Soru

A $46$-year-old man presents to the clinic with an $8$-month history of progressive fatigue, decreased libido, and generalized joint pain. He notes that the joint pain is most pronounced in the index and middle fingers of both hands. He also reports that his skin has become diffusely darker, despite working indoors year-round. He has no significant past medical history and takes no medications.

Physical examination reveals a firm liver edge palpable $3$ cm below the right costal margin. Testes are small and soft bilaterally. There is bony enlargement and tenderness over the second and third metacarpophalangeal joints of both hands.

Laboratory results are as follows:
Fasting blood glucose: $138$ mg/dL
Sodium: $139$ mEq/L
Potassium: $4.1$ mEq/L
Thyroid-stimulating hormone (TSH): $1.1$ $\mu$ U/mL (Normal: $0.5 - 5.0$ $\mu$ U/mL)
Free thyroxine (T4): $0.5$ ng/dL (Normal: $0.9 - 1.7$ ng/dL)
Total testosterone ($8:00$ AM): $140$ ng/dL (Normal: $300 - 1000$ ng/dL)
Luteinizing hormone (LH): $1.2$ mIU/mL (Normal: $1.5 - 9.3$ mIU/mL)
Prolactin: $12$ ng/mL (Normal: $2 - 18$ ng/mL)

Which of the following is the most appropriate next step in establishing the underlying etiology of this patient's condition?