A 28-year-old man with newly diagnosed Graves' disease presents with palpitations, tremor, and anxiety. He is started on propranolol 40 mg BD. After 2 weeks, his symptoms persist. Thyroid function tests show TSH 0.01 mIU/L, free T4 28 pmol/L (normal 10–20), and free T3 9.5 pmol/L (normal 3.5–7.0). What is the most appropriate next step in management?