A 28-year-old woman presents with a 3-month history of progressive fatigue, weight gain of 8 kg, cold intolerance, and dry skin. On examination, she has a regular pulse of 58 bpm, delayed ankle reflexes, and a diffuse, non-tender thyroid enlargement. Laboratory investigations show: TSH 18.5 mIU/L (normal 0.4–4.0), free T4 0.8 ng/dL (normal 0.8–1.8), free T3 2.1 pg/mL (normal 2.3–4.2). Thyroid peroxidase (TPO) antibodies are positive at 450 IU/mL. What is the most appropriate immediate next step in management?