Soru

A 60-year-old man visits his primary care physician because of progressive fatigue, constipation, and an unintended weight gain of $5 \text{ kg}$ ($11 \text{ lbs}$) over the past eight months. He says he constantly feels cold, even when his house is well heated, and reports a recently decreased libido. He has no significant past medical history and takes no medications. Vital signs show a temperature of $36.2^\circ\text{C}$ ($97.2^\circ\text{F}$), blood pressure of $112/74 \text{ mmHg}$, and a heart rate of $54/\text{min}$. Physical examination reveals dry, coarse skin and a noticeable delay in the relaxation phase of the Achilles tendon reflexes. Laboratory studies show:

Thyroid-stimulating hormone (TSH): $0.2 \text{ } \mu\text{U/mL}$ (Normal: $0.5 - 5.0 \text{ } \mu\text{U/mL}$)
Free thyroxine (T4): $0.6 \text{ ng/dL}$ (Normal: $0.9 - 1.7 \text{ ng/dL}$)

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