A 35-year-old woman presents with a 3-month history of fatigue, weight gain of 8 kg, cold intolerance, and dry skin. On examination, she has a non-tender diffuse goitre and delayed relaxation of deep tendon reflexes. TSH is 18 mIU/L (normal 0.4–4 mIU/L) and free T₄ is 8 pmol/L (normal 10–20 pmol/L). Anti-TPO antibodies are positive. What is the most appropriate next step in management?