A 55-year-old woman presents with weight gain, proximal muscle weakness, thin skin, and facial rounding. Plasma ACTH is undetectable. Abdominal CT shows bilateral adrenal hyperplasia. High-dose dexamethasone suppression test shows NO cortisol suppression. What is the most likely cause of Cushing syndrome in this patient?

• A Pituitary microadenoma
• B Ectopic ACTH secretion from small cell lung carcinoma
• C ACTH-independent macronodular adrenal hyperplasia (AIMAH) ✓
• D Exogenous glucocorticoid use

Explanation

Undetectable ACTH indicates ACTH-independent Cushing syndrome. Bilateral macronodular hyperplasia with ACTH independence points to AIMAH, where adrenal cortex is stimulated by non-ACTH factors. Pituitary adenoma and ectopic ACTH both produce HIGH ACTH levels. Exogenous glucocorticoids cause ACTH-independent Cushing syndrome but lead to adrenal atrophy, not hyperplasia. The failure of high-dose dexamethasone suppression is consistent with ACTH-independent disease.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.