Abstract
BackgroundSevere iodine deficiency impacts fertility and reproductive outcomes. The potential effects of mild-to-moderate iodine deficiency are not well known. The aim of this study was to examine whether iodine intake was associated with subfecundity (i.e. > 12 months trying to get pregnant), foetal growth, and adverse pregnancy outcomes in a mild-to-moderately iodine-deficient population.MethodsWe used the Norwegian Mother, Father and Child Cohort Study (MoBa) and included 78,318 pregnancies with data on iodine intake and pregnancy outcomes. Iodine intake was calculated using an extensive food frequency questionnaire in mid-pregnancy. In addition, urinary iodine concentration was available in a subsample of 2795 pregnancies. Associations were modelled continuously by multivariable regression controlling for a range of confounding factors.ResultsThe median iodine intake from food was 121 μg/day and the median urinary iodine was 69 μg/L, confirming mild-to-moderate iodine deficiency. In non-users of iodine supplements (n = 49,187), low iodine intake (< 100–150 μg/day) was associated with increased risk of preeclampsia (aOR = 1.14 (95% CI 1.08, 1.22) at 75 vs. 100 μg/day, p overall < 0.001), preterm delivery before gestational week 37 (aOR = 1.10 (1.04, 1.16) at 75 vs. 100 μg/day, p overall = 0.003), and reduced foetal growth (− 0.08 SD (− 0.10, − 0.06) difference in birth weight z-score at 75 vs. 150 μg/day, p overall < 0.001), but not with early preterm delivery or intrauterine death. In planned pregnancies (n = 56,416), having an iodine intake lower than ~ 100 μg/day was associated with increased prevalence of subfecundity (aOR = 1.05 (1.01, 1.09) at 75 μg/day vs. 100 μg/day, p overall = 0.005). Long-term iodine supplement use (initiated before pregnancy) was associated with increased foetal growth (+ 0.05 SD (0.03, 0.07) on birth weight z-score, p < 0.001) and reduced risk of preeclampsia (aOR 0.85 (0.74, 0.98), p = 0.022), but not with the other adverse pregnancy outcomes. Urinary iodine concentration was not associated with any of the dichotomous outcomes, but positively associated with foetal growth (n = 2795, p overall = 0.017).ConclusionsThis study shows that a low iodine intake was associated with restricted foetal growth and a higher prevalence of preeclampsia in these mild-to-moderately iodine-deficient women. Results also indicated increased risk of subfecundity and preterm delivery. Initiating iodine supplement use in pregnancy may be too late.
Highlights
Severe iodine deficiency impacts fertility and reproductive outcomes
This study shows that a low iodine intake was associated with restricted foetal growth and a higher prevalence of preeclampsia in these mild-to-moderately iodine-deficient women
Non-iodine supplement users who consumed less than 3 dL milk/yoghurt per day (25% of all participants) had a median urinary iodine concentration (UIC) of 48 μg/L, and those who excluded dairy products entirely from their diet (1.5% of all participants) had a median UIC of 32 μg/L
Summary
Severe iodine deficiency impacts fertility and reproductive outcomes. > 12 months trying to get pregnant), foetal growth, and adverse pregnancy outcomes in a mild-to-moderately iodine-deficient population. Recent findings in two population-based cohort studies have indicated that even mild-to-moderate iodine deficiency may affect thyroid function in pregnant women [4, 5]. An abrupt increase in iodine intake caused by introduction of salt iodization programmes or iodine supplement use might temporarily affect thyroid function in populations that are mild-to-moderately iodine deficient [4, 6]. While it is well documented that severe iodine deficiency poses reproductive risks, including abortions, stillbirths, and impaired neurodevelopment, the potential impact of mild-to-moderate iodine deficiency on fertility and pregnancy outcomes remains largely unknown [7]
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