The NHANES 2003-2006 has assessed iron and iodine status, 2 trace element nutrients of continued public health interest, in the U.S. population. We investigated associations of sociodemographic (age, sex, race-ethnicity, education, income) and lifestyle (smoking, alcohol consumption, BMI, physical activity, dietary supplement use) variables with the iron status indicators serum ferritin, soluble transferrin receptor (sTfR), and body iron in women aged 20-49 y (n = 2539, 2513, and 2509, respectively) and with urine iodine, a biomarker of iodine intake, in adults aged ≥ 20 y (n = 3066). Significant correlations between the study variables and biomarkers were weak (|r| ≤ 0.24). Urine creatinine (uCr) was moderately significantly correlated with urine iodine (r = 0.52). The individual variables explained ≤ 5% of the variability in biomarker concentrations in bivariate analysis. In multiple regression models, sociodemographic and lifestyle variables together explained 4-13% of the variability in iron indicators and 41% of the variability in urine iodine (uCr in the model). The adjusted estimated body iron was ≈ 1 unit (mg/kg) lower in non-Hispanic black vs. non-Hispanic white women and ≈ 1 unit higher in women who smoked vs. those who did not and in women consuming 1 vs. 0 alcoholic drinks/d. The adjusted estimated urine iodine concentration (uCr in the model) was 34% lower in non-Hispanic blacks vs. non-Hispanic whites, 22% higher in supplement users vs. nonusers, and 11% higher with every 10-y increase in age. In summary, after adjusting for sociodemographic and lifestyle variables (and uCr in the iodine model), race-ethnicity retained a strong association with sTfR, body iron, and urine iodine; smoking and alcohol consumption with iron biomarkers; and supplement use and age with urine iodine.