Glucose-Responsive Insulinoma in a Patient with Postprandial Hypoglycemia in the Morning

We report the case of an obese 79-year-old woman who experienced postprandial hypoglycemia in the morning. The serum immunoreactive insulin (IRI) and C-peptide levels responded in parallel with her serum glucose level during a 75-g oral glucose tolerance test. A prolonged fast test lowered her serum glucose level to 30 mg/dL, but serum IRI was not fully suppressed. Abdominal computed tomography revealed a tumor in the uncinate process of the pancreas. The tumor was histologically diagnosed as benign insulinoma after surgery. Therefore, glucose-responsive insulinoma as well as reactive hypoglycemia should be considered in patients who exhibit postprandial hypoglycemia.