Insulinoma in a patient with normal results from prolonged fast and glucagon-induced hypoglycemia

To describe a man with a functioning insulinoma and normal results from two 72-hour fasts who developed hypoglycemia secondary to exaggerated insulin response following glucagon stimulation.

We report the patient's clinical findings, laboratory findings, and clinical course. We also review the literature for previously reported cases and possible mechanisms.

A 49-year-old man presented with hypoglycemic symptoms initially occurring after jogging and well-documented symptomatic hypoglycemia occurring during an evening meal. A 72-hour fast was associated with a serum glucose concentration of 50 mg/dL, suppressed insulin and C-peptide levels, and mildly elevated β-hydroxybutyrate. Another documented episode of hypoglycemia occurring 3 hours postprandially was associated with elevated insulin and C-peptide and suppressed β-hydroxybutyrate. A second 72-hour fast provoked asymptomatic hypoglycemia (glucose concentration at 60 hours: 32 mg/dL) with suppressed insulin and measurable β-hydroxybutyrate. After 72 hours of fasting, glucagon administration led to a decrease in glucose from 50 to 18 mg/dL, elevations in insulin and C-peptide, and suppression of β-hydroxybutyrate. Computed tomography revealed a mass lesion in the pancreatic tail. Distal pancreatectomy was performed, and the resected specimen demonstrated immunostaining for insulin. Hypoglycemic symptoms resolved postoperatively.

Normal results from a prolonged fast do not preclude an insulinoma and may demonstrate exaggerated insulin secretion from the insulinoma following glucagon administration. In addition to examining the glucose response to glucagon as a surrogate for insulinoma diagnosis, measurement of serum insulin levels following glucagon administration may provide a further clue to the diagnosis of insulinoma.