Clearing up the misinterpretation on iodine intake in pregnancy

Abstract

We appreciate the opportunity to clarify our work in response to the concerns raised.1Verkaik-Kloosterman J. de Jong M. Ocké M. Assessing prevalence of inadequacy and excessive iodine intake: misinterpretation is lying in ambush.Am J Obstet Gynecol. 2021; 224: 418Abstract Full Text Full Text PDF Scopus (1) Google Scholar It was asserted that we applied a cut point recommended by the World Health Organization (WHO) for assessing iodine status of populations to the estimated average requirement (EAR) or tolerable upper intake level method developed by the Institute of Medicine. In fact, we employed the 2 different methods separately and illustrated differences between them. The WHO recommends using median urinary iodine concentration (UIC) as a biomarker of dietary iodine to assess iodine status of populations (median UIC cut point for pregnant women=150 μg/L).2World Health OrganizationUrinary iodine concentrations for determining iodine status deficiency in populations.https://www.who.int/vmnis/indicators/urinaryiodine/en/Date: 2013Date accessed: October 28, 2020Google Scholar If we had used only this method, our conclusion would have been that the population level iodine status in our mid-Michigan pregnancy cohort was sufficient (overall median UIC=176 μg/L with no difference across gestation). However, because we collected multiple urine samples per woman, we were able to use the EAR for iodine for pregnant women of 160 μg/L3Institute of Medicine (US) Panel on MicronutrientsDietary reference intakes for vitamin A, vitamin K, arsenic, boron, chromium, copper, iodine, iron, manganese, molybdenum, nickel, silicon, vanadium, and zinc.https://www.ncbi.nlm.nih.gov/books/NBK222330/Date: 2001Google Scholar in conjunction with techniques described previously4Zimmermann M.B. Hussein I. Al Ghannami S. et al.Estimation of the prevalence of inadequate and excessive iodine intakes in school-age children from the adjusted distribution of urinary iodine concentrations from population surveys.J Nutr. 2016; 146: 1204-1211Crossref PubMed Scopus (27) Google Scholar to estimate prevalence of inadequate and excessive iodine intakes. As described in our methods, we used statistical modeling to separate within-person variation from between-person variation and then removed the excess within-person variation. In addition, we used a complex transformation to approximate normality by using specialized software developed for dietary intake distribution estimation. The WHO median UIC cut points are designed to identify populations at highest risk of iodine deficiency when resources are limited. In contrast, pregnancy cohort studies with multiple urine specimens per participant can use more sophisticated statistical methods that allow for more precise prevalence estimates. Although it is true that we do not have daily urine volume, a 24-hour urine collection is impractical for population research, and more importantly, it has been shown that spot UICs are a reasonable estimate of a 24-hour UIC.5Perrine C.G. Cogswell M.E. Swanson C.A. et al.Comparison of population iodine estimates from 24-hour urine and timed-spot urine samples.Thyroid. 2014; 24: 748-757Crossref PubMed Scopus (59) Google Scholar Furthermore, we found stable UICs across gestation even though glomerular filtration rate increases across pregnancy, giving some assurance that results were not impacted by large differences in urine volume. The importance of iodine in brain development calls for attempts to improve precision when estimating population prevalence of inadequate intake. By using the techniques first described for use in pediatric populations4Zimmermann M.B. Hussein I. Al Ghannami S. et al.Estimation of the prevalence of inadequate and excessive iodine intakes in school-age children from the adjusted distribution of urinary iodine concentrations from population surveys.J Nutr. 2016; 146: 1204-1211Crossref PubMed Scopus (27) Google Scholar and replicating it here in a pregnancy cohort, we have illustrated a method that can be used to characterize iodine status in different regions of the United States among different population subgroups with varied dietary patterns. Importantly, we now have a national consortium of pregnancy cohorts (https://www.nih.gov/echo) to which these methods could be applied. Assessing prevalence of inadequacy and excessive iodine intake: misinterpretation is lying in ambushAmerican Journal of Obstetrics & GynecologyVol. 224Issue 4PreviewWe read with interest the article of Kerver et al1 about the iodine status of pregnant women in the United States. The overall conclusion of the authors is that “the high prevalence of inadequate iodine intake coupled with low prevalence of excessive iodine intake in this US pregnancy cohort indicates a need for at least some women to increase their iodine intake.” However, in our opinion, based on the analyses presented in this paper, this conclusion cannot be confirmed because of several limitations. Full-Text PDF