Exams and Tests for Cushing Syndrome

• Measurement of 24-hour cortisol levels: The amount of cortisol excreted in the urine during a 24-hour time period is measured. Values higher than three to four times the upper limit of normal are suggestive of Cushing syndrome.

• Overnight 1-mg dexamethasone suppression test: For this test, 1mg of dexamethasone (exogenous corticosteroid) is administered at 11 pm, with measurement of serum cortisol the next morning at 8 am. In healthy individuals, the serum cortisol should be less than 2-3 mcg/dL. Cushing syndrome may be excluded with a cortisol level less than 1.8 mcg/dL.

• The 48-hour low-dose dexamethasone suppression test: In many persons, overnight 1mg dexamethasone suppression test may be inconclusive. In such persons, 48-hour low-dose dexamethasone suppression test is performed. In this test, 0.5mg dexamethasone is administered every six hours for eight doses and serum cortisol levels are measured.

• Corticotropin-releasing hormone (CRH) stimulation test: This test can detect mild corticosteroid excess. It combines the 48-hour low-dose dexamethasone suppression test with CRH stimulation. CRH is administered intravenously two hours after the eighth dose of 0.5mg dexamethasone. Corticosteroid levels are measured 15 minutes after CRH administration. A level of greater than 1.4 mg/dL is suggestive of Cushing syndrome.

• Brain CT scan: In persons with pituitary adenoma, enlarged pituitary gland can be seen on brain CT scan.

• Abdominal CT scan: It is recommended if a primary adrenal problem is suggested by the symptoms and clinical examination. The presence of an adrenal mass larger than 4-6 cm in the CT scan raises the possibility that the mass is an adrenal carcinoma.

• Chest and abdominal CT scans: They should be performed in patients with suspected ACTH production from tumors present elsewhere in the body (for example, lung).

• Magnetic resonance imaging (MRI) of the brain: If a pituitary source of excess ACTH is suspected, MRI of the brain is performed.