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Iodine
A trace mineral the thyroid uses to make its hormones, found in seaweed, fish, dairy and iodised salt, and taken as a supplement in pregnancy.
Explained simply · Last reviewed
What it is
Iodine is a trace element and an essential part of the thyroid hormones. ODS, the US Office of Dietary Supplements, says it is naturally present in some foods, is added to some types of salt, and is available as a dietary supplement. ODS says thyroid hormones are required for proper skeletal and central nervous system development in fetuses and infants, and it calls iodine deficiency the most common cause of preventable intellectual disability in the world.
More about Iodine
ODS, the US Office of Dietary Supplements, says it is naturally present in some foods, is added to some types of salt, and is available as a dietary supplement. ODS says thyroid hormones are required for proper skeletal and central nervous system development in fetuses and infants, and it calls iodine deficiency the most common cause of preventable intellectual disability in the world.
ODS names seaweed, such as kelp, nori, kombu and wakame, as one of the best food sources of iodine, and fish, other seafood and eggs as other good sources. It says dairy products contain iodine, though the amount varies with whether cows were given iodine feed supplements and how cows and milk equipment were cleaned, and that most fruits and vegetables are poor sources. According to ODS, the salt iodization programs that many countries have implemented have dramatically reduced the prevalence of iodine deficiency worldwide.
ODS reports that the general US population has adequate iodine intake, but that some pregnant women may be at risk of deficiency. Several national and international groups recommend iodine supplementation during pregnancy, lactation and early childhood, it says. ODS reports that, in US survey data from 2011 to 2014, 72.2% of pregnant women took a dietary supplement and 17.8% of them took one containing iodine.
In supplements, iodine is often present as potassium iodide or sodium iodide, according to ODS. Many multivitamins contain it, some but not all prenatal supplements do, and supplements containing kelp are also sold. ODS adds that many iodine-only supplements contain high doses, sometimes above the upper limit.
The research below keeps apart what its sources keep apart: areas of severe iodine deficiency and areas of mild to moderate deficiency, and pregnancy, childhood and babies born very early.
Also known as: potassium iodide, sodium iodide, iodide, kelp
At a glance
Safety: known risks, 3 groups who should avoid it or take extra care and 3 documented interactions. See Safety ↓Claims made about Iodine, and how strong the evidence is for each:
- Iodine supplements taken before, during or after pregnancy improve thyroid health and birth outcomes for mothers and babiesEvidence rating: Limited
- Iodine taken before or during pregnancy improves children’s mental developmentEvidence rating: Limited
- Iodine supplements or iodised salt improve growth before birth and during childhoodEvidence rating: Limited
- Iodine supplements improve thinking skills in children with mild iodine deficiencyEvidence rating: Limited
- Iodine given to babies born very preterm reduces deaths before hospital discharge or improves development at age twoEvidence rating: Evidence against
- Iodine supplements reduce breast pain and tenderness in women with fibrocystic breast diseaseEvidence rating: Limited
Each rating follows our published rules and the sources cited for it. We don’t add our own medical judgment. How we rate
Safety
Read this before the ratings below.
Risks
- ODS, the US Office of Dietary Supplements, states that high intakes of iodine can cause some of the same symptoms as iodine deficiency, including goitre (an enlarged thyroid), elevated TSH levels and hypothyroidism (an underactive thyroid). It states that high intakes can also cause iodine-induced hyperthyroidism (an overactive thyroid), including when iodine is given to treat deficiency, and that studies have shown excessive iodine intakes cause thyroiditis and thyroid papillary cancer.
- ODS states that responses to excess iodine, and the doses that cause adverse effects, vary. It says some people, such as those with autoimmune thyroid disease and iodine deficiency, may experience adverse effects with iodine intakes considered safe for the general population. It adds that long-term intakes above the upper limit increase the risk of adverse health effects.
- ODS reports that cases of acute iodine poisoning are rare and are usually caused by doses of many grams. It lists burning of the mouth, throat and stomach, fever, abdominal pain, nausea, vomiting, diarrhea, weak pulse and coma as symptoms.
- ODS describes a 2010 study of pregnant women in Spain in which those taking higher-dose iodine supplements had raised TSH (hyperthyrotropinemia) more often than those taking low doses. It says these findings suggest that “taking higher doses of supplemental iodine during pregnancy could induce thyroid dysfunction in some women”, and that more research is needed.
- In the 2017 Cochrane review of iodine before, during or after pregnancy, one trial of 76 women found that 33.3% of those given iodine dropped out because of nausea or vomiting, against 2.2% of those not given iodine. The reviewers rated this very low certainty. In another trial, excessive iodine intake during pregnancy was more common with supplements (7.0% against 1.6%). Differences in raised thyroid antibodies and in an overactive thyroid during pregnancy were not statistically significant, which is too little evidence to show a difference either way.
Who should avoid it or take extra care
- People with autoimmune thyroid disease, and people who are iodine deficient: ODS states they may experience adverse effects with iodine intakes considered safe for the general population.
- Infants: the upper-limit table in the ODS fact sheet states that formula and food should be the only sources of iodine for infants.
- People who regularly take antithyroid medicines, ACE inhibitors or potassium-sparing diuretics: ODS says they should discuss their iodine intakes with their health care providers.
Documented interactions
- Antithyroid medicines (for example methimazole): ODS states that taking high doses of iodine with antithyroid medications can have an additive effect and could cause hypothyroidism. Source [1]
- ACE inhibitors (for example benazepril, lisinopril, fosinopril): ODS states that taking potassium iodide with ACE inhibitors can increase the risk of hyperkalemia (high blood potassium). Source [1]
- Potassium-sparing diuretics (for example spironolactone, amiloride): ODS states that taking potassium iodide with potassium-sparing diuretics can increase the risk of hyperkalemia (high blood potassium). Source [1]
Safety sources: Source [1]Source [2]
What the evidence says
Each claim is rated on its own evidence. Ratings follow the scale on our methodology page.
Iodine supplements taken before, during or after pregnancy improve thyroid health and birth outcomes for mothers and babies Evidence rating: Limited
Women who are planning a baby, pregnant or breastfeeding take iodine hoping it will keep them and their baby healthy.
A 2017 Cochrane review of trials, mostly in areas of mild to moderate deficiency, found too little evidence for firm conclusions and rated it low to very low certainty.
A 2017 Cochrane review looked at iodine given to women before, during or after pregnancy, alone or with other vitamins and minerals. Eleven trials with over 2,700 women contributed data. Most gave daily tablets, capsules or drops; the three oldest gave injections of iodised oil. Almost all of the evidence came from areas with mild or moderate iodine deficiency.
For mothers, the reviewers found no clear differences in an underactive thyroid during pregnancy or after birth, or in preterm birth. Women given iodine had an overactive thyroid after birth less often (1.6% against 4.9%; three trials, 543 women). The review’s abstract and text give this result a 95% confidence interval of 0.11 to 0.91. Its summary table gives 0.11 to 1.91. This page reports both.
For babies, the differences in low birthweight, deaths around birth and newborn thyroid problems were not statistically significant. That is too little evidence to show an effect either way. All the deaths around birth came from one trial in an area of severe deficiency. The abstract counts two trials for deaths around birth and for newborn thyroid problems, while the summary table lists one trial for each. No trial reported growth or infant deaths.
The reviewers rated how sure they could be (certainty) as low for the mothers’ thyroid and birth outcomes, very low for nausea and vomiting (see the safety section), and low to very low for the babies’ outcomes. They concluded that there were “insufficient data to reach any meaningful conclusions on the benefits and harms of routine iodine supplementation in women before, during or after pregnancy”. This page rates the claim on the lowest certainty, very low.
Sources: Source [1]
Iodine taken before or during pregnancy improves children’s mental development Evidence rating: Limited
Pregnant women take iodine hoping it will help their baby’s brain develop.
Older studies of iodised oil in areas of severe deficiency found better scores in young children; ODS calls the effects of iodine supplements in pregnancy inconclusive.
A 2013 review pooled studies of children aged five and under. Eight of its ten intervention studies gave iodised oil in areas of severe iodine deficiency, all but one in the 1960s and 1970s. The other two, in Spain, gave daily potassium iodide. Only two of the ten were randomized. The reviewers report “a consistently better outcome for children of mothers who were supplemented with iodine before or during pregnancy compared to placebo or no supplementation”.
The reviewers write that the results “must be interpreted with caution”. They name small samples, possible dropouts, analyses that ignored whole villages being given iodine together, and little attention to other differences between families. They call for well-designed randomized trials. A 2014 correction changed some of the review’s effect estimates, and states that the changes have no material impact on its conclusions.
ODS, the US Office of Dietary Supplements, says that despite iodine’s importance for fetal development, the effects of iodine supplements during pregnancy on infant and child neurodevelopment “are inconclusive”. It describes two randomized trials that gave iodine from early pregnancy to delivery. Iodine had no effect on the children’s cognitive, language or motor scores at 1.5 or 2 years. One of the trials also found no benefit at 5 to 6 years.
The sources differ, so this page takes the lower rating, limited.
Sources: Source [1]Source [2]
Iodine supplements or iodised salt improve growth before birth and during childhood Evidence rating: Limited
Parents hope extra iodine will help their babies and children grow.
A 2018 review is uncertain whether iodine in pregnancy increases infant growth and found no clear effects on children’s growth; it rated most evidence low or very low certainty.
A 2018 review included 18 studies with 5,729 participants: 13 randomized trials, four non-randomized trials and one before-and-after study. It compared iodine supplements or iodised salt with placebo, non-iodised salt or nothing. The pregnancy studies gave iodised oil or daily iodine tablets. The five trials in school-age children gave iodised oil capsules.
In two non-randomized studies in areas of severe deficiency, babies of women given iodine weighed 200 g more at birth. The reviewers rated this very low certainty. In four randomized trials in areas of mild to moderate deficiency, birthweight did not differ (very low certainty). In school-age children given iodised oil, the pooled results showed no difference in height, height for age or weight (low or very low certainty).
Two trials in school-age children found higher blood levels of two growth factors, IGF-1 and IGFBP-3, which the reviewers call indirect markers of growth (low certainty). The reviewers stress that their review shows “no evidence of an effect”, not “evidence of no effect”. They add that no study, and none of their pooled analyses, was large enough to detect an effect on growth.
One cluster-randomized trial of iodised salt in Ethiopian preschool children found slightly higher weight for age. The reviewers rated it very low certainty and say it should be interpreted with caution. This page rates the claim on the lowest certainty, very low.
Sources: Source [1]
Iodine supplements improve thinking skills in children with mild iodine deficiency Evidence rating: Limited
Parents give children iodine hoping it will help them think and learn.
ODS describes one trial in mildly deficient children in New Zealand that found better reasoning scores with iodine, and says more research is needed.
ODS, the US Office of Dietary Supplements, says the effects of severe iodine deficiency on neurological development are well documented. It says the effects of mild deficiency in childhood are more difficult to quantify. According to ODS, some research suggests that iodine supplementation “might improve cognitive function in mildly iodine-deficient children”.
ODS describes a 2009 randomized, placebo-controlled trial of 184 children aged 10 to 13 in New Zealand with low iodine levels. After 28 weeks, iodine improved their iodine status. It also significantly improved perceptual reasoning and overall cognitive score compared with placebo. ODS concludes that “additional research is required to fully understand the effects of mild iodine deficiency and iodine supplementation on cognitive function”.
This page could not read in full a review that pools the trials in children, so it reports what ODS says. With one trial described, it rates the evidence limited.
Sources: Source [1]
Iodine given to babies born very preterm reduces deaths before hospital discharge or improves development at age two Evidence rating: Evidence against
Parents of babies born very early hope extra iodine will help them survive and develop well.
A 2019 Cochrane review found iodine for preterm babies did not change deaths before discharge or development scores at age two, and rated this high certainty.
This claim is about babies born very early and given iodine in hospital, not about supplements for adults. A 2019 Cochrane review found two randomized trials with 1,394 babies. In the larger, 1,273 babies born before 31 weeks of pregnancy received sodium iodide or a sodium chloride placebo, through a drip or their feeds. The smaller, of babies born before 33 weeks, compared formula with and without added iodide. The reviewers say most babies in the trials were born very preterm.
Iodine made no difference to deaths before hospital discharge (two trials, 1,380 babies). It made no difference to cognitive, motor or language scores at two years, corrected for prematurity (one trial, 1,259 babies). The reviewers rated both outcomes high certainty. They concluded that the trials “do not show any evidence of beneficial effects of iodine supplementation for preterm infants”.
In the larger trial, a collapsed lung (pneumothorax) was less common with iodine. The reviewers say this was one of many secondary outcomes and should be interpreted with caution. They add that further trials in this group are unlikely to be research priorities.
Sources: Source [1]
Iodine supplements reduce breast pain and tenderness in women with fibrocystic breast disease Evidence rating: Limited
Women with lumpy, painful breasts take iodine hoping it will ease the pain.
ODS describes two small placebo-controlled trials with less breast pain on high doses of iodine, calls the results promising and says more research is needed.
ODS, the US Office of Dietary Supplements, describes fibrocystic breast disease as a benign condition with lumpy, painful breasts. It says some research suggests iodine supplementation might be helpful, although “a specific mechanism of action has not been established” and data are limited.
ODS describes two double-blind trials. In one, of 56 women over six months, 65% of those given iodine reported less pain, against 33% on placebo. In the other, of 111 women over five months, the two higher iodine doses reduced breast pain, tenderness and nodularity compared with placebo or the lowest dose. ODS reports that none of the doses was associated with major adverse events or changes in thyroid function tests.
ODS says more research is needed. It notes that the doses in these trials were several times higher than the adult upper limit, and that “doses of this magnitude should only be used under the guidance of a physician”.
Sources: Source [1]
How and when to take it
What the cited sources say about taking it. Ask your doctor or pharmacist what applies to you.
Check the label of iodine-only and kelp products
ODS, the US Office of Dietary Supplements, states that dietary supplements containing only iodine are available and that many contain high doses, sometimes above the upper limit. It says the Supplement Facts label shows whether iodine is present and at what dose. Source [1]
Several bodies recommend iodine in pregnancy and breastfeeding
ODS reports that the American Thyroid Association recommends that women who are planning a pregnancy, currently pregnant, or lactating “should supplement their diet with 150 mcg/day iodine in the form of potassium iodide”. ODS adds that some, but not all, prenatal supplements contain iodine. The 2017 Cochrane review notes that medical bodies in several countries have concluded the potential benefits of supplementation in pregnancy and breastfeeding outweigh the potential harms, while finding its own trials too few for meaningful conclusions. Source [1]Source [2]
The upper limits do not cover medical treatment
ODS states that the upper limits do not apply to individuals receiving iodine for medical treatment, “but such individuals should be under the care of a physician”. Source [1]
Which form the evidence used
Supplements are sold in several forms. This is what the sources on this page say about them — which form the research used, and any comparison those sources themselves made. Where they made none, this page does not rank the forms.
Potassium iodide or sodium iodide, What the sources supportDetails
ODS, the US Office of Dietary Supplements, states that iodine in dietary supplements is often present as potassium iodide or sodium iodide, and that a small study found people absorb potassium iodide almost completely (96.4%). The 2017 Cochrane review of iodine in pregnancy reports that, where trials named the compound, it was mostly potassium iodide. The larger trial in very preterm babies used sodium iodide. None of the cited reviews compared one compound with another. Source [1]Source [2]Source [3]
Kelp and other seaweed, Details
ODS names seaweed, such as kelp, nori, kombu and wakame, as one of the best food sources of iodine, and says supplements containing kelp are available. It reports that the iodine in seaweeds varies greatly: commercially available seaweeds in whole or sheet form ranged from 16 mcg/g to 2,984 mcg/g. None of the trials in the cited reviews tested kelp or seaweed supplements. Source [1]
Reference intake values
Quoted exactly as published by the cited authority. These are population reference values, not a recommendation for you. Ask your doctor or pharmacist what applies to your situation.
Adults, 19+ years
- Recommended intake
- 150 mcg
- Upper limit
- 1,100 mcg
- Source
- Source [1]
Pregnancy, 19+ years
- Recommended intake
- 220 mcg
- Upper limit
- 1,100 mcg
- Source
- Source [1]
Lactation, 19+ years
- Recommended intake
- 290 mcg
- Upper limit
- 1,100 mcg
- Source
- Source [1]
Teens, 14–18 years
- Recommended intake
- 150 mcg
- Upper limit
- 900 mcg
- Source
- Source [1]
Pregnancy, 14–18 years
- Recommended intake
- 220 mcg
- Upper limit
- 900 mcg
- Source
- Source [1]
Lactation, 14–18 years
- Recommended intake
- 290 mcg
- Upper limit
- 900 mcg
- Source
- Source [1]
Children, 9–13 years
- Recommended intake
- 120 mcg
- Upper limit
- 600 mcg
- Source
- Source [1]
Children, 4–8 years
- Recommended intake
- 90 mcg
- Upper limit
- 300 mcg
- Source
- Source [1]
Children, 1–3 years
- Recommended intake
- 90 mcg
- Upper limit
- 200 mcg
- Source
- Source [1]
What the upper limits count: “The FNB has established iodine ULs for food and supplement intakes” Source [1]
Sources
- Iodine supplementation for women during the preconception, pregnancy and postpartum periodCochrane Database of Systematic Reviews, 2017 · Systematic review · Primary source · Certainty: very low
- Iodine and Mental Development of Children 5 Years Old and Under: A Systematic Review and Meta-AnalysisNutrients, 2013 · Meta-analysis · Primary source
- Iodine: Fact Sheet for Health ProfessionalsNIH Office of Dietary Supplements, 2024 · Health authority summary · Primary source
- Effects of Iodized Salt and Iodine Supplements on Prenatal and Postnatal Growth: A Systematic ReviewAdvances in Nutrition, 2018 · Systematic review · Primary source · Certainty: very low
- Iodine supplementation for the prevention of mortality and adverse neurodevelopmental outcomes in preterm infantsCochrane Database of Systematic Reviews, 2019 · Systematic review · Primary source · Certainty: high
Page history
Show all 2 changes to this page
- — Published.
- — First drafted. Iodine in pregnancy for mothers and babies, for children’s mental development, for growth, for children’s thinking and for breast pain rated limited; iodine for very preterm babies rated evidence against.