Nonautonomous Function of a Pancreatic Insulinoma

A 56-year-old woman with symptoms of weakness, visual blurring, and sweating underwent diagnostic studies to evaluate the etiology of her hypoglycemia. Fasting hypoglycemia was never documented; in diagnostic studies performed during her two hospitalizations and several outpatient glucose tolerance tests, the lowest fasting plasma glucose recorded was 56 mg/dl. The patient displayed exaggerated plasma insulin responses following oral glucose (peak response: 447 muU/ml at 30 min) and following 1 gm of iv tolbutamide (peak response: 719 muU/ml at 5 min) with symptomatic profound hypoglyceria during both tests. Basal per cent proinsulin was elevated at 49% (normal range 5-22%). Throughout a 72 h fast, values for plasma glucose, insulin, and glucose/insulin ratios were all within the normal range. During the infusion of exogenous insulin (0.1 U/kg for 60 min) serum C-peptide reactivity suppressed to less than 1.3 ng/ml when the plasma glucose fell below 40 mg/dl representing normal suppression. At surgery, a pancreatic beta cell adenoma was found and removed. This patient represents the uncommon circumstances in which stimulation tests with tolbutamide and glucose were more helpful in establishing a preoperative diagnosis than were the suppression tests.