Iodine

Pregnancy

Iodine is needed to make thyroid hormone, and ODS says the fetus depends on maternal thyroid hormone early in pregnancy. U.S. RDAs are 220 mcg/day in pregnancy and 290 mcg/day in lactation. ACOG’s pregnancy FAQ, last updated March 2026, also lists 220 mcg/day and food sources but does not specify a 150 mcg iodine supplement. Current ATA guidance says to aim for 250 mcg/day total from food and supplements as needed, and suggests a 150 mcg/day supplement for women at risk of deficiency. Older ATA/AAP advice, summarized by CDC and ODS, recommends a 150 mcg/day supplement for pregnant or lactating women. The 2026 systematic review included 10 publications from eight prospective cohorts; suboptimal iodine status was associated with modestly lower child neurodevelopment scores (Hedges' g -0.13, 95% CI -0.20 to -0.06), with high overall heterogeneity (I2 95.2%). The review's nonlinear dose-response findings were exploratory. ODS reports that randomized trials in mildly deficient women did not show a clear child neurodevelopment benefit from supplementation; a 2017 trial of 832 women given 200 mcg/day or placebo found no difference in child IQ or executive-function measures at age 5 to 6. For miscarriage, MotherToBaby describes a possible pathway through low thyroid hormone. A nested case-control study of 2,105 pregnancies found higher first-trimester TSH was associated with miscarriage, but it did not test iodine intake or supplementation. A cross-sectional study of 1,844 pregnant women found higher odds of TSH above 3 mU/L among those taking at least 200 mcg/day supplemental iodine versus under 100 mcg/day (adjusted OR 2.5, 95% CI 1.2 to 5.4); this does not establish a causal threshold. Primary reports describe three infants with congenital hypothyroidism after maternal supplement intake of 12.5 mg/day and a separate fetal hypothyroid goiter case involving excess iodine from a supplement. These high-exposure reports do not establish harm from ordinary dietary amounts.

Breastfeeding

LactMed describes iodine as a normal part of breast milk and says milk iodine concentration varies with maternal iodine status. U.S. lactation RDA is 290 mcg/day. LactMed and MotherToBaby describe temporary infant thyroid underactivity in reports involving very high maternal iodine exposure, including heavy seaweed intake; these cases are not comparable to usual nutritional intake. A 2011 report described two infants with hypothyroidism in the context of maternal seaweed consumption during pregnancy or breastfeeding. A 2025 follow-up included 684 mothers from a prior survey; high postpartum iodine intake was not significantly associated with maternal thyroid disease, subclinical hypothyroidism, or child growth and development. A 2026 report followed one mother who used potassium iodide 150 mg three times daily before thyroidectomy; breastfeeding was interrupted during treatment, so the report does not establish infant exposure during treatment. The report estimated very high milk iodine during treatment and no reported infant adverse event after partial breastfeeding resumed. These small reports do not define a high-dose intake limit or overturn the usual-amount recommendation.

What was tested

Evidence reviewed includes clinical guidelines, nutrient-reference values, observational pregnancy cohorts, randomized supplement trials, systematic reviews, milk-level studies, and infant case reports. The 2026 neurodevelopment meta-analysis combined 10 publications from eight cohorts; its overall association had high heterogeneity and its dose-response curve was exploratory. The 2017 randomized trial tested 200 mcg/day supplementation in mildly iodine-deficient pregnant women in India and Thailand. Lactation evidence includes case reports of excess exposure and a 2025 postpartum follow-up of 684 mothers. The 2026 high-dose potassium iodide milk report concerns one mother and breastfeeding was interrupted during treatment.