An insulinoma without fasting hypoglycemia.

A patient with a 25-year history of episodic irritability, confusion, and unconsciousness was suspected of having organic hypoglycemia. He was fasted for a 72-hour period without developing symptoms. At the end of the fast his blood sugar was 85 mg/100 ml and the immunoreactive insulin (IRI) concentraion was elevated to 350 microunits/ml. Surgical exploration demonstrated a 4 x 3 centimeter insulinoma in the tail of the pancreas. Since proinsulin-like components (PLC) are known to be elevated in patients with islet cell tumors and have only 5 to 10 per cent of the biologic activity of insulin, it is suggested that a major portion of the IRI was PLC. This case demonstrates that the inability to withstand prolonged fasting does not exclude the diagnosis of an insulinoma and suggests a mechanism whereby such patients can tolerate fasting.