Thyroglobulin level at week 16 of pregnancy is superior to urinary iodine concentration in revealing preconceptual and first trimester iodine supply

Abstract

Pregnant women are prone to iodine deficiency due to the increased need for iodine during gestation. Progress has recently occurred in establishing serum thyroglobulin (Tg) as an iodine status biomarker, but there is no accepted reference range for iodine sufficiency during pregnancy. An observational study was conducted in 164 pregnant women. At week 16 of gestation urinary iodine concentration (UIC), serum Tg, and thyroid functions were measured, and information on the type of iodine supplementation and smoking were recorded. The parameters of those who started iodine supplementation (≥150 μg/day) at least 4 weeks before pregnancy (n = 27), who started at the detection of pregnancy (n = 51), and who had no iodine supplementation (n = 74) were compared. Sufficient iodine supply was found in the studied population based on median UIC (162 μg/L). Iodine supplementation ≥150 μg/day resulted in higher median UIC regardless of its duration (nonusers: 130 μg/L vs. prepregnancy iodine starters: 240 μg/L, and pregnancy iodine starters: 205 μg/L, p < .001, and p = .023, respectively). Median Tg value of pregnancy starters was identical to that of nonusers (14.5 vs. 14.6 μg/L), whereas prepregnancy starters had lower median Tg (9.1 μg/L, p = .018). Serum Tg concentration at week 16 of pregnancy showed negative relationship (p = .010) with duration of iodine supplementation and positive relationship (p = .008) with smoking, a known interfering factor of iodine metabolism, by multiple regression analysis. Serum Tg at week 16 of pregnancy may be a promising biomarker of preconceptual and first trimester maternal iodine status, the critical early phase of foetal brain development.

The Effects of Iodine Supplementation in Pregnancy on Iodine Status, Thyroglobulin Levels and Thyroid Function Parameters: Results from a Randomized Controlled Clinical Trial in a Mild-to-Moderate Iodine Deficiency Area

“…Our data, obtained in a placebo-controlled prospective setting, suggest that iodine supplementation is not harmful for mildly-to-moderately iodine-deficient pregnant women, and has no appreciable detrimental effect on the thyroid gland or thyroid hormone production—a cause of concern in various studies [16,28,37]. A previous study in the same setting found lower FT4 levels in the third trimester in iodine-supplemented pregnant women, but they were not in the range of mild-to-moderate deficiency before starting iodine supplementation [27].…”

“…Our data suggest that Tg could be a good marker of iodine sufficiency in mid–late pregnancy, and the better correlation found in the third trimester suggests that it particularly reflects long-term iodine intake rather than recent intake [14,16,30]. Our randomized placebo-controlled study correlated Tg and UI/Creat in all three trimesters of pregnancy and post-partum in a population of mildly-to-moderately iodine-deficient pregnant women, some of them in a supplemented iodine-sufficient group.…”

“…Therefore, Tg at mid-pregnancy was the most reliable marker of iodine status before conception and in the first trimester of pregnancy—the most crucial period for fetal brain development. [16].…”

Iodine Supplementation in Mildly Iodine-Deficient Pregnant Women Does Not Improve Maternal Thyroid Function or Child Development: A Secondary Analysis of a Randomized Controlled Trial

“…One study reported that women who started taking 150 μg iodine/day (as part of a multivitamin/mineral tablet) had a significantly lower free thyroxine concentrations during pregnancy than women who started using iodized salt before pregnancy ( 31 ). Most cross-sectional studies associating iodine supplements with maternal thyroxine concentrations found no difference in free thyroxine ( 32 – 34 ) or total thyroxine concentrations ( 35 ), although two studies reported lower free thyroxine ( 36 , 37 ) in iodine-supplement users. A large cross-sectional Chinese study (in an iodine-sufficient region) reported higher mUICs during early pregnancy were associated with mild maternal hypothyroidism ( 9 ): among pregnant women ( n = 7,190) at 4–8 wk gestation, spot UICs of 250–499 μg/L and ≥500 μg/L were associated with a 1.7-fold and a 2.2-fold increased risk of subclinical hypothyroidism, respectively, and the latter UIC with a 2.9-fold increased risk of isolated hypothyroxinemia ( 9 ).…”

The effect of iodine deficiency during pregnancy on child development

“…The evidence from the observational studies suggests iodine supply (either use of iodised salt or supplements) prior to pregnancy is associated with lower TSH ( 63 – 65 ) , higher fT4 ( 63 ) , lower thyroglobulin ( 64 ) (suggesting a lower thyroid volume) and lower thyroid volume ( 62 ) than either supplementation or iodised salt use, which commenced in pregnancy. More recently, the idea that pre-pregnancy iodine supply is important has been extended to study the relationship between pre-pregnancy iodine status and child cognition in a UK-based cohort study ( 66 ) .…”