Diagnosis and differential diagnosis of Cushing's syndrome

The diagnosis and differential diagnosis of Cushing's syndrome has become the most challenging diagnostic problem in clinical endocrinology. Distinguishing patients with true spontaneous Cushing's syndrome from the large number of patients with the Cushing's phenotype often is difficult clinically, especially when the degree of hypercortisolism is mild. Measurements of cortisol secretion (urine-free cortisol or late night salivary cortisol) currently provide the best sensitivity and specificity for the diagnosis of Cushing's syndrome. The dexamethasone-CRH test is indicated in patients with equivocal biochemical data. Introduction of reliable plasma ACTH measurements, the use of inferior petrosal sinus ACTH sampling, and the improved techniques of pituitary and adrenal imaging have made the differential diagnosis of Cushing's syndrome relatively straightforward.