Background: Hypoglycaemia is more frequent among diabetic patients as a result of hypoglycaemic lowering therapies, making the diagnosis and management straightforward. However, when documented in nondiabetic people, it becomes a diagnostic problem. Despite their seemingly benign behaviour, the aetiology and posterior treatment is of crucial importance, since recurrences are frequent and prolonged hypoglycaemia can cause serious brain lesions, making mortality higher among nondiabetic patients. Case Presentation: A 36-year-old man was admitted to our hospital due to an episode of altered mental status with documented hypoglycaemia. The patient had no medical conditions known and was only on zonidamide. He denied drinking alcoholic beverages and had no other symptoms. Physical examination, on the emergency department, as well as blood test was normal, and he was admitted to study the cause of hypoglycaemia. After the 72-hour fast test, endogenous hyperinsulinism was diagnosed and the team initiated the investigation of a possible insulinoma, which was later found to be in pancreatic location. The patient was referred for surgical resection of the insulinoma and was discharged home 7 days after the surgery. He recovered well without further symptoms of hypoglycaemia. Conclusion: The authors want to highlight the rarity of hypoglycaemia among non-diabetic patients emphasising endogenous hyperinsulinism as an unusual cause of it, infrequently as a result of an insulinoma. Despite its rarity, once the correct treatment is chosen the prognosis is good and the overall survival rate does not differ from that expected in the general.

Keywords: Spontaneous hypoglycaemia, endogenous hyperinsulinism, Insulinoma