Pseudoinsulinoma syndrome from inadvertent tolazamide ingestion.

A 70-year-old woman with rheumatoid arthritis presented with fasting hypoglycemia, inappropriately elevated insulin and C-peptide levels, and negative insulin antibodies, all compatible with the diagnosis of an insulinoma. However, results of a 72-hour fast were subsequently negative. Medication identification revealed that the patient had been taking tolazamide (Tolinase; The Upjohn Company, Kalamazoo, Michigan) instead of tolmetin (Tolectin; McNeil Laboratories, Inc., Fort Washington, Pennsylvania).