Two siblings were diagnosed with autoimmune hypothyroidism at age 5 and 16 months, presenting with classic symptoms of hypothyroidism, abnormal thyroid function tests (TSH: 200 and 660 mU/L; reference range (RR): 0.73–8.4 mU/L; Free T4: 5.9 and <1.3 pmol/L; RR: 11.9–25.6 pmol/L), and high thyroid peroxidase antibody levels. The gene discussed is TPO; the disease is hypothyroidism.