A 44-year-old man on chronic glucocorticoid therapy for polymyalgia rheumatica is being tapered. He develops fatigue, nausea, hypotension, and hyponatremia. Cortisol at 8 AM is 2.8 mcg/dL. Which additional electrolyte abnormality would most favor primary adrenal insufficiency over secondary?
- A Hypokalemia
- B Hypernatremia
- C Hyperkalemia ✓
- D Normal potassium
Correct answer: C. Hyperkalemia
Explanation
Hyperkalemia distinguishes primary adrenal insufficiency (Addison's disease) from secondary (ACTH deficiency). In primary AI, both cortisol AND aldosterone are deficient; aldosterone deficiency causes sodium loss, potassium retention, and hyperkalemia. In secondary AI (including iatrogenic from glucocorticoid withdrawal), the renin-angiotensin-aldosterone axis is intact, so aldosterone secretion is preserved, and potassium remains normal. This patient likely has secondary AI from HPA axis suppression, but hyperkalemia would suggest primary disease.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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