A 33-year-old female presents to the emergency department with cough and fever for 1 week, and coma for half a day. She has a history of Addison's disease. Physical examination: blood pressure (BP) 80/60 mmHg, respiratory rate 23/min, generalized skin hyperpigmentation, heart rate 110/min with regular rhythm, and few rales in the left lower lung. Which laboratory test is most urgently needed to manage this patient?