An insulinoma refers to a tumour of the beta cells within the Islets of Langerhan of the pancreas. It is a favourite medical school topic to assess in written examinations.
are normally benign tumours (rarely malignant) that cause an excessive secretion of insulin
. This leads to recurrent fasting hypoglycaemia due to an inappropriately high level of insulin for the blood glucose concentration. Insulinomas may occur de novo
, or in association with multiple endocrine neoplasia (MEN), an autosomal dominant inherited condition. See our notes on MEN syndromes
The diagnosis is usually made through a combination of both functional testing (confirms endogenous hyperinsulinaemia) and imaging (localises the tumour).
This involves a prolonged 72 hour fast, which aims to induce an episode of hypoglycaemia with subsequent measurement of insulin and C-peptide levels. C-peptide is part of the precursor proinsulin, which is released by beta cells and then cleaved to form insulin and C-peptide. In an insulinoma, there is an inappropriately high level of insulin and C-peptide. In the absence of an underlying disorder, a 72 hour fast should not lead to hypoglycaemia due to counterregulatory hormones and a reduction in insulin secretion.
Following functioning testing, there are a variety of imaging modalities that can be used to localise the tumour (e.g. computed tomography, magnetic resonance imaging). If these are negative, more invasive investigations may be required.