A 35-year-old man with a known history of Graves' disease, currently on propranolol monotherapy, presents with palpitations, tremor, and heat intolerance for 2 weeks. His thyroid is diffusely enlarged and tender. On examination: HR 110 bpm, BP 145/85 mmHg, fine tremor, warm moist skin. Laboratory results: TSH <0.01 mIU/L (suppressed), free T4 6.2 ng/dL (normal 0.8–1.8), free T3 8.5 pg/mL (normal 2.3–4.2), TSI (TSH receptor antibodies) strongly positive. He reports poor compliance with antithyroid medications. What is the most appropriate next step in management?