Soru

A 54-year-old man presents to the clinic reporting a 4-month history of persistent fatigue, sluggishness, and increased sensitivity to cold. He also mentions a recent decline in his sexual drive and difficulty maintaining an erection. He has no past medical history and takes no medications. His temperature is $36.2^{\circ}\text{C}$ ($97.2^{\circ}\text{F}$), blood pressure is $108/68\text{ mm Hg}$, and pulse is $55\text{/min}$. Physical examination demonstrates pale, dry skin and a noticeable delay in the relaxation phase of the patellar reflexes. Confrontational visual field testing detects a deficit in the temporal fields bilaterally.

Laboratory results are as follows:
Thyroid-stimulating hormone (TSH): $1.8\text{ }\mu\text{IU/mL}$ (normal: $0.4-4.0\text{ }\mu\text{IU/mL}$)
Free thyroxine (T4): $0.5\text{ ng/dL}$ (normal: $0.9-1.7\text{ ng/dL}$)
Total testosterone: $140\text{ ng/dL}$ (normal: $300-1,000\text{ ng/dL}$)

Which of the following is the most likely underlying mechanism for this patient's findings?