Answer 2 MRI of the pituitary gland. This patient’s serum cortisol levels after both the low-dose and high-dose dexamethasone suppression testing as well as the serum ACTH level suggest a pituitary source of hypercortisolism (ACTH-dependent Cushing’s syndrome). Thus, MRI of the pituitary is the next step. If serum cortisol is not suppressed with low-dose dexamethasone but is suppressed with high-dose dexamethasone, a pituitary tumor is the likely cause. In the case of pituitary adenomas, cortisol levels will suppress by more than 50% with high-dose dexamethasone (8 mg/day for 2 days). Elevated or normal serum ACTH levels in the presence of elevated serum cortisol indicate a pituitary or ectopic source, whereas low levels of ACTH indicate an adrenal source. Click here to return to the questions
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