Dexamethasone suppression test

Low Dose:

• Overnight: 8 a.m. plasma cortisol < 5 mcg/dl
• Standard: Urinary free cortisol on day 3 < 10 mcg/day

High Dose:

• Overnight: > 50 % reduction in plasma cortisol
• Standard: > 90% reduction in urinary free cortisol

Normal value ranges may vary slightly among different laboratories.

If there is not a normal response on the low-dose test, abnormal secretion of cortisol is likely (Cushing's Syndrome). This could be a result of a cortisol-producing adrenal tumor, a pituitary tumor that produces ACTH, or a tumor in the body that inappropriately produces ACTH. The high-dose test can help distinguish a pituitary cause (Cushing's Disease) from the others.

• Low dose: no change
• High dose: no change

Cushing's syndrome related to ectopic ACTH-producing tumor

• Low dose: no change
• High dose: no change

Cushing's Syndrome caused by pituitary tumor (Cushing's Disease)

• Low dose: no change
• High dose: normal suppression

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