A 74-year-old man with a history of smoking notices blood in his chronic daily sputum production. He has no fever or chills, but has lost 10 lb in the past 6 months. On examination, he has bilateral expiratory wheezes, and his fingers are clubbed. There are no lymph nodes and the remaining examination is normal. CXR reveals a left hilar mass. Which of the following suggests that the tumor is a small cell lung cancer?