Urinary phytoestrogens were associated with reduced sensitivity to thyroid hormone in NHANES 2007-1010: A large cross-sectional study

Background Estrogen and thyroid hormone interact extensively in vivo, but the relationship between phytoestrogens and thyroid function and thyroid hormone sensitivity remains unclear. Methods Using 2007–2010 NHANES data of United States, we analysed the association of 6 types of urinary phytoestrogens with thyroid function and thyroid hormone sensitivity using a multivariate linear regression model. Results A total of 2,521 participants were included in the final analysis. After adjustment for multiple factors, we found that urinary phytoestrogens were associated with thyroid function and thyroid hormone sensitivity, and the associations were sex and type specific. For thyroid function: Phytoestrogens were negatively associated with FT4 and TT4 in overall analysis. After sex stratification, Equol, Enterodiol and TT4 were negatively associated in men. In women, Daidzein, O-Desmethylangolensin (O-DMA) and Genistein were positively associated with FT4, while Enterodiol was negatively associated with FT4. In addition, Enterolactone was negatively associated with TT4. For thyroid hormone sensitivity: Phytoestrogens were associated with Thyroid feedback quantile-based index (TFQI) in overall analysis. After sex stratification, Genistein was positively associated with FT3/FT4 ratio and Enterolactone was positively associated with TFQI in men. In women, Daidzein and O-DMA were negatively associated with FT3/FT4 ratio and positively associated with TFQI. Conclusion Urinary phytoestrogens were associated with FT4 and TT4 levels and may adversely affect the central thyroid hormone sensitivity. Given the risk of hypothyroidism and SCH associated with reduced thyroid hormone sensitivity, more health monitoring of people with high phytoestrogens intake is warranted.