A 74-year-old man presents to the emergency department with new symptoms of blood tinged sputum. For the past week he has noticed streaks of blood in his chronic daily sputum production. He reports no fever or chills, but has lost 10 lb in the past 6 months involuntarily. His past medical history is significant for hypertension, dyslipidemia, and a 40-pack year history of smoking.On physical examination, he has bilateral expiratory wheezes, and there is clubbing of his fingers. No lymph nodes are detected and the remaining examination is normal. A CXR reveals a left hilar mass. Which of the following suggests that the tumor is a small cell lung cancer?