A 49-year-old man with no remarkable past medical history presents to the clinic complaining of feeling faint when he stands up quickly. He reports worsening fatigue and generalized weakness over the past year. His appetite has decreased, and he has lost 10 lbs over the past few months. The patient is cachectic, and his skin appears well tanned. Vital signs include blood pressure of 100/60 mm Hg and heart rate of 100 beats per minute. A metabolic panel shows mild hyponatremia and hyperkalemia. Which of the following is the most appropriate next step in diagnosis?