An insulinoma is a rare tumour derived from the beta cells of the pancreas producing large amounts of insulin. As explained in the physiology section, insulin is released from the normal pancreas in response to increased glucose levels in the blood (e.g. after a meal). The insulin allows tissues such as muscle to take up glucose. Excessive insulin production causes a persistently low glucose blood concentration (hypoglycaemia) that results in a diverse array of symptoms
99% of Insulinomas are in the pancreas, with 1% arising outside it but always in near structures (such as the duodenum). They are generally small (1-2 cm in diameter).
The majority (90-95%) are benign, the remainder being malignant.
Although called insulinomas, such tumours are often capable of secreting a large number of other active hormones in addition to insulin. These include human chorionic gonadotrophin (HCG), corticotrophin (ACTH), serotonin (5-HT), gastrin, glucagon, somatostatin and pancreatic peptide (PP). They also occur as part of the syndrome Multiple Endocrine Neoplasia type 1 (MEN-1).
The estimated prevalence is around 1 in 1,125,000 people. There appears to be no preponderance in either sex and it occurs in all age groups, but incidence peaks between 40 and 60 years of age.
An insulinoma is essentially a genetic condition. Specific gene abnormalities are being examined at the moment, particularly in its connection with MEN-1 but no consensus has yet been reached.
No environmental factors that predispose to the development of insulinomas have been found.