A 42-year-old woman has progressive fatigue, reduced appetite and dizziness when standing. Her serum sodium is 127 mmol/L (135–145), potassium is 5.8 mmol/L (3.5–5.2), and glucose is 3.4 mmol/L (3.5–7.8). An 08:00 serum cortisol is 85 nmol/L (reference range 140–690), and she is haemodynamically stable after oral fluids. Which investigation is most appropriate to confirm the suspected diagnosis?
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