A 52-year-old man has an ulcerated area on the sole of his foot that has not healed for 2 months. He is 180 cm tall, weighs 126 kg, and has continued to gain weight gradually. He has not had any major illnesses. His blood pressure is normal. Which of the following laboratory tests performed on serum from a blood sample would be most useful in elucidating the underlying cause for his problem: