Evidence explainerIodine (potassium iodide, kelp)
Iodine Before and During Pregnancy: Why Many Prenatals Leave It Out, and What an 832-Woman Trial Found
Thyroid and pediatric groups advise 150 mcg a day, yet only 34 of 59 best-selling prenatal vitamins checked in 2016 to 2017 contained any. The largest trial of the question had a surprising result.
Key takeaways
- The iodine RDA rises from 150 mcg/day to 220 mcg in pregnancy and 290 mcg while breastfeeding [1].
- The American Thyroid Association advises 150 mcg/day as potassium iodide when planning a pregnancy, during pregnancy and while breastfeeding [1].
- Of 59 best-selling prenatal vitamins in 2016 to 2017, only 34 contained iodine [1].
- In a trial of 832 mildly deficient pregnant women, 200 mcg/day did not change child IQ or executive function at age 5 to 6 [2].
- A 2020 review found too little good-quality evidence to support or rule out the advice in mild-to-moderate deficiency [3].
Safety at a glance
From our Iodine (potassium iodide, kelp) monograph. Published interactions and cautions only, each with its source.
Interactions
- Antithyroid medicines. High iodine doses with antithyroid drugs can have an additive effect and could cause low thyroid function.[1]
- ACE inhibitors. Potassium iodide with ACE inhibitors can raise the risk of high blood potassium.[1]
- Potassium-sparing diuretics. Potassium iodide with these diuretics can raise the risk of high blood potassium.[1]
Who should be careful
- High intakes. Too much iodine can cause some of the same problems as too little, including goiter, raised TSH and low thyroid function, and can also trigger an overactive thyroid or thyroid inflammation. The adult UL is 1,100 mcg/day; some iodine-only supplements exceed it.[1]
- Thyroid conditions. People with autoimmune thyroid disease, or who were previously iodine deficient, may have adverse effects at intakes considered safe for the general population.[1]
- Pregnancy. ODS reports that the American Thyroid Association and American Academy of Pediatrics advise 150 mcg/day of iodine before and during pregnancy and while breastfeeding, while noting a Spanish study linking ≥200 mcg/day supplements in pregnancy to raised TSH. Discuss amounts with a prenatal clinician.[1]
- Before you start. Talk to your pharmacist or clinician before adding this supplement, especially if you take prescription medicines, are pregnant or breastfeeding, or have a chronic health condition.[1]
The evidence, graded
Pulled from our monograph evidence tables. Grades follow our evidence-grading policy.
- No effectNo effectChild IQ and executive function after supplementing mildly deficient pregnant womenNo difference in verbal IQ (difference -0.7; 95% CI -2.9 to 1.5), performance IQ (-1.6) or executive function. Adverse events did not differ between groups.
Why the need goes up
Iodine is needed to make thyroid hormone, and needs rise in pregnancy: the RDA is 220 mcg/day in pregnancy and 290 mcg/day while breastfeeding, compared with 150 mcg for other adults [1]. The WHO advises 250 mcg/day during pregnancy and breastfeeding [1].
ODS reports that a substantial share of pregnant women in the US may be iodine insufficient, and that women who eat no dairy may be at particular risk [1]. A meta-analysis of 6,180 mother-child pairs from three European birth cohorts linked lower iodine status in the first trimester with lower verbal IQ in children aged 1.5 to 8 years [1]. That is an association, not proof that supplements change the outcome.
What the professional groups say
The American Thyroid Association recommends that women who are planning a pregnancy, pregnant or breastfeeding take 150 mcg/day of iodine as potassium iodide [1]. The American Academy of Pediatrics recommends a daily supplement with at least 150 mcg and the use of iodized salt [1].
Check your label. Of 59 best-selling prenatal multivitamins on the market in 2016 to 2017, only 34 contained iodine [1]. ODS notes that some, but not all, prenatal supplements include it [1].
What the biggest trial found
The question was tested directly in Bangalore, India, and Bangkok, Thailand. Mildly iodine-deficient pregnant women took 200 mcg of iodine a day or placebo from early pregnancy until delivery, starting at an average of 10.7 weeks [2].
Of 832 women who entered, 313 children were assessed for IQ at age 5 to 6 [2]. Verbal IQ averaged 89.5 with iodine versus 90.2 with placebo, and performance IQ was 97.5 versus 99.1, differences too small to rule out chance [2]. Executive function did not differ either, and adverse events were similar between groups [2].
A 2020 systematic review found that only 1 of 3 trials on child development was adequately powered, and pooling 2 trials showed no effect on cognitive, language or motor scores [3]. Its authors concluded there is insufficient good-quality evidence to support the recommendations in areas of mild-to-moderate deficiency, and called for trials in more deficient women [3].
Why "more" is not the answer
Higher doses carry their own question mark. ODS describes a 2010 Spanish study in which pregnant women taking 200 mcg/day or more of supplemental iodine were more likely to have raised TSH than those taking under 100 mcg/day [1]. The adult upper limit is 1,100 mcg/day, and some iodine-only supplements exceed it [1].
The practical takeaway is a conversation, not a dose: ask your prenatal clinician whether your prenatal vitamin contains iodine and whether its amount fits your diet. For everyday sources, read sea salt, pink salt and iodine, and see the iodine monograph for the full evidence. Folate questions are covered in methylfolate vs folic acid.
Quick answers
Do all prenatal vitamins contain iodine?
No. Only 34 of 59 best-selling prenatal multivitamins in 2016 to 2017 contained iodine, so it is worth checking the label [1].
Keep reading
Full monographs
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References
- [1]Iodine: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements, 2024.
- [2]Effect of iodine supplementation in pregnant women on child neurodevelopment: a randomised, double-blind, placebo-controlled trial. Lancet Diabetes Endocrinol, 2017.
- [3]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. Am J Clin Nutr, 2020.