Insulin Autoimmune Syndrome (IAS) is a rare cause of hypoglycemia. We present the case of a 54-year-old male patient who was admitted to the emergency department with blurred consciousness approximately 6 weeks after the initiation of the methimazole therapy due to the diagnosis of Graves' disease. The blood glucose level was 40 mg/dL and levels of fasting insulin, C-peptide, and anti-insulin antibodies were increased. The patient was considered to have IAS and the methimazole therapy was terminated. Following the termination of the treatment, the complaints disappeared and the insulin antibodies gradually decreased to normal levels in approximately 8 months.