Cushing Syndrome (cont.)
Exams and Tests for Cushing's 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's 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's 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's syndrome.
- 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.
George P Chrousos, MD, FAAP, MACP, MACE
Arthur B Chausmer, MD, PhD, FACP, FACE, FACN, CNS
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