Context: The mechanisms leading to thyroid autoimmunity are largely unknown.

Objective: To assess the role of environment in the development of thyroid autoimmunity.

Design: Prevalence of thyroid autoantibodies in two neighboring populations living in completely different socio-economic circumstances (Russian Karelia and Finland).

Setting: Two population-based cohorts sharing partly the same ancestry.

Patients or Other Participants: 532 schoolchildren from Russian Karelia and 532 schoolchildren in Finland matched for the age, gender, and season of the blood sampling.

Interventions: None.

Main Outcome Measures: Prevalence of thyroid peroxidase antibodies (TPOAb), thyroglobulin antibodies (TGAb), and HLA-DQ alleles.

Results: The prevalence of TPOAb was significantly lower in Russian Karelian than in Finnish children (0.4% vs. 2.6%, P=0.006). A similar difference was observed for TGAb (0.6% vs. 3.4%, P=0.002). Finnish girls tested more frequently positive for both TPOAb (4.3% vs. 0.4%, P=0.01) and TGAb (5.3% vs. 0.9%, P=0.01) than boys. Seven of the 23 tested subjects with signs of thyroid autoimmunity (30%) had increased serum TSH concentrations as a sign of subclinical hypothyroidism. The frequency of HLA genotypes did not differ between the two countries or between autoantibody-positive and -negative subjects.

Conclusions: The prevalence of thyroid autoimmunity is lower in Russian Karelia than in Finland. This difference was not related to ethnic background or HLA-DQ alleles. The results support the idea that the Russian Karelian environment which is characterized by inferior prosperity and standard of hygiene may provide protection against thyroid autoimmunity.