A 38-year-old unmarried woman presents with 2 months of heat intolerance, hyperhidrosis, and weight loss. Physical exam: thyroid gland enlarged to degree I, no bruit, heart rate 120/min with regular rhythm. Thyroid iodine uptake: 36% at 3 hours; 90% at 24 hours. After 6 weeks of methimazole (tapazole) treatment, symptoms significantly improved with T3/T4 normalized, but WBC is 3.8×10^9/L (neutrophils 55%). The most appropriate next step is: