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Few small-scale studies have reported a genetic and familial predisposition in Hashimoto’s thyroiditis (HT), however, quantified familial risk estimates from population-level data are unavailable. We aimed to estimate the incidence and familial risk of HT among first degree relatives (FDR) according to age, sex, and family relationships.
We conducted a population-based study in the general population of Korea from 2002-2017. Using the nationwide health insurance database, which has full population coverage and family relationship information, a cohort of 22 million individuals with blood related FDR comprising 12 million families were followed up for a familial occurrence of HT. Age- and sex- adjusted incidence risk ratios (IRRs) were calculated in individuals with an affected FDR compared to those without an affected FDR.
Among 21,940,795 individuals, 234,912‬ had an HT affected FDR, of whom 2,425 familial cases developed HT with an incidence of 7.12/10,000 person-years. The familial risk for HT was 6.5-fold (95% CI: 6.24-6.78) higher in individuals with versus without affected FDR. According to relationship, familial risks were IRR102.71, IRR7.80, IRR5.54 and IRR5.52 with an affected twin, sibling, mother and father, respectively, and the corresponding incidence (/10,000 person-years) were 115.57, 10.66, 5.73 and 5.91. Same sex twins had a three-fold higher risk of developing HT than opposite sex twins (IRR 121.01 vs 21.46). The sex-specific familial risk was higher in males than females. The risks demonstrated age dependence, being higher in younger age groups.
This study represents the largest population-based study of familial HT risk in Asia. We demonstrated elevated familial risk of incident HT among FDR, but with lower magnitude as those observed in previous studies. Familial risk increased with the degree of genetic relatedness among FDR indicating a prominent role of genetic factors in the familial aggregation of HT. Elevated risks in the younger age groups should motivate clinicians to screen people with a family history, especially those < 30 years.