"Please see this man with a 69-year history of hypoglycaemia".

Mutations in the HNF4A gene are associated with hyperinsulinaemic hypoglycaemia in infants, frequently evolving into relative deficiency of insulin in adulthood ---as maturity onset diabetes of the young (MODY). A 69-year-old male with a strong family history of adult-onset diabetes was referred with lifelong hypoglycaemia, found to be due to a pathogenic HNF4A mutation. HbA1c levels were low, continuous glucose monitoring demonstrated frequent low glucose events in the early morning, and he was successfully treated with diazoxide. This case represents a new phenotype of a known mutation associated more commonly with MODY. The same mutation in one family led to profoundly different manifestations. Genetic causes of hyperinsulinaemic hypoglycaemia can present late in life and identifying such cases is important to allow the correct treatment to be established.