A 28-year-old woman from Mumbai presents with a 3-month history of progressive fatigue, weight gain of 8 kg, cold intolerance, and dry skin. On examination, she has a non-tender, diffuse thyroid enlargement (goitre) and delayed relaxation of deep tendon reflexes. Serum TSH is 18 mIU/L (normal: 0.5–5 mIU/L), free T4 is 8 pmol/L (normal: 10–20 pmol/L), and anti-TPO antibodies are positive. A thyroid ultrasound shows heterogeneous echotexture with increased vascularity. Which of the following best explains the elevated TSH despite low free T4 in this patient?