Q. A 45-year-old male presents to your clinic with complaints of decreased libido, erectile dysfunction, fatigue, and depression. He reports that he has been experiencing these symptoms for the past few months, and they have been affecting his daily life and work. Upon further questioning, he also reports a decrease in his muscle mass and strength, and has gained weight despite not changing his diet or exercise habits. He denies any history of diabetes or cardiovascular disease. His medical history is significant for hypertension, which is well-controlled with medications. On physical examination, he appears well-nourished and in no acute distress. His blood pressure is within normal limits. Genital examination reveals small testes and absence of hair growth in the pubic and axillary regions. Further examination reveals a normal thyroid gland and no signs of other hormonal imbalances. Given his symptoms and physical exam findings, you decide to order a comprehensive hormone panel to assess the patient for primary hypogonadism. Which of the following tests is LEAST useful at this time?