A 35-year-old woman from Delhi presents with a 3-month history of progressive fatigue, weight gain of 8 kg, cold intolerance, and dry skin. On examination, she has a diffuse, non-tender thyroid enlargement and delayed relaxation of deep tendon reflexes. Her serum TSH is 18.5 mIU/L (normal 0.4–4.0), free T4 is 6.2 pmol/L (normal 10–20), and anti-TPO antibodies are positive at 450 IU/mL (normal <35). What is the primary mechanism responsible for the elevated TSH in this patient?