Proinsulinomas are extremely rare pancreatic neuroendocrine tumours (NETs) that can be a diagnostic dilemma. These may be missed without methodical, extensive work up and specialised imaging is necessary to arrive at a final diagnosis. We present, a 34 year old gentleman, misdiagnosed with an anxiety disorder for two years before seeing an endocrinologist.
Our patient reported episodes of severe neuroglycopaenic symptoms rectified on eating. He reported low capillary glucose levels, down to 1.1 mmol/L. To prevent episodes of hypoglycaemia, he was consuming carbohydrates every two hours, with over 10kg of weight gain. Physical examination revealed a high body mass index of 32 kg/m2 but no features suggestive of an endocrinopathy. Biochemistry revealed, normal fasting insulin, C-peptide and beta hydroxybutyrate levels following a 12 hour fast, but a raised proinsulin level of > 99.9pmol/L (<13.3). A CT of his abdomen did not demonstrate focal pancreatic lesions. MRI along with DOTATE-PET localised a small circumscribed lesion within the neck of pancreas. He proceeded to have a Whipple’s procedure, and now two years following surgery, he remains well, with no suggestion of recurrence clinically or radiologically.
Pancreatic NETS are a heterogenous group of islet-cell tumours with a tendency for hormonal production. Among this group, proinsulinomas are very rare. Whilst insulinomas and proinsulin-secreting NETs have many parallels it is important to understand their differences. Proinsulinomas have elevated proinsulin levels with low or normal insulin levels, as opposed to insulinomas, which mostly presents with elevated insulin levels. Diagnosis of organic hypoglycaemia can be challenging in the absence of elevated fasting insulin levels, hence the need to include proinsulin levels in the work up1. Tumour localisation is often challenging and may require multiple sequential imaging modalities2. Surgery is the mainstay of treatment1. Long-term follow up is important due to potential for recurrence and metastasis.