A 40-year-old woman has slowly progressive fatigue and weight gain of 8kg over the last 3 months. she has no history of medical problems. On examination, HR 80 bpm, and BP 120/80mmHg. Her thyroid is firm and slightly enlarged but non-tender. Her TSH is 50mU/L and serum freeT4 is 8pmol/L. TPO antibody titer is high. Which of the following is the best therapy for this patient?