A 55-year-old man has proximal muscle wasting, central obesity with purple striae, glucose intolerance, and profound hypokalemia of 2.4 mEq/L. Plasma ACTH is 320 pg/mL, cortisol fails to suppress even with high-dose dexamethasone, and pituitary MRI shows no adenoma. Chest imaging reveals a hilar mass. The most likely source of ACTH is:

• A Bronchial carcinoid tumor secreting CRH
• B Medullary thyroid carcinoma secreting calcitonin cross-reacting in the ACTH assay
• C Adrenal cortical adenoma secreting cortisol autonomously
• D Small cell lung carcinoma secreting ACTH ✓

Explanation

Profound hypokalemia, rapid onset, weight loss, very high ACTH, and failure of high-dose dexamethasone suppression indicate ectopic ACTH secretion, most commonly from small cell lung carcinoma. High-dose dexamethasone suppresses corticotroph adenomas of Cushing disease through residual glucocorticoid receptors but does not suppress ectopic sources, which kills the pituitary interpretation. Autonomous adrenal lesions suppress ACTH, contradicting the measured value here.

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

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Written and medically reviewed by the StethoPrep medical team.