Systematic review and meta-analysis of the effects of iodine supplementation on thyroid function and child neurodevelopment in mildly-to-moderately iodine-deficient pregnant women.

Systematic review and meta-analysis of 37 publications evaluating pregnant women with mild-to-moderate iodine deficiency, mainly from Europe with additional studies from Asia, Australia, and other regions. Evidence included randomized trials, nonrandomized interventions, and observational studies; only 3 publications contributed to the meta-analyses, with 2 RCTs pooled for maternal thyroid markers and 2 RCTs pooled for child cognitive, language, or motor outcomes.

Iodine supplementation during pregnancy, generally given orally as potassium iodide or iodine-containing multivitamin/mineral tablets, with doses in the included trials ranging from 50 to 300 μg/day and most randomized comparisons using 150 to 225 μg/day from early pregnancy through late gestation or term. Some studies initiated supplementation before 10 to 16 weeks, while others started at 12 to 14 weeks or at term; a few comparisons used iodized salt or no-iodine control conditions.

Overall, the evidence was insufficient to support routine iodine supplementation during pregnancy in mildly-to-moderately iodine-deficient women. Some studies suggested improved maternal thyroid biomarkers, especially lower thyroglobulin and smaller thyroid-volume increases, but effects on TSH and free thyroxine were inconsistent. In the pooled RCT data, maternal TSH, FT4, and Tg showed small changes during pregnancy, but child neurodevelopment did not improve: meta-analysis of 2 RCTs found no effect on cognitive, language, or motor scores. The sole adequately powered trial also found no benefit for motor, mental, or behavioral outcomes, and some observational findings were mixed or potentially adverse in specific contexts.

The evidence base was heterogeneous in design, dosing, timing of initiation, baseline iodine status, and outcome measurement, limiting comparability across studies. Child neurodevelopment was assessed in few studies, most trials were small or not powered for clinical outcomes, and many estimates came from nonrandomized or observational data with inconsistent results. These limitations reduce confidence in any apparent maternal biomarker changes and make it difficult to generalize findings to all iodine-deficient pregnant populations.