A 46-year-old woman presents with progressive fatigue, weight loss, postural dizziness and hyperpigmentation of the palmar creases. BP is 90/60 mmHg lying, 70/50 mmHg standing. Sodium is 128 mmol/L, potassium 5.8 mmol/L. Morning serum cortisol is 80 nmol/L (low). What is the most appropriate next diagnostic test to confirm the diagnosis?