has been confirmed. This process of differential diagnosis relies on a combination of dynamic endocrine testing and directed radiologic imaging. The causes of CS can be divided broadly into adrenocorticotropin (ACTH)–dependent or –independent forms (Box 1). Sustained exposure to excess cortisol suppresses the normal production of corticotropin-releasing hormone (CRH) and ACTH. As a result, circulating plasma ACTH concentrations are low in primary adrenal disorders, and normal or increased in patients with excess ACTH secretion from a pituitary tumor (Cushing’s disease [CD]) or an ectopic source. These physiologic differences lead to measurement of plasma ACTH as the initial focus of differential diagnosis.