A 2-year-old girl with Down syndrome is brought to the pediatric outpatient clinic for routine follow-up. Her parents report she has been increasingly irritable over the past 3 months, with poor feeding and constipation. Physical examination reveals a firm, enlarged thyroid gland. Laboratory investigations show TSH 8.2 mIU/L (normal 0.5–5.0) and free T4 2.1 ng/dL (normal 0.8–1.8). What is the most appropriate next step in management?