Comparison of urinary iodine levels in women of childbearing age during and after pregnancy

Purpose Median urinary iodine concentration (UIC) is used to describe the iodine status of a population. However, the link between UIC and iodine intake may vary during pregnancy. The aim of this study was to compare UIC during and after pregnancy, adjusting for factors that affect iodine intake. Methods Two repeated measures of UIC and data on maternal iodine intake estimated through questionnaires were collected during pregnancy and 1–4 years after pregnancy in a subsample of women (n = 598) from a mother and child cohort study in Spain. Random-effects interval regression was used to assess the changes in UIC according to pregnancy status. Results Median UIC was similar during (133 μg/L) and after pregnancy (139 μg/L). After adjusting for iodised salt, iodine supplement consumption, and socio-demographic related variables, UIC was 24.0% (95% CI 11.3, 38.2) higher after than during pregnancy. This difference was maintained in a subsample of women with exhaustive information on diet (n = 291): 26.2%, 95% CI 10.3, 44.4. Conclusions In an iodine sufficient area for the general population, iodine excretion was lower during than after pregnancy when factors affecting iodine intake were controlled for. Current recommendations of median UIC during pregnancy are based on the equivalence between iodine intake and UIC estimated from studies in non-pregnant populations, which might lead to overestimation of iodine deficiency during gestation. Further studies should evaluate the equivalence between iodine intake and its urinary excretion during pregnancy