A 45-year-old hypertensive patient has an incidentally detected 2.8 cm adrenal mass. Biochemical workup reveals a 1 mg overnight dexamethasone suppression test with cortisol 3.2 µg/dL, normal 24-hour urine catecholamines, and normal aldosterone-renin ratio. What is the most likely diagnosis?

• A Non-functioning adrenal adenoma
• B Mild autonomous cortisol secretion (MACS) ✓
• C Pheochromocytoma
• D Primary aldosteronism

Explanation

Post-DST cortisol 1.8–5.0 µg/dL defines mild autonomous cortisol secretion (previously subclinical Cushing's), per Endocrine Society 2023 guidelines on adrenal incidentaloma. A non-functioning adenoma would suppress to <1.8 µg/dL. Pheochromocytoma is excluded by normal catecholamines. Primary aldosteronism requires an elevated aldosterone-renin ratio. MACS is associated with increased cardiometabolic risk and warrants close follow-up and consideration of adrenalectomy if comorbidities worsen.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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