Systematic review of 8 RCTs found that in areas of severe iodine deficiency, supplementation reduced cretinism risk & showed some motor development benefits; in mild-to-moderate deficiency areas, no RCTs provided data on childhood cognitive or developmental outcomes; effects on maternal thyroid function were inconsistent.

A systematic review of 10 RCTs & 23 observational studies found inconsistent effects of iodine supplementation on maternal or infant TSH & no significant improvement in childhood cognitive, language, or motor scores; authors concluded evidence is insufficient to support universal supplementation in mild-to-moderate deficiency regions but supplementation remains important in severe deficiency.

A comprehensive review confirmed that the thyroid gland contains the highest selenium concentration per gram of any organ; selenium is indispensable for the synthesis of thyroid hormones (as deiodinase cofactor), & deficiency impairs T4-to-T3 conversion; supplementation in autoimmune thyroiditis reduces antibody titers & may improve quality of life.

Meta-analysis of 35 RCTs (n=2,358) found selenium supplementation significantly decreased TPOAb levels in patients not on thyroid hormone replacement, lowered TSH, & reduced malondialdehyde; adverse effects were comparable to placebo (moderate certainty of evidence).