Diagnosis is based on a review of the patient's medical history, physical examination and laboratory tests. Often x-ray exams of the adrenal or pituitary glands are useful for locating tumors. These tests help to determine if excess levels of cortisol are present and why.
No single lab test is perfect, and usually, several are needed. The three most common tests used to diagnose Cushing's syndrome are the 24-hour urinary free cortisol test, the measurement of midnight plasma cortisol or late-night salivary cortisol, and the low-dose dexamethasone suppression test (LDDST). Another test, the dexamethasone-corticotropin-releasing hormone (dexamethasone-CRH) test, may be needed to distinguish Cushing's syndrome from other causes of excess cortisol.