Cumulative Incidence of Type 1 Diabetes in 10,168 Siblings of Finnish Young-Onset Type 1 Diabetic Patients

Cumulative Incidence of Type 1 Diabetes in 10,168 Siblings of F innish Young-Onset Type 1 Diabetic Patients Valma Harjutsalo 1 , Toomas Podar 2 and Jaakko Tuomilehto 1 3 4 1 Diabetes and Genetic Epidemiology Unit, Department of Epidemiology and Health Promotion, National Public Health Institute, Helsinki, Finland 2 Endocrinology Center, Tartu, Estonia 3 Department of Public Health, University of Helsinki, Helsinki, Finland 4 South Ostrobotnia Central Hospital, Seinäjoki, Finland Address correspondence and reprint requests to Valma Harjutsalo, National Public Health Institute, Mannerheimintie 166, FIN-00300, Helsinki, Finland. E-mail: valma.harjutsalo{at}ktl.fi Abstract The aims of our analysis were to obtain the empirical risk estimates for type 1 diabetes in the siblings of a Finnish population-based cohort of childhood-onset diabetic patients and search for demographic and other factors predicting the risk of type 1 diabetes in siblings. We defined the diabetes status of all siblings of all probands who are included in the nationwide register of Finnish cases for whom type 1 diabetes was diagnosed before age 18 years between 1965 and 1979. Siblings’ diabetes status was ascertained by a record search of nationwide registries through 2001, and the type of diabetes and date of its manifestation were obtained from medical records. The total number of person-years during the follow-up was 405,685. Of the 10,168 siblings at risk, 647 (6.4%) had been diagnosed with type 1 diabetes by 2001. The cumulative incidence of type 1 diabetes by ages 10, 20, 30, 40, and 50 years in all siblings was 1.5, 4.1, 5.5, 6.4, and 6.9%, respectively. A young age at diagnosis in the index case, paternal young-onset diabetes, male sex, and older parental age at delivery considerably increased the risk of type 1 diabetes for siblings. This large prospective family study of type 1 diabetes in siblings of childhood-onset diabetic patients provides reliable empirical estimates for the sibling recurrence risk. CDR, Central Drug Register CPR, Central Population Registry HDR, National Hospital Discharge Register Footnotes Accepted October 28, 2004. Received July 7, 2004. DIABETES