Clinical meaning
Insulinoma is the most common functioning pancreatic neuroendocrine tumor, arising from pancreatic beta cells that autonomously secrete insulin independent of blood glucose levels. The incidence is approximately 1-4 per million person-years. Understanding the pathophysiology and diagnostic approach is essential for the NP evaluating recurrent hypoglycemia.
Normal insulin secretion is tightly regulated by blood glucose: as glucose falls below 70 mg/dL, insulin secretion is suppressed (essentially turned off) while counterregulatory hormones (glucagon, epinephrine, cortisol, growth hormone) are activated. In insulinoma, neoplastic beta cells continue to secrete insulin autonomously despite hypoglycemia, creating a pathological state of endogenous hyperinsulinism. The ongoing insulin secretion drives glucose into cells, suppresses hepatic gluconeogenesis and glycogenolysis, and inhibits lipolysis — all of which prevent the normal counterregulatory response to hypoglycemia.
Whipple's triad is the clinical hallmark used to confirm symptomatic hypoglycemia: (1) symptoms consistent with hypoglycemia (neuroglycopenic — confusion, visual changes, behavioral changes, seizures, loss of consciousness; and adrenergic — tremor, diaphoresis, palpitations, anxiety), (2) documented low blood glucose at the time of symptoms (<55 mg/dL during a monitored fast), and (3) resolution of symptoms upon glucose administration. All three criteria must be met to establish the diagnosis of clinically significant hypoglycemia.
The 72-hour supervised fast is the gold standard diagnostic test. The patient fasts under continuous observation with serial glucose, insulin, C-peptide, and proinsulin measurements every 6 hours (and every 1-2 hours when glucose falls below 60 mg/dL). In insulinoma, the fast reveals: low glucose (<55 mg/dL), inappropriately elevated insulin (≥3 mcU/mL when glucose is low — normally insulin would be suppressed to <3), elevated C-peptide (≥0.6 ng/mL — confirms endogenous insulin source, distinguishes from exogenous insulin administration where C-peptide would be low), elevated proinsulin (≥5 pmol/L), and negative sulfonylurea/meglitinide screen (rules out surreptitious medication use). Approximately 95% of insulinoma patients develop symptomatic hypoglycemia within 48 hours of fasting, and 99% within 72 hours.