Iodine deficiency could lead to significant delays in becoming pregnant, according to data from a prospective cohort study published online in Human Reproduction.
Researchers followed 501 couples that were discontinuing contraception to become pregnant, for 12 months, with the woman’s iodine levels measured at the time of enrollment.
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This negative impact on fecundity remained even after researchers controlled for hypo/hyperthyroidism and adjusted for body mass index and cotinine as an indicator of smoking status.
“The significant delay in time to pregnancy in that group raises serious concerns given the high prevalence of iodine deficiency in women of childbearing age,” wrote James L. Mills, MD, of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, and his coauthors. Previous research has found that around one-third of American women of childbearing age have urinary iodine concentrations below 100 mcg/L, and iodine deficiency may be present in more than two-thirds of British schoolgirls.
“Although it seems incongruous that deficiency would be common in a population with high sodium intake, the likely explanation is that most sodium in the diet comes from processed food, and it appears that most salt in processed food is not iodized,” they wrote.
Women with iodine-creatinine ratios in the 50-99 mcg/g range – categorized as mildly deficient – had a smaller but nonsignificant increase in time to pregnancy, compared with women with ratios above 100 mcg/g.
Iodine deficiency is known to have effects on thyroid function, and hypothyroidism in particular is associated with infertility, the authors wrote.
“Low thyroid hormone concentrations are associated with thyrotropin-releasing hormone elevations that stimulate prolactin, which in turn interferes with GnRH pulsatility,” they wrote. “They also cause decreased granulosa cell steroid production and alterations in androgen and estrogen concentrations.”
The researchers selected couples that had recently stopped using contraception to rule out individuals with long-term fertility problems. They also used sensitive HCG pregnancy tests, and the women kept daily journals so that the time to pregnancy could be calculated accurately.
However, they did note that iodine levels were measured only at enrollment and may have varied over the course of the study. They also did not measure thyroid levels during the study.
The study was funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health. No conflicts of interest were declared.
SOURCE: Mills JL et al. Hum Reprod. 2018 Jan 11. doi: 10.1093/humrep/dex379.