A 28-year-old woman presents with a 3-month history of fatigue, weight gain (4kg despite no change in diet), constipation, and feeling cold. Her periods have become heavier and irregular. On examination, her skin is dry, and she has some mild facial puffiness. Her heart rate is 58 bpm. Thyroid-stimulating hormone (TSH) is 12.5 mU/L (0.4-4.0 mU/L) and free T4 (FT4) is 7.2 pmol/L (10-22 pmol/L). What is the most appropriate management?