Potassium levels in ectopic Cushing syndrome are typically:

• A. Normal
• B. High
• C. Low
• D. Very high

Answer: (C)

Excess cortisol in ectopic ACTH syndrome behaves like a mineralocorticoid in the kidney. When cortisol levels are very high, it activates mineralocorticoid receptors in the distal tubule, promoting sodium reabsorption at the expense of potassium and hydrogen ion loss. This potassium wasting leads to low potassium levels (hypokalemia), which is a typical finding in this condition. Normal potassium would miss the pronounced mineralocorticoid effect, and higher potassium would contradict the expected renal potassium loss.