Supplementary Material for: Smoothed Urinary Iodine Percentiles for the US Population and Pregnant Women: National Health and Nutrition Examination Survey, 2001-2010

Background: Iodine intake is essential for normal growth, development and metabolism throughout life, especially for women during gestation and lactation. The present study applies a novel statistical approach, providing smoothed urinary iodine (UI) percentile curves for the total US population as well as the categories of sex, race/ethnicity, women of childbearing age and pregnant women who were participants in the National Health and Nutrition Examination Survey (NHANES) 2001-2010. To our knowledge, this is the first application of this technique to NHANES nutritional biomarker data. Methods: We used UI and urinary creatinine that were measured in participants aged 6 and older in the NHANES survey periods 2001-2002, 2003-2004, 2005-2006, 2007-2008 and 2009-2010. A nonparametric double-kernel method was applied to smooth percentile curves for UI and creatinine-corrected results. Results: The UI population estimates showed a U-shaped distribution by age for the total US population. Overall, females had lower UI concentrations and median values compared to males (median UI for females, 141.8 µg/l; median UI for males, 176.1 µg/l; p < 0.0001). Non-Hispanic blacks had the lowest median UI concentrations compared to other racial/ethnic groups (p < 0.0001). Among women of childbearing age (15-44 years), UI concentrations mostly declined with increasing age. Pregnant women aged 35 years and older tended to have higher UI concentrations than younger pregnant women at similar percentiles. Conclusions: The smoothed reference distribution of UI concentrations provides an improved and visual display of the entire distribution of values for the US population and specific demographic categories.