Can Mild-to-Moderate Iodine Deficiency during Pregnancy Alter Thyroid Function? Lessons from a Mother-Newborn Cohort

Abstract

Severe iodine deficiency during pregnancy has substantial hormonal consequences, such as fetal brain damage. Data on the potential effects of mild-to-moderate iodine deficiency on the thyroid function of pregnant women and their newborns are scarce and divergent. We investigated the association between iodine intake in pregnancy and maternal and neonatal thyroid function in a region with mild-to-moderate iodine deficiency. Pregnant women’s iodine status was evaluated using an iodine food frequency questionnaire, serum thyroglobulin (Tg), and urinary iodine concentration (UIC). Neonatal thyrotropin (nTSH) values were measured after birth. Obstetrics and anthropometric data were also collected. Among the 178 women (median age 31 years) included in the study, median (interquartile range) estimated dietary iodine intake, Tg and UIC were 179 (94−268) μg/day, 18 (11−33) μg/L, and 60 (41−95) μg/L, respectively. There was a significant inverse association of iodine intake with Tg values among the study population (β = −0.2, F = 7.5, p < 0.01). Women with high free triiodothyronine (FT3) values were more likely to exhibit an estimated iodine intake below the estimated average requirement (160 μg/day, odds ratio [OR] = 2.6; 95% confidence interval [CI], 1.1−6.4; p = 0.04) and less likely to consume iodine-containing supplements (OR = 0.3, 95% CI, 0.1−0.8; p = 0.01). It is possible that thyroid function may be affected by iodine insufficiency during pregnancy in regions with mild-to-moderate iodine deficiency. The relatively small sample size of the studied population warrants further investigation.

Keywords: free triiodothyronine; iodine insufficiency; maternal dietary iodine intake; neonatal thyrotropin; thyroglobulin.

Figure 1

Study flow chart. Abbreviations: BUMCA = Barzilai University Medical Center, Ashkelon, nTSH = neonatal thyrotropin.

Figure 2

Available UIC (a) levels (N = 165) and logTg (b) values (N = 189) by estimated iodine intake obtained from sIFFQ. The regression line equation, R² and p value for associations of UIC and logTg are shown in the left-upper area of (a) and (b), respectively. The solid line represents the estimated linear fit, and the shaded areas are the 95% CI. After adjusting for participants’ age, smoking status, and gravidity, the correlation between logTg and estimated iodine intake remained significant [β (95% CI) = −0.198 (−0.0005, −0.003), p = 0.04]. Abbreviations: UIC, urinary iodine concentration; sIFFQ, semi-quantitative iodine food frequency questionnaire; log Tg, serum log-transformed thyroglobulin; CI, confidence interval.

Figure 3

Regression lines of FT3 (y-axis) against log Tg (x-axis). The solid lines represent the estimated linear fit, and the shaded areas illustrate the 95% CIs. (a) Pregnant women in their third trimester (N = 138): y = 3.4 − 0.2(x), and R² = 0.045; β (95% CI) = 0.19 (0.03, 0.27), p = 0.024; and (b) All pregnant women (N = 178): y = 3.6 − 0.1 × (x), and R² = 0.032; β (95% CI) = 0.17 (0.02, 0.24), p = 0.024. After adjusting for participants’ age, smoking status, and gravidity, both correlations remained significant (p = 0.04 for both). Abbreviations: CI, confidence interval; FT3, triiodothyronine; log Tg, serum log-transformed thyroglobulin.