Iodine Intake is Associated with Thyroid Function in Mild to Moderately Iodine Deficient Pregnant Women

Abstract

Background: Studies indicate that mild to moderate iodine deficiency in pregnancy may have a long-term negative impact on child neurodevelopment. These effects are likely mediated via changes in maternal thyroid function, since iodine is essential for the production of thyroid hormones. However, the impact of iodine availability on thyroid function during pregnancy and on thyroid function reference ranges are understudied. The aim of this study was to investigate the association between iodine intake and thyroid function during pregnancy.

Design: In a population-based pregnancy cohort including 2910 pregnant women participating in The Norwegian Mother and Child Cohort Study, we explored cross sectional associations of maternal iodine intake measured (1) by a food frequency questionnaire and (2) as iodine concentration in a spot urine sample, with plasma thyroid hormones and antibodies.

Results: Biological samples were collected in mean gestational week 18.5 (standard deviation 1.3) and diet was assessed in gestational week 22. Median iodine intake from food was 121 μg/day (interquartile range 90, 160), and 40% reported use of iodine-containing supplements in pregnancy. Median urinary iodine concentration (UIC) was 59 μg/L among those who did not use supplements and 98 μg/L in the women reporting current use at the time of sampling, indicating mild to moderate iodine deficiency in both groups. Iodine intake as measured by the food frequency questionnaire was not associated with the outcome measures, while UIC was inversely associated with FT3 (p = 0.002) and FT4 (p < 0.001). Introduction of an iodine-containing supplement after gestational week 12 was associated with indications of lower thyroid hormone production (lower FT4, p = 0.027, and nonsignificantly lower FT3, p = 0.17). The 2.5th and 97.5th percentiles of TSH, FT4, and FT3 were not significantly different by groups defined by calculated iodine intake or by UIC.

Conclusion: The results indicate that mild to moderate iodine deficiency affect thyroid function in pregnancy. However, the differences were small, suggesting that normal reference ranges can be determined based on data also from mildly iodine deficient populations, but this needs to be further studied. Introducing an iodine-containing supplement might temporarily inhibit thyroid hormone production and/or release.

Keywords: MoBa; The Norwegian Mother and Child Cohort Study; dietary supplements; iodine; pregnancy; thyroid function.

FIG. 1.

Flow chart of inclusion. *TPOAb positive according to manufacturer-defined cutoff for nonpregnant women (TPOAb >4.11 IU/mL). FFQ, food frequency questionnaire; FT4, free thyroxine; MoBa, Norwegian Mother and Child Cohort Study; TPOAb, thyroid peroxidase antibodies; TSH, thyrotropin.

FIG. 2.

Association between iodine from food (measured by the food frequency questionnaire covering gestational weeks 0–22) and spot UIC measured at mean gestational week 18.5 (SD 1.2) in nonusers of iodine supplements (n = 1738). The solid line represents the estimated median, the dotted line shows the 90th percentile, and shaded areas are the 95% confidence intervals. The predicted values were estimated by quantile regression adjusting for calculated energy intake. The histogram illustrates the distribution of iodine intake from food. UIC, urinary iodine concentration.

FIG. 3.

Adjusted associations between UIC (column 1; n = 2900), creatinine-adjusted UIC (column 2; n = 2900), and iodine from food (column 3, restricted to nonusers of iodine supplement; n = 1730) and TSH, FT4, and FT3. For TSH, the geometric mean was modelled (by square root transformation), and for FT3 and FT4, the highest first percentile was excluded to obtain ∼normal distribution. Including the upper first percentile of FT3 and FT4 did not change the results (Supplementary Fig. S5). FT3, free triiodothyronine.

FIG. 4.

Prevalence of thyroid antibody positivity by UIC (column 1; n = 2900), creatinine-adjusted UIC (column 2; n = 2900), and by iodine from food (column 2, restricted to nonusers of iodine supplements; n = 1730), adjusted models. Antibody positivity was defined as antibody plasma concentration >92nd percentile.

FIG. 5.

Timing of initiation of iodine-containing supplements in pregnancy and measures of maternal thyroid function, adjusted models. Timing was categorized as: no reported supplement use (never; n = 1738), first use in weeks 1–26 before conception (preconception; n = 347), first use during first trimester (GW 0–12; n = 323), and first use during second trimester (GW 13–20; n = 194). Participants who reported use of iodine-containing supplements in the FFQ but did not report timing of use (n = 308) were not included in this analysis. For TSH, the geometric mean was modelled (by square root transformation), and for FT3 and FT4 the highest first percentile was excluded to obtain ∼normal distribution. Crude models are included in Supplementary Fig. S10, and models excluding antibody positive women in Supplementary Fig. S9. *p < 0.05. GW, gestational week.