MON-084 - Insulinoma: A Missed Diagnosis That Almost Cost A Life

Insulinoma is a rare functional pancreatic neuroendocrine tumour characterised by autonomous insulin secretion, resulting in recurrent hypoglycaemia. Clinical features vary including adrenergic and neuroglycopenic symptoms such as tremor, syncope, and seizures. Diagnosis is based on inappropriately elevated insulin and C-peptide levels during hypoglycaemia, usually demonstrated during a supervised prolonged fast. Tumour localisation is done by cross-sectional imaging, and surgical resection is curative in most non-metastatic cases.
We present the case of a man in his 50s with recurrent hypoglycaemia who was misdiagnosed with reactive hypoglycaemia for almost a decade, resulting in significant physical morbidity and severe psychological sequelae, including a suicide attempt.
He first presented in 2016 with collapse and loss of consciousness, associated with hypoglycaemia, but was discharged with cardiology follow-up. He subsequently experienced frequent symptomatic hypoglycaemia, managed with carbohydrate intake every 1–2 hours, and was referred to endocrinology. At initial assessment, his BMI was 23 kg/m² with no significant comorbidities. A supervised fast demonstrated hypoglycaemia, including overnight, but paired insulin (56 pmol/L) and C-peptide (605 pmol/L) levels (with insulin: C-peptide ratio 0.09) were deemed non-diagnostic. CT imaging was normal, and he was labelled as having reactive hypoglycaemia despite no bariatric surgery or clear postprandial triggers.
Over subsequent years, he continued dietary management but developed severe weight gain due to frequent and nocturnal eating, with BMI increasing to 39 kg/m². His quality of life deteriorated markedly, culminating in a suicide attempt in February 2025.
Shortly thereafter, reassessment by a different endocrinologist raised suspicion of insulinoma. Continuous glucose monitoring (CGM) revealed frequent fasting and nocturnal hypoglycaemia. A repeat supervised fast induced hypoglycaemia within 3–4 hours, with glucose as low as 1.6 mmol/L and markedly elevated insulin (580 pmol/L) and C-peptide (1963 pmol/L), with an insulin: C-peptide ratio of 0.29, consistent with endogenous hyperinsulinism. MRI demonstrated a 17-mm pancreatic lesion, and endoscopic biopsy confirmed a well-differentiated insulinoma (Ki-67 2%).
The patient underwent pylorus-preserving pancreatoduodenectomy in December 2025, with complete resolution of hypoglycaemia and significant improvement in physical and mental health.
This case highlights the diagnostic challenges of insulinoma and the consequences of attributing recurrent hypoglycaemia to reactive causes. Fasting and nocturnal hypoglycaemia should prompt reconsideration of insulinoma, even with equivocal biochemistry. CGM and repeat evaluation are valuable tools, and delayed diagnosis can result in profound physical and psychological harm.

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