Background and aims  Hepatocyte nuclear factor‐1 alpha (HNF1A) gene mutations are the commonest cause of monogenic diabetes, but patients are often misdiagnosed as having Type 1 diabetes and started on insulin treatment. Patients with HNF1A diabetes are particularly sensitive to the glucose‐lowering effect of sulphonylureas, which are the pharmacological treatment of choice. We aimed to assess if patients do change from insulin to sulphonylurea treatment when HNF1A diabetes is confirmed and the impact of this treatment change on long‐term glycaemic control.

Methods  We investigated the clinical course of 43 patients who were insulin treated from diagnosis for a median 4 years (range 1–14) before an HNF1A gene mutation was identified.

Results  Thirty‐four patients (79%) stopped insulin following genetic testing and transferred to sulphonylureas. Twenty‐four of them (71%) remained off insulin at a median 39 months (range 17–90) post‐transfer. The 10 patients who recommenced insulin had a trend towards a longer duration of diabetes (18 vs. 7 years, P = 0.066) compared with those remaining on tablets. The median glycated haemoglobin (HbA_1c) was good (6.9%; interquartile range 6.3–8.0%) in the patients who remained off insulin and 19/24 patients (79%) achieved HbA_1c < 7.5% or improved their pre‐genetic diagnosis HbA_1c by > 1.0%. Transfer off insulin was not attempted in eight patients: one of these was planning pregnancy and two chose to remain on insulin.

Conclusion  In this observational study we found that a molecular genetic diagnosis of HNF1A diabetes does alter treatment in clinical practice, with 79% attempting transfer to sulphonylureas. Transfer to sulphonylureas was successful in the majority of patients without deterioration in glycaemic control.