Hypoglycemia due to a monoclonal insulin-binding antibody in multiple myeloma

Recurrent, severe hypoglycemia in the absence of exogenous insulin administration is uncommon. Reported causes include islet-cell adenomas or carcinomas, surreptitious use of insulin or sulfonylurea medications, stimulatory insulin-receptor antibodies, and various non-islet-cell tumors. We describe a patient who presented with a hypoglycemic seizure and who had many of the features of the autoimmune insulin syndrome. On investigation, however, it became clear that the patient had multiple myeloma and that the hypoglycemia was due to a monoclonal insulin-binding antibody produced by the tumor.