Abstract
CONTEXT: Autoimmune hypothyroidism, commonly known as Hashimoto thyroiditis (HT), is an autoimmune thyroid disorder affecting approximately 5% of the US population. Previous relative risk studies have suggested that first-degree relatives of individuals with HT are at ∼4.5 to 32 times higher risk for developing HT than the general population. Twin studies estimate high heritability for the development of HT (∼65%). OBJECTIVE: In this study, we aimed to better estimate the HT relative risk in first-, second-, and third-degree relatives in the Utah Population Database. METHODS: From the Utah Population Database, a total of 92 405 HT probands and 184 810 matched controls were identified, with 2 960 650 relatives of HT probands and 5 730 159 relatives of controls, making this the largest relative risk study of HT. RESULTS: Females with HT in this cohort were 2.71-fold more common than males. The odds ratio (OR) of HT in the first-degree relatives of affected individuals is 1.77 (95% CI, 1.74-1.80). The OR of HT in second-degree relatives is 1.23 (95% CI, 1.22-1.27) and 1.11 (95% CI, 1.10-1.12) in third-degree relatives of HT probands. We also identified an increased OR of spouses to develop HT of 1.50 for husbands of affected wives (95% CI, 1.39-1.61) and 1.58 for wives of affected husbands (95% CI, 1.47-1.70), suggesting a significant environmental component contributing to HT development. CONCLUSION: This is the first study to estimate an increased risk of HT for second- and third-degree relatives, who are less likely to share common environments than first-degree relatives.