A 52-year-old man from Mumbai presents with progressive fatigue, weight gain (8 kg over 6 months), and easy bruising. Physical examination reveals central obesity, proximal muscle weakness, and purple striae on the abdomen. Investigations show: fasting glucose 156 mg/dL, serum potassium 3.1 mEq/L, 24-hour urinary free cortisol 285 µg/24h (normal 20–90), morning cortisol 28 µg/dL (normal 5–25). Low-dose dexamethasone suppression test (1 mg overnight) shows cortisol 24 µg/dL. High-dose dexamethasone suppression test (8 mg overnight) shows cortisol 8 µg/dL. Which of the following best explains the suppression of cortisol at high-dose dexamethasone?