Islet autoimmune syndrome (IAS) is an autoimmune disease caused by high concentrations of insulin autoantibodies (IAA) in the blood. It is characterized by hyperinsulinemia and spontaneous hypoglycemia. The incidence of IAS is low, and the hypoglycemia symptom is usually mild. Hence, the severe manifestations (up to seizures and coma) are rarely reported. Here, we reported two cases of Graves' disease who developed insulin autoimmune syndrome after methimazole treatment. The patients exhibited sudden hypoglycemic coma after receiving methimazole treatment for approximately 2 or 6 months. The patients' serum glucose levels were below 2.8 mmol/L, and laboratory tests showed high levels of serum insulin and high titers of insulin autoantibodies. Patient 1 discontinued methimazole treatment and the hypoglycemic symptoms disappeared after 7 days. However, patient 2 experienced severe hypoglycemia after discontinuation of methimazole, and the patient condition improved after glucocorticoid therapy. He developed thyroid storm during the treatment, and his condition improved after receiving standard treatment procedures for thyroid storm. To the best of our knowledge, this is the first case report of IAS in Graves' disease with thyroid storm.