A 40-year-old man presents with fatigue, weight loss, nausea, and hyperpigmentation of skin creases and buccal mucosa. Labs show hyponatremia, hyperkalemia, low morning cortisol, and markedly elevated plasma ACTH. Which is the most likely diagnosis?

• A Primary adrenal insufficiency (Addison disease) ✓
• B Secondary adrenal insufficiency
• C Cushing syndrome
• D Pituitary adenoma

Explanation

Primary adrenal insufficiency (Addison disease) causes destruction of the adrenal cortex, leading to low cortisol and loss of negative feedback, resulting in markedly elevated ACTH. High ACTH stimulates melanocytes, causing hyperpigmentation of skin creals and mucous membranes. Secondary adrenal insufficiency has low ACTH and no hyperpigmentation. Cushing syndrome shows high cortisol. B pituitary adenoma causing ACTH excess would cause hypercortisolism, not insufficiency.

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

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