Background: Insulinoma classically presents with fasting hypoglycaemia, whereas isolated postprandial hypoglycaemia is uncommon and may delay diagnosis. Ectopic insulinoma arising from heterotopic pancreatic tissue is an exceptionally rare cause of endogenous hyperinsulinaemic hypoglycaemia.
Case: A 52-year-old man presented with a three-year history of recurrent neuroglycopenic episodes occurring 30–60 minutes after ingestion of rapidly absorbed carbohydrates. Capillary glucose during symptomatic episodes was as low as 1.3 mmol/L, while overnight fasting remained largely asymptomatic. Mixed-meal testing demonstrated profound endogenous hyperinsulinaemia, with insulin concentrations exceeding 1000 mU/L and markedly elevated C-peptide despite falling glucose concentrations. Continuous glucose monitoring (CGM) revealed persistent asymptomatic hypoglycaemia (mean glucose 3.6 mmol/L; 79% of readings <3.8 mmol/L), consistent with hypoglycaemic unawareness, while HbA1c was <4.2%. Contrast-enhanced CT identified an equivocal pancreatic lesion without characteristic enhancement. Subsequent 68Ga-DOTATATE PET/CT demonstrated intense uptake within an 18.5 mm exophytic perigastric lesion, with no pancreatic avidity. Endoscopic ultrasound confirmed a lesion external to the stomach, and biopsy demonstrated a WHO grade 1 well-differentiated neuroendocrine tumour. Surgical resection identified a 2 cm tumour arising within heterotopic pancreatic tissue adjacent to the lesser curvature of the stomach. Histopathology confirmed an insulinoma with background endocrine microadenomatosis. Following resection, hypoglycaemia resolved completely, with normalisation of CGM profiles. Postoperative evaluation demonstrated elevated parathyroid hormone with normal calcium, raising suspicion for an underlying hereditary endocrine neoplasia syndrome, and genetic testing is ongoing.
Conclusion: This case highlights that insulinoma should remain a differential diagnosis in patients with isolated postprandial hypoglycaemia. CGM may uncover clinically significant hypoglycaemic unawareness, while multimodal imaging is critical when pancreatic localisation is inconclusive. The presence of ectopic insulinoma with endocrine microadenomatosis should prompt consideration of an underlying hereditary endocrine tumour syndrome