A 38-year-old woman from Delhi presents with progressive weight loss (8 kg over 3 months), heat intolerance, and tremor. Laboratory investigations reveal elevated free T4 (18 pg/mL, normal 7.5–11) and suppressed TSH (0.01 mIU/L). She denies palpitations or dyspnea at rest. Physical examination shows a non-tender, diffuse thyroid enlargement without nodules. On further questioning, she reports no recent iodine exposure, no neck radiation history, and no family history of autoimmune thyroid disease. What is the most appropriate next step in management?