Vitamins & Supplements · Mineral
Iodine
Iodine (I) · also called Iodide, potassium iodide
Iodine is a trace mineral your thyroid needs to make its hormones. Too little can swell the thyroid and slow it down, and getting enough prevents that. In pregnancy, trials of iodine pills in mildly short women found no gain in the child's brain development. The main caution is that too much, including from kelp products, can also upset the thyroid.
General information, not medical advice. Last substantive update · Not medically reviewed. A named clinician has not signed off on this page. How this page is made
About Iodine
Iodine is an essential trace mineral that the thyroid uses to make hormones. It comes from iodized salt, dairy, eggs, fish and seaweed. Correcting low intake prevents goitre and an underactive thyroid. Extra iodine pills have not been shown to help people who already get enough, and too much can harm the thyroid.
What it is
Iodine is a mineral the body cannot make. The thyroid takes it up and builds it into thyroid hormones, which help control metabolism, growth and brain development. A review describes the steps as transport, oxidation and binding inside thyroid cells.
Supplements and salt use forms such as potassium iodide. Foods and kelp products give iodine in very different amounts.
Where iodine comes from
MedlinePlus names iodized salt as the main food source, then seafood such as cod, sea bass, haddock and perch, kelp, dairy and plants grown in iodine-rich soil. NHS names milk and dairy, eggs, sea fish and shellfish, and says cereals vary with soil.
Per serving, a university table (Linus Pauling Institute, May 2024) lists: cod, 3 ounces, 146 micrograms; nori, 2 tablespoons, 116; steamed oysters, 3 ounces, 93; 1% milk, 1 cup, 88; plain nonfat Greek yogurt, three-quarter cup, 87; iodized salt, quarter teaspoon, 78; fish sticks, 2, 57; full-fat cottage cheese, half cup, 50; Swiss cheese, 1 ounce, 41; a hard-boiled egg, 31. Amounts vary by brand and soil. Food sources are not the same as pills: they come with other nutrients, and amounts are less exact.
Official amounts and limits
These are the regulators' statements, not advice. MedlinePlus (reviewed January 2025) lists recommended amounts of 150 micrograms a day for ages 14 and older, 220 in pregnancy and 290 when breastfeeding. NHS (reviewed August 2020) says UK adults need 140 micrograms. The Linus Pauling Institute reports National Academies upper limits of 1,100 micrograms for adults, 900 for ages 14 to 18, 600 for 9 to 13, 300 for 4 to 8 and 200 for 1 to 3. The NIH Office of Dietary Supplements page could not be opened.
What the research shows
Low iodine causes goitre and an underactive thyroid, and reviews agree that salt iodisation prevents this. In mild to moderate deficiency, the thyroid works harder, which over time raises nodular goitre and overactive thyroid in a population.
In pregnancy, a UK cohort linked low iodine to lower child verbal IQ and reading. But trials of iodine pills in mildly short women found no benefit: one trial of 832 women at age 5 to 6, and meta-analyses up to age 2. Trials in severely deficient pregnant women have not been done. A Cochrane review found too few data on routine supplements.
In children and teenagers, a 2026 Cochrane review of 8 trials found a slight rise in thinking scores, fewer cases of underactive thyroid, and fewer infant deaths, each from a single study. Benefits in people who already get enough iodine are not shown.
At a glance
Traditional uses history, not proof
- Added to table salt in many countries to prevent goitre
- Eaten as seafood, dairy and eggs; long use as food is not proof that extra pills help
Studied for research
- Thyroid hormone and goitre prevention in low-iodine settings, high certainty
- Children's thinking scores, moderate certainty (one study)
- Child brain development after supplements in pregnancy, evidence against a benefit in mild deficiency
- Postpartum thyroid problems, low certainty
- Infant deaths, low certainty (one study)
- Birth size and early development, low certainty (no difference)
Main compounds
- Iodide and potassium iodide (the forms in salt and supplements)
- Thyroid hormones (made from iodine)
Common forms
- Iodized salt and foods such as fish, dairy and eggs (food sources)
- Iodine tablets, alone or in multivitamins (as studied in trials)
- Kelp and seaweed products, which vary enormously in iodine
Research links: 2026 Cochrane review: iodine for children and teenagers · 2017 trial: iodine in pregnancy and child brain development · 2024 review: risks of iodine excess · Research Library · Genes & Biomarkers Library
What it is and its history
- Accepted scientific name
- Iodine (chemical element, symbol I)
- Common names
- Iodine (English); Iodide (English); Potassium iodide (English)
- Family or group
- Essential trace mineral.
- Harvest and processing
- Iodine is added to table salt (iodized salt). Supplements and kelp products are other sources; kelp content varies hugely (see Safety).
