Insulinoma and Whipple Triad
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Direct answer
Insulinoma is a neuroendocrine tumour of pancreatic beta cells that secretes insulin autonomously, causing recurrent neuroglycopenic and adrenergic symptoms. The Whipple triad, comprising symptoms of hypoglycaemia, a documented low plasma glucose during those symptoms, and relief after glucose, supports the diagnosis. Confirmation rests on a supervised fast in which insulin and C-peptide are inappropriately high at a glucose of about 50 to 55 mg/dL.
What you must remember
- Whipple triad: symptoms of hypoglycaemia, a documented low plasma glucose during those symptoms, and relief of symptoms after glucose intake.
- Supervised fast: performed in hospital for up to 72 hours, with regular glucose checks, and stopped when hypoglycaemia with symptoms develops or at 72 hours.
- Insulinoma criteria: insulin of at least 3 µU/mL, C-peptide of at least about 0.6 ng/mL, proinsulin of at least 5 pmol/L and beta-hydroxybutyrate at or below 2.7 mmol/L at the time of hypoglycaemia.
- Factitious hypoglycaemia: exogenous insulin raises insulin while suppressing C-peptide, whereas sulfonylurea use raises both and is detected by a drug screen.
- Localisation: CT or MRI is usually done first, and endoscopic ultrasound is highly sensitive for small lesions, with functional PET used when the findings are equivocal.
- Nature: most insulinomas are benign and solitary, and a minority occur as part of multiple endocrine neoplasia type 1.
- Treatment: surgical enucleation or resection is definitive, while diazoxide (octreotide in selected patients) controls symptoms before surgery.
Working up a suspected insulinoma
The diagnosis depends on capturing the hormone pattern at the moment of hypoglycaemia. The steps below set out how the sample is taken and read.
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Document the Whipple triad. Record symptoms with a measured low glucose and confirm that glucose rises after intake, since spontaneous hypoglycaemia is the key observation.
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Run the supervised fast. Admit the patient, check glucose at regular intervals, and stop the test when symptomatic hypoglycaemia occurs, ideally with samples taken at that point.
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Send the full panel together. Insulin, C-peptide, proinsulin, beta-hydroxybutyrate and a sulfonylurea screen are measured on the same sample to separate endogenous from exogenous causes.
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Interpret the pattern. Inappropriately high insulin with high C-peptide and proinsulin, and low ketones, supports an insulinoma. A glucagon response on testing can add support.
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Localise and plan surgery. CT or MRI is usually done first, endoscopic ultrasound is the most sensitive localising test, and a multidisciplinary review decides between enucleation and resection.
| Scenario | Insulin | C-peptide | Key clue |
|---|---|---|---|
| Insulinoma | High | High | Ketones low, proinsulin raised |
| Exogenous insulin | High | Suppressed | Insulin injection history or vial |
| Sulfonylurea use | High | High | Positive drug screen |
The table shows that C-peptide is suppressed by exogenous insulin but high in insulinoma and sulfonylurea use. The drug screen and proinsulin then complete the distinction.
Where the exam tries to catch you
Insulinoma questions usually test the fasting protocol, the C-peptide interpretation or the first-line treatment.
- Trap: treating a normal short-duration glucose test as excluding insulinoma. Correct: the diagnosis requires a supervised fast, since spontaneous hypoglycaemia may appear only after prolonged fasting.
- Trap: reading high insulin with high C-peptide as exogenous insulin. Correct: exogenous insulin suppresses endogenous C-peptide, so high C-peptide points to endogenous insulin from an insulinoma or a sulfonylurea.
- Trap: choosing octreotide over surgery as definitive therapy. Correct: surgical enucleation or resection is definitive for a localised insulinoma, while diazoxide, and octreotide in selected patients, only controls symptoms beforehand.
Frequently asked questions
What does the insulin and C-peptide pattern show in insulinoma?
In insulinoma, insulin and C-peptide are both inappropriately high during hypoglycaemia, reflecting autonomous endogenous insulin secretion from the tumour. This pattern separates insulinoma from exogenous insulin use, where C-peptide is suppressed. Proinsulin and beta-hydroxybutyrate measurements add further support to the diagnosis.
Why is a supervised 72-hour fast used to diagnose insulinoma?
A supervised fast of up to 72 hours provokes hypoglycaemia in patients whose tumour secretes insulin autonomously, and it allows samples to be taken during that episode. Glucose is checked regularly and the fast is stopped when symptomatic hypoglycaemia develops. Supervision makes the test safe and the samples meaningful.
How does factitious hypoglycaemia from sulfonylureas differ from insulinoma?
Sulfonylurea use raises insulin and C-peptide together, which can mimic insulinoma, so a drug screen is needed to detect the sulfonylurea in serum or urine. Exogenous insulin, by contrast, raises insulin while suppressing C-peptide. The drug screen and C-peptide pattern therefore distinguish factitious causes from insulinoma.
Which imaging is used to localise an insulinoma?
Endoscopic ultrasound is highly sensitive for small pancreatic insulinomas, and CT or MRI is used for larger or extrapancreatic lesions. Functional PET imaging using a GLP-1 receptor tracer may help when conventional imaging is equivocal. Localisation is confirmed before surgical planning.