Background
US iodine intake, estimated from the median urinary iodine concentration of population representative data, has declined by half since the 1970s which is problematic because maternal iodine intake is critical for fetal neurodevelopment. Relying on median urinary concentrations to assess iodine intake of populations is standard practice but does not describe the number of individuals with insufficient intake. Prevalence estimates of inadequate and excessive intake are better for informing public health applications but require multiple urine samples per person; such estimates have been generated in pediatric populations but not yet among pregnant women.
Objective
Our aims were: 1) to assess median urinary iodine concentrations across pregnancy for comparison to national data; and 2) to estimate the prevalence of inadequate and excessive iodine intake among pregnant women in mid-Michigan.
Study Design
Data were collected in 2008-2015 as part of a prospective pregnancy cohort where women were enrolled at their first prenatal clinic visit. Few exclusion criteria (<18 y, or non-English speaking) resulted in a sample of women generally representative of the local community, unselected for any specific health conditions. Urine specimens were obtained as close as practicable to at least one specimen per trimester during routine prenatal care throughout pregnancy (n=1-6 specimens/woman) and stored at -80°C until urinary iodine was measured to estimate iodine intake (n=1,014 specimens from 464 women). We assessed urinary iodine across pregnancy by each gestational week of pregnancy and by trimester. We used multiple urine specimens per woman, accounted for within-person variability, performed data transformation to approximate normality, and estimated the prevalence of inadequate and excessive iodine intake using a method commonly employed for assessment of nutrient status.
Results
Maternal characteristics reflected the local population in racial and ethnic diversity and socio-economic status: 53% non-Hispanic White, 22% non-Hispanic Black and 16% Hispanic; 48% had ≤ high school education and 71% had an annual income < $25,000. Median urinary iodine concentrations in the 1st, 2nd, and 3rd trimester—including some women contributing more than one specimen per trimester—were 171 μg/L (n=305 specimens), 181 μg/L (n=366 specimens), and 179 μg/L (n=343 specimens), respectively, with no significant difference by trimester (p=.50, Kruskall-Walllis test for equality of medians). The estimated prevalence of inadequate and excessive iodine intake was 23%, and <1%, respectively.
Conclusions
Median urinary iodine concentrations in each trimester were above the World Health Organization cut-off of 150 μg/L indicating iodine sufficiency at the group level across pregnancy. However, the estimated prevalence of inadequate iodine intake was substantial at 23%, while prevalence of excessive intake was <1%, indicating a need for at least some women to increase consumption of iodine during pregnancy. The American Thyroid Association, the Endocrine Society, and the American Academy of Pediatrics recommend that all pregnant and lactating women receive a daily multivitamin/mineral supplement that contains 150 μg of iodine. The data presented here should encourage the collection of similar data from additional US population samples for the purpose of informing the American Council of Obstetrics and Gynecology’s own potential recommendations for prenatal iodine supplementation.