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Summary Activating mutation of glucokinase gene (GCK) causes resetting of insulin inhibition at a lower glucose threshold causing hyperinsulinaemic hypoglycaemia (GCK-HH). This is the first reported case who tolerated years of regular fasting during Ramadhan, presenting only with seizure and syncope now. We describe a case with GCK gene variant p.T65I diagnosed in a 51-year-old woman with hypoglycaemia unawareness even at glucose level of 1.6 mmol/L. Insulin and C-peptide levels during hypoglycaemia were suggestive of hyperinsulinism, but at a day after intravenous glucagon, hypoglycaemia occurred with low insulin and C-peptide levels, pointing against insulinoma as the underlying aetiology. Imaging studies of the pancreas and calcium arterial stimulation venous sampling were unremarkable. A review of old medical records revealed asymptomatic hypoglycaemia years ago. Genetic testing confirmed activating mutation of GCK. Hypoglycaemia was successfully controlled with a somatostatin analogue. This case highlights the importance of consideration of genetic causes of hypoglycaemia in adulthood, especially when imaging is uninformative. Learning points Consider genetic causes of endogenous hyperinsulinism hypoglycaemia in adulthood, especially when imaging is uninformative. Late presentation of activating mutation of GCK can occur because of hypoglycaemia unawareness. Long-acting somatostatin analogue may be useful for the treatment of activating mutation of GCK causing hypoglycaemia. Depending on the glucose level when the blood was taken, and the threshold of glucose-stimulated insulin release (GSIR), the serum insulin and C-peptide levels may be raised (hyperinsulinaemic) or low (hypoinsulinaemic) in patients with activating mutation of GCK. Glucagon may be useful to hasten the process of unmasking the low insulin level during hypoglycaemia below the GSIR level of which insulin released is suppressed.