Late postprandial hypoglycemia due to bioactive insulin dissociation from autoantibody leading to unconsciousness in a patient with insulin autoimmune syndrome.

We report here the case of an 83-year-old man who was treated for unconsciousness and hypoglycemia (39 mg/dL) accompanied by marked elevation of serum immunoreactive insulin (IRI) (4,760 μIU/mL). We diagnosed his condition as insulin autoimmune syndrome (IAS, Hirata disease) because of a high insulin autoantibody (IAA) titer (>90%: bound/total) and no history of exogenous insulin administration. Reactive hypoglycemia occurred due to immediate association followed by dissociation between insulin and insulin autoantibodies after glucose or food intake. An α-glucosidase inhibitor in combination with frequent small meals reduced the postprandial hyperglycemia (glucose spike) and ameliorated the reactive hypoglycemia.