A 28-year-old woman from rural Maharashtra presents with a 6-month history of progressive fatigue, weight gain of 8 kg, cold intolerance, and dry skin. On examination, she has a firm, diffuse thyroid enlargement (grade II goiter) with no nodules. Her heart rate is 52 bpm and reflexes are sluggish. Laboratory investigations show TSH 18.2 mIU/L (normal 0.4–4.0), free T4 0.8 ng/dL (normal 0.8–1.8), and free T3 2.1 pg/mL (normal 2.3–4.2). Serum iodine is low at 35 μg/L. Which of the following best explains the thyroid hormone synthesis defect in this patient?