A 32-year-old woman presents with increased appetite and weight gain over 1.5 years. Physical exam: height 160 cm, weight 75 kg, thickened abdominal and gluteal fat, with faint red striae on the lower abdomen and upper thighs. BP 150/95 mmHg. Further tests show elevated 24-hour urinary free cortisol and plasma cortisol levels: 8 AM 414 nmol/L (15 μg/dL); 4 PM 331.2 nmol/L (12 μg/dL); midnight 281.5 nmol/L (10.2 μg/dL). Which test should be performed next to confirm the diagnosis?