A population-based study of chronic autoimmune hypothyroidism in Danish twins

Hashimoto's thyroiditis (HT), atrophic thyroiditis (AT), and Graves' diseas e are autoimmune thyroid diseases in which genetic factors are suspected to play an important role in disease susceptibility. In a recent population-b ased twin study we rendered it probable that a substantial part of the susc eptibility to Graces' disease is attributable to genetic factors. At presen t there are no population-based twin studies supporting such a genetic infl uence in the etiology of HT/AT. To elucidate whether there is a genetic influence in the etiology of HT/AT, we studied the distribution of HT/AT in a population-based sample of 2945 Danish female-female twin pairs (5890 individuals) born between 1953 and 19 72. Information on hypothyroidism was obtained from a nationwide questionna ire survey in 1994. Information from hospitals, out-patient clinics, genera l practitioners, and specialists was sought to verify the diagnosis. The overall prevalence of autoimmune hypothyroidism was 0.41% (24 of 5890). The prevalence did not differ between monozygotic and dizygotic twins (0.4 2% and 0.40%, respectively). The crude proband-wise concordance rates were significantly higher for monozygotic compared to dizygotic twin pairs: 0.55 (95% confidence interval, 0.23-0.83) us. 0.0 (95% confidence interval, 0.0 -0.25; P = 0.01). All of the healthy cotwins (n = 15) of twins with clinica lly overt autoimmune hypothyroidism were biochemically euthyroid. Overall, regardless of zygosity 53% (8 of 15) of the healthy cotwins were positive f or antithyroid antibodies. The prevalence of autoantibodies among the monoz ygotic cotwins was 80% (4 of 5) and 40% (4 of 10) among dizygotic cotwins ( P = 0.36). In conclusion the higher concordance rate in monozygotic compared to dizygo tic pairs indicates that genetic factors play a role in the etiology of HT/ AT among Caucasian women living in areas with borderline iodine deficiency. However, the fact that the concordance rate among MZ twins was below 1 sug gests that environmental factors also are of etiological importance.