Proinsulin-secreting neuroendocrine tumor of the pancreas

Insulinoma is characterized by spontaneous fasting hypoglycemia. Diagnosis relies on inappropriately increased insulin levels (>6 microU/ml), high insulin/glucose ratio (IGR >0.3), raised proinsulin values (>5 pMol/l). A 74-yr-old man was referred to us for episodes of symptomatic hypoglycemia without hyperinsulinemia and imaging [abdominal computed tomography (CT) and magnetic resonance scans] negative for neuroendocrine tumor (NET). During hospitalization severe hypoglycemic crises persisted requiring continuous glucose iv infusion. Insulin values (immunofluorimetric method) were not inappropriately increased, accordingly IGR was normal but C-peptide was in the upper-normal range. Proinsulin levels measured with specific radioimmunoassay were remarkably high. Octreoscan study was negative whereas endoscopic ultrasound disclosed a 10 mm lesion in the body of the pancreas, confirmed by rapid spiral CT scanning with dynamic images. Increased proinsulin levels allowed diagnosis of a secreting NET. After removal of the lesion, the patient experienced hyperglycemia. Histology confirmed a benign NET positively staining for insulin. In conclusion, proinsulin assay is of particular help when immunoreactive insulin, measured by specific new immunometric assays (immunoenzymometric and immunofluorimetric assays), is normal. These methods have good precision and specificity (no cross reactivity with intact or Des 31,32 proinsulin), but rare insulinomas secreting most, or all, of their insulin-like activity as proinsulins would go undetected if insulin levels alone were measured.