The consequent mobilization of stored fuels and enhancement of gluconeogenesis serve to maintain glucose levels. A failure of insulin to decrease as glucose levels fall therefore heralds inappropriate insulin secretion. Several findings suggest inappropriate insulin secretion by an insulinoma as the cause of fasting hypoglycemia. Symptoms are often neuroglycopenic in character.
Exercise can precipitate hypoglycemia during the fast.
Approximately 65 percent of patients with an insulinoma will develop Whipple's triad within the first 12 hours of a fast. Thus, the fast should not be discontinued until Whipple's triad occurs or 72 hours have elapsed. Several more complicated diagnostic tools, including insulin administration, are also used in evaluating patients with suspected islet cell tumors. The presence of such a tumor is indicated by the failure of the levels of proinsulin or C-peptide to be suppressed when exogenous insulin is given and causes hypoglycemia. Most insulinomas release excessive amounts of insulin after the intravenous administration of 1 g of tolbutamide.