Thyroid Function and Phytoestrogens in General US Population

Data from National Health and Nutrition Examination Survey for the years 2007-2010 were used to evaluate the association between serum levels of six thyroid variables, namely, thyroid stimulating hormone (TSH), free and total triiodothyronine (FT3, TT3), free and total thyroxin (FT4, TT4), and thyroglobulin with the urinary levels of six phytoestrogens, namely, daidzein (DAZ), o-desmethylangolensin (DMA), equol (EQU), enterodiol (ETD), enterolactone (ETL), and genistein (GNS). Regression models with log10 transformed values of thyroid variables as dependent variables and age, gender, race/ethnicity, smoking status, iodine sufficiency status, body mass index, fasting time, urinary creatinine, and log10 transformed values of phytoestrogens as independent variables were fitted. Alternate models with tertiles of phytoestrogens in place of log10 transformed values of phytoestrogens were also fitted. Results of the tertile based models showed a negative association between the levels of FT3 and first and third tertiles of DMA, EQU, ETL and ETD. FT4 levels were positively associated with the levels of DAZ (β=0.004, p< 0.01) and DMA (β= 0.003, p< 0.01). TT4 levels were found to be negatively associated with the levels of EQU (β=-0.007, p=0.01), ETD (β=-0.009, p< 0.01), and ETL (β=-0.008, p< 0.01). Levels of urinary iodine were positively associated with the levels of every phytoestrogen.