A 41-year-old man complains of a decreased libido for three months, in association with easy fatigability.

Upon further assessment, he reveals that he has also experienced erectile dysfunction during the same time period. He is otherwise healthy, and denies headaches or visual disturbances.

His medical history is unremarkable and he is not on any medications. There is no history of exposure to radiation. He only drinks socially, and has never smoked or used recreational drugs.

At a routine checkup a month ago, a hemoglobin A1c (HbA1c) and lipid profile were normal. A complete blood count reveals normal hemoglobin levels (12.9 g/dL).