During a low dose (1 mg) dexamethasone suppression test, total cortisol level at 8:35 am is 2.8 μg/dL. How is this interpreted?

During a low dose (1 mg) dexamethasone suppression test, total cortisol level at 8:35 am is 2.8 μg/dL. How is this interpreted?

• A. The patient is normal.
• B. The patient has Cushings' syndrome.
• C. The patient has a nonpituitary tumor causing ectopic ACTH syndrome.
• D. The patient has an ACTH-secreting pituitary adenoma.

Answer: (B)

The test hinges on how cortisol is controlled by feedback. After a small dose of dexamethasone at night, a normal pituitary-adrenal axis is suppressed, so morning cortisol falls to a low level (typically below about 1.8 μg/dL). A morning level of 2.8 μg/dL shows inadequate suppression, meaning the axis is not properly inhibited. That pattern supports a diagnosis of Cushing's syndrome rather than a normal response to dexamethasone. This does not specify the exact source of excess ACTH or cortisol, but it indicates hypercortisolism. Further testing (such as high-dose dexamethasone suppression or ACTH measurement) would be used to distinguish between pituitary, ectopic, or adrenal causes.

The test hinges on how cortisol is controlled by feedback. After a small dose of dexamethasone at night, a normal pituitary-adrenal axis is suppressed, so morning cortisol falls to a low level (typically below about 1.8 μg/dL). A morning level of 2.8 μg/dL shows inadequate suppression, meaning the axis is not properly inhibited. That pattern supports a diagnosis of Cushing's syndrome rather than a normal response to dexamethasone. This does not specify the exact source of excess ACTH or cortisol, but it indicates hypercortisolism. Further testing (such as high-dose dexamethasone suppression or ACTH measurement) would be used to distinguish between pituitary, ectopic, or adrenal causes.