A 46-year-old woman with a history of Graves' disease presented with the chief complaints of appetite loss, weight loss, fatigue, nausea, and sweating. She was diagnosed with diabetic ketoacidosis (DKA), thyroid storm, and influenza A. She was treated with an intravenous insulin drip, intravenous fluid therapy, intravenous hydrocortisone, oral potassium iodine, and oral methimazole. As methimazole-induced neutropenia was suspected, the patient underwent thyroidectomy. It is important to maintain awareness that thyroid storm and DKA can coexist. Furthermore, even patients who have relatively preserved insulin secretion can develop DKA if thyroid storm and infection develop simultaneously.