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.