- Look-alikes and adulterants
- Name: Kelp and seaweed products; Note: A natural source with a very wide iodine range; not the same as a measured iodine supplement.
What is inside, and how it may work
What it contains and how those compounds act in cells and animals. Laboratory and animal mechanisms are not proven effects in people.
Main compounds
| Compound | Family | Where it is concentrated | Activity studied | Level of evidence |
|---|---|---|---|---|
| Iodide / potassium iodide | mineral salt | Form used in supplements and salt (existing nutrient summary; not re-verified). | Taken up by the thyroid and used to make thyroid hormones. 11 14 | Human data |
| Thyroid hormones (thyroxine and related) | hormone | Not applicable | Made by the thyroid using iodine; iodine is a building block. 14 9 | Human data |
Mechanisms, receptors and pathways
| Target or pathway | Gene or receptor | What was found | Level |
|---|---|---|---|
| Thyroid hormone synthesis (iodide cycle) | Not specified in the sources read | The iodide cycle in thyroid cells involves transport, oxidation, organification and binding or coupling steps. 9 | Human data |
| Wolff-Chaikoff effect and Jod-Basedow phenomenon | Not specified | In susceptible people an iodine load can cause underactive thyroid (failure to escape the Wolff-Chaikoff effect) or overactive thyroid (Jod-Basedow phenomenon). 9 | Human data |
| Thyroid compensation in low iodine | Not specified | In mild to moderate deficiency the thyroid works harder to keep hormone normal, which over time raises toxic nodular goitre. 12 | Human data |
Absorption, metabolism and elimination
- Absorption
- Not found in the sources read.
- Distribution
- Taken up by the thyroid (review).
- Metabolism
- Not found.
- Elimination
- Urinary iodine of a population reflects total iodine intake (Farebrother 2019), which is why urine is used to judge intake.
- Bioavailability
- Not established for kelp or supplements in the sources read.
Isolated compounds versus whole extracts
Foods (dairy, fish, eggs, iodized salt), measured tablets and kelp products give iodine in different amounts. Kelp products range from 5 to 62,400 micrograms per portion or dose, so kelp is not interchangeable with a measured source. Trials used measured iodine tablets or single doses.
Nutritional composition
Per serving, the Linus Pauling Institute table (a university source, updated May 2024; the table's own original source was not opened) lists: cod, 3 ounces, 146 micrograms; nori, 2 tablespoons, 116; oysters, steamed, 3 ounces, 93; 1% milk, 1 cup, 88; plain nonfat Greek yogurt, three-quarter cup, 87; iodized salt, quarter teaspoon, 78; fish sticks, 2, 57; full-fat cottage cheese, half cup, 50; Swiss cheese, 1 ounce, 41; a hard-boiled egg, 31. Food amounts vary by brand and soil. MedlinePlus also names cod, sea bass, haddock, perch, kelp, dairy and iodized salt (78 micrograms in a quarter teaspoon). NHS says cereals contain iodine depending on soil. The USDA, FDA and NIH iodine database (Pehrsson 2022) is the US source of food values; its values were not read here.
1 high certainty1 moderate certainty5 low certainty1 evidence against
What was looked at
The 8 benefits and uses checked for Iodine, each with how strong the evidence is. Tap one to open it.
- Thyroid hormone, goitre and underactive thyroid from low iodineHigh certainty
- Children's thinking skillsModerate certainty
- Pregnancy iodine level and the child's brain development (observational)Low certainty
- Iodine supplements in pregnancy and the child's brain developmentEvidence against
- Thyroid problems around pregnancy and after birthLow certainty
- Deaths in infantsLow certainty
- Birth size and baby developmentLow certainty
- Children's growth (weight)Low certainty
Lab markers tracked in the studies
Urinary iodine concentration (UIC)TSH (thyroid-stimulating hormone) and free thyroxineThyroglobulin (a thyroid protein)Thyroid size (volume)
What is Iodine good for?
The main studied uses and how strong the human evidence is for each. A normal job in the body is not proof that extra pills add benefit.
Benefits, one by one
Each card is a research question, not a promise. Tap a goal to filter. Open a card to see the studies, doses and limits.
Thyroid hormone, goitre and underactive thyroid from low iodine
High certaintyThe thyroid needs iodine to make its hormones. When intake is too low, the gland can swell (goitre) and can become underactive. Fixing low intake, for example with iodized salt, prevents most of this. Extra iodine in people who already get enough is not shown to help.
5 studiesIodine from iodized salt, food and supplements; in the children's tri…
Studies, doses and limits
What the trials found
A 2009 review says deficiency has many effects, called iodine deficiency disorders, and that universal salt iodisation is the best way to control it in most countries. A 2015 review says severe deficiency causes goitre and underactive thyroid, while mild to moderate deficiency makes the gland work harder and raises toxic nodular goitre and overactive thyroid across a population. In a 2026 Cochrane review of 8 trials in children and teenagers, iodine given as a large single dose or a daily dose cut the risk of underactive thyroid (1 study) and, after removing one high-risk study, goitre (1 study).
Results
Correcting low iodine prevents the problems that low iodine causes. These findings are about deficiency, not about adding iodine to a diet that already has enough.
Effect sizes as reported
Children and teenagers (Cochrane 2026): underactive thyroid RR 0.03 (95% CI 0.00 to 0.23), 1 study, 264 children, moderate certainty by the authors. Goitre: very uncertain overall (RR 0.68, 0.15 to 3.07, 2 studies), but RR 0.31 (0.22 to 0.45) after removing the high-risk study, moderate certainty.
Why this rating
High that low iodine causes thyroid problems and that correcting it prevents them (consistent reviews, long public-health record). The trial figures rest on single studies in children and are moderate by the Cochrane authors' own rating.
Form and dose studied
Iodine from iodized salt, food and supplements; in the children's trials, single large oral or injected doses or one daily dose (as studied).
Who was studied
People and populations with low iodine intake, often in places with iodine-poor soil.
Limits
Few trials, old trials (1982 to 2009), several results from one study each. Not tested in people who already get enough iodine.
Why researchers looked
Iodine is a building block of thyroid hormones, so scientists have studied what happens when it is missing and when it is replaced.
Traditional use
Iodine is a nutrient the thyroid needs, not a traditional remedy. Its history as a medicine is not covered by the sources read.
Proposed mechanism Authoritative guidance
Iodine is taken into the thyroid and built into thyroid hormones. Without enough, the gland cannot make enough hormone and grows to try to catch more.
A proposed mechanism does not show that the effect happens in people.
Too much iodine can also upset the thyroid; see the Safety tab.
Children's thinking skills
Moderate certaintyIn trials in children and teenagers, iodine supplements gave a slight rise in thinking-test scores. This came from one trial, in children who were likely short of iodine.
2 studiesOne trial: iodine versus placebo; the doses across the 8 trials were …
Studies, doses and limits
What the trials found
A 2026 Cochrane review of 8 randomised trials (2,528 children and teenagers, 7 countries) found a slight increase in cognitive function with iodine versus placebo. Only one study (166 children) measured it. Zimmermann's 2009 review also says childhood deficiency reduces growth, thinking and movement.
Results
A small gain in test scores in one trial. It does not show that extra iodine helps children who already get enough.
Effect sizes as reported
Standardised mean difference 0.19 (95% CI 0.04 to 0.34), 1 study, 166 participants.
Why this rating
The Cochrane authors rate this high-certainty, but it comes from a single study of 166 children, so moderate is stated here. Applies to iodine-short children.
Form and dose studied
One trial: iodine versus placebo; the doses across the 8 trials were a single large dose or a daily dose for 28 weeks (as studied).
Who was studied
Children and teenagers, mostly in places with iodine deficiency.
Limits
One study for this outcome. The authors call for more trials in children at risk and with longer follow-up.
Why researchers looked
Thyroid hormone supports brain growth, so researchers asked whether giving iodine to children improves thinking.
Traditional use
Iodine is a nutrient the thyroid needs, not a traditional remedy. Its history as a medicine is not covered by the sources read.
Proposed mechanism Authoritative guidance
Not shown in the sources read. The reviews tie the effect to thyroid hormone needed for growth and development.
A proposed mechanism does not show that the effect happens in people.
Children's upper limits are lower than adults'; see the Safety tab.
Pregnancy iodine level and the child's brain development (observational)
Low certaintyIn a UK study, children of women with low iodine early in pregnancy were more likely to score in the lowest group for verbal IQ and reading at ages 8 and 9. This is a link, not proof that low iodine caused it.
1 studiesMeasured iodine status, not a supplement.
Studies, doses and limits
What the trials found
In the Avon study (ALSPAC), 1,040 first-trimester urine samples were matched to the child's IQ at age 8. The group was mildly to moderately iodine deficient (median 91.1 micrograms/L). Children of women with a low iodine-to-creatinine ratio (under 150 micrograms/g) were more likely to be in the lowest quartile for verbal IQ, reading accuracy and reading comprehension, and scores worsened as the ratio fell.
Results
A consistent link in one UK cohort. It does not prove that low iodine caused the lower scores.
Effect sizes as reported
Lowest-