1. Low iodine intake associated with decreased subfecundity, reduced fetal growth, increased preterm delivery, and increased preeclampsia.
Evidence Rating Level: 2 (Good)
The thyroid gland is important in fertility for females and improper functioning has been shown to have adverse effects on fertility and pregnancy. Although iodine is known to be critical to thyroid functioning, associations between iodine deficiency and adverse gestational or fetal outcomes have primarily been studied and reported in low quality or underpowered studies with inconclusive results. In this cohort study, using the Norwegian Mother, Father and Child Cohort Study, 78 318 pregnancies (68 166 women) were assessed to investigate the association between iodine intake and adverse pregnancy outcomes. Multivariable regression analyses were used and iodine intake of 100 ug per day was used as reference (aOR=1.0) as intakes of 100-150 ug/d were usually associated with the least adverse outcomes. Of non-supplement users, 75 ug iodine intake per day pregnancies were associated with higher risk of preeclampsia (aOR 1.14, 95% CI 1.08-1.22), preterm delivery <37 weeks (aOR 1.10, 95% CI 1.04-1.16), and reduced fetal growth z-scores (-0.08 SD, 95% CI -0.10 to -0.06) but not with early preterm delivery <32 weeks and intrauterine death. These effect sizes became more substantial with lower intakes of iodine (50 ug/d, 25 ug/d) compared to reference (100ug/d). There was a similar observation with planned pregnancies and increasingly lower iodine intake was associated with higher rates of subfecundity (>12 months trying to get pregnant) (75 ug/d aOR 1.05 95% CI 1.01-1.09, 50 ug/d aOR 1.14 95% 1.04-1.26, 25 ug/d aOR 1.25, 95% CI 1.07-1.46 versus reference 100 ug/d). Further, supplementation use that was started before pregnancy was associated with reduced risk of preeclampsia (aOR 0.85, 95% CI 0.74-0.98), increased fetal growth z scores (+0.05SD, 95% CI 0.03-0.07) but not with other outcomes. Study findings suggest that sufficient iodine intake may be an important factor for positive pregnancy outcomes, and that there may be benefits associated with antenatal iodine supplementation.
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