A 30-year-old woman has recurrent symptomatic hypoglycaemia after prolonged fasting. Supervised 72-hour fast: glucose 41 mg/dL, insulin 18 µU/mL, C-peptide 3.8 ng/mL, proinsulin 22 pmol/L, β-hydroxybutyrate 0.2 mmol/L, negative sulfonylurea screen. Which is the most likely diagnosis?
- A Surreptitious insulin injection
- B Non-islet cell tumour hypoglycaemia (IGF-II mediated)
- C Insulinoma ✓
- D Insulin autoimmune syndrome (Hirata disease)
Correct answer: C. Insulinoma
Explanation
Insulinoma produces endogenous hyperinsulinaemia: elevated insulin, C-peptide, and proinsulin with suppressed β-hydroxybutyrate and absent sulfonylurea. C-peptide is co-secreted with insulin by beta cells, so elevated C-peptide excludes exogenous insulin injection. Non-islet cell tumour hypoglycaemia is driven by big-IGF-II with suppressed insulin, and Hirata disease shows very high total insulin with high-molecular-weight complexes. Diagnostic Whipple's triad is fulfilled here with the biochemical pattern consistent with insulinoma.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.