A 41-year-old man presents to the outpatient clinic with a 3-month history of progressive fatigue, weight loss, and dizziness on standing. He completed a full 6-month course of treatment for pulmonary tuberculosis one year ago. On examination, he appears thin and lethargic, with generalized skin hyperpigmentation involving the palmar creases and buccal mucosa. His blood pressure is 90/55 mmHg with a postural drop. Routine blood tests reveal hyponatraemia and mild hyperkalaemia. The clinical team suspects adrenal insufficiency secondary to previous tuberculosis. What is the most appropriate investigation to confirm the diagnosis?