title: Type 1 diabetes and multiple sclerosis: A Danish population-based cohort study.
abstract: BACKGROUND
  Type 1 diabetes mellitus (T1D) and multiple sclerosis (MS) contribute considerably to the burden of autoimmune diseases in young adults. Although HLA patterns of T1D and MS are considered mutually exclusive, individual and familial co-occurrence of the 2 diseases has been reported.
  
  
  OBJECTIVE
  To assess the co-occurrence of T1D and MS by estimating the risk for MS in patients with T1D and the risk for T1D in first-degree relatives of patients with MS.
  
  
  METHODS
  Two population-based disease registers, the Danish Hospital Discharge Register and the Danish Multiple Sclerosis Register were used to identify patients with T1D, defined as patients in whom diabetes was diagnosed before age 20 years (N = 6078), and patients with MS (N = 11 862). First-degree relatives (N = 14,771) of patients with MS were identified from family information in the Danish Civil Registration System.
  
  
  METHODS
  Patients with T1D and first-degree relatives of patients with MS were followed up for occurrence of MS and T1D, respectively, and the relative risks were expressed as standardized incidence ratios, that is, ratios of observed to expected numbers of outcomes based on national age, sex, and period-specific MS and T1D incidence rates.
  
  
  RESULTS
  Patients with T1D were at more than 3-fold increased risk for development of MS (relative risk, 3.26; 95% confidence interval, 1.80-5.88; n = 11). First-degree relatives of patients with MS were at 63% increased risk (relative risk, 1.63; 95% confidence interval, 1.26-2.12; n = 56) for development of T1D. However, adjusting for familial relationship to patients with T1D reduced the excess risk to 44% (relative risk, 1.44; 95% confidence interval, 1.11-1.88; n = 56).
  
  
  CONCLUSIONS
  The present nationwide cohort study demonstrates an intraindividual and, to a lesser degree, an intrafamilial co-occurrence of MS and T1D.
authors.affiliation: Department of Epidemiology Research, Statens Serum Institut, Copenhagen S Denmark. NMN@ssi.dk
authors.forename: Nete M
authors.initials: NM
authors.lastname: Nielsen
authors.forename: Tine
authors.initials: T
authors.lastname: Westergaard
authors.forename: Morten
authors.initials: M
authors.lastname: Frisch
authors.forename: Klaus
authors.initials: K
authors.lastname: Rostgaard
authors.forename: Jan
authors.initials: J
authors.lastname: Wohlfahrt
authors.forename: Nils
authors.initials: N
authors.lastname: Koch-Henriksen
authors.forename: Mads
authors.initials: M
authors.lastname: Melbye
authors.forename: Henrik
authors.initials: H
authors.lastname: Hjalgrim
mesh_terms: D000293:Adolescent; D000328:Adult; D015331:Cohort Studies; D015897:Comorbidity; D003718:Denmark / Q000453:epidemiology; D003922:Diabetes Mellitus, Type 1 / Q000453:epidemiology*; D005192:Family Health; D005260:Female; D006801:Humans; D008297:Male; D009103:Multiple Sclerosis / Q000453:epidemiology*; D010290:Parents; D012042:Registries; D012307:Risk Factors; D035781:Siblings