Urinary Isoflavones Levels in Relation to Thyroid Function in Adults from NHANES 2007-2010

Abstract

Background: Isoflavones are phytoestrogens commonly found in plant-based foods and nutritional supplements. Some studies have reported inverse associations between isoflavone intake and selected health outcomes, including cancer, cardiovascular disease, and osteoporosis. However, there is growing concern that isoflavones may affect thyroid status. Experimental studies have shown a relationship between isoflavones and hypothyroidism, but human studies have reported conflicting results.Method: Using cross-sectional data from the National Health and Nutrition Examination Survey (2007-2010), we examined the association between urinary isoflavone concentrations and serum measures of thyroid function in adults free of thyroid disease. Specifically, we conducted multivariable analyses relating the concentrations of each urinary isoflavone (genistein, daidzein) and their metabolites (equol, O-desmethylangolensin [O-DMA]) to serum concentrations of thyroid stimulating hormone [TSH], free triiodothyronine [T3], total T3, free thyroxine [T4], total T4, thyroperoxidase [TPO] antibodies, adjusting for sociodemographics, creatinine, iodine and thiocyanate.Results: We observed positive associations between daidzein and O-DMA and free T4, and an inverse association between equol and total T4. Daidzein and O-DMA associations were stronger among women: a 10-fold increase in daidzein was related to a 3.2% (95% CI: 1.9%, 4.5%) increase in free T4, and a 10-fold increase in O-DMA was related to a 2.0% (95% CI: 1.1%, 2.9%) increase in free T4. The equol association was stronger among men: a 10-fold increase in equol was related to a 1.8% decrease (95% CI: -3.4%, -0.13%) in total T4.Conclusion: In the present study, isoflavone intake was associated with selected thyroid hormones in a sex-dependent fashion.