A 32-year-old woman presents to her GP with a 3-month history of fatigue, weight gain, and feeling cold all the time. She also reports constipation and dry skin. She has no significant past medical history and is not on any medications. On examination, her blood pressure is 120/80 mmHg, heart rate is 60 bpm, and she has a slightly enlarged thyroid gland. Laboratory tests reveal a low free T4 level and an elevated TSH level. Which of the following is the most appropriate initial treatment for this patient?