A 38-year-old woman presents with progressive fatigue, weight loss, and hyperpigmentation over 3 months. On examination, she has generalized darkening of skin, particularly in palmar creases and buccal mucosa. Her BP is 95/60 mmHg. Laboratory findings show Na⁺ 128 mEq/L, K⁺ 5.8 mEq/L, and fasting glucose 65 mg/dL. A morning cortisol level is 2 µg/dL (normal >10 µg/dL). What is the most appropriate next step in management?