A 40-year-old alcoholic man is being treated for tuberculosis, but he has been only intermittently compliant with his medications despite the health department's best efforts at directly observed therapy. He complains of increasing weakness, fatigue, weight loss, and nausea over the preceding 3 weeks. He appears thin, and his blood pressure is 80/50 mm Hg. There is increased pigmentation over the elbows and in the palmar creases. Cardiac examination is normal. Which of the following is the best next step in evaluation?