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Objective: Vegetarian diets may be associated with low prevalence of autoimmune disease, as observed in rural sub-Saharan Africans. Graves' disease, an autoimmune disorder, is the most common cause of hyperthyroidism. We studied prevalence of hyperthyroidism according to dietary pattern in a population with a high proportion of vegetarians., Design: Cross-sectional prevalence study. The association between diet and prevalence of hyperthyroidism was examined using multivariate logistic regression analyses controlling for sociodemographic characteristics and salt use., Setting: The Adventist Health Study-2 conducted in the USA and Canada., Subjects: Church members (n 65 981) provided demographic, dietary, lifestyle and medical history data by questionnaire., Results: The prevalence of self-reported hyperthyroidism was 0·9 %. Male gender (OR=0·32; 95 % CI 0·26, 0·41) and moderate or high income (OR=0·67; 95 % CI 0·52, 0·88 and OR=0·73; 95 % CI 0·58, 0·91, respectively) protected against hyperthyroidism, while obesity and prevalent CVD were associated with increased risk (OR=1·25; 95 % CI 1·02, 1·54 and OR=1·92; 95 % CI 1·53, 2·42, respectively). Vegan, lacto-ovo and pesco vegetarian diets were associated with lower risk compared with omnivorous diets (OR=0·49; 95 % CI 0·33, OR=0·72, 0·65; 95 % CI 0·53, 0·81 and OR=0·74; 95 % CI 0·56, 1·00, respectively)., Conclusions: Exclusion of all animal foods was associated with half the prevalence of hyperthyroidism compared with omnivorous diets. Lacto-ovo and pesco vegetarian diets were associated with intermediate protection. Further study of potential mechanisms is warranted.