A 35-year-old woman from Mumbai presents with a 3-month history of progressive fatigue, weight gain of 8 kg despite reduced appetite, and constipation. She reports feeling cold even in warm weather and has noticed her skin becoming dry and coarse. On examination, her heart rate is 52 bpm, blood pressure 128/82 mmHg, and there is a non-tender, diffuse thyroid enlargement. Laboratory investigations show: TSH 18.5 mIU/L (normal 0.4–4.0), free T4 2.1 ng/dL (normal 0.8–1.8), and elevated anti-TPO antibodies (>500 IU/mL). What is the primary mechanism underlying the elevated TSH despite low-normal free T4 in this patient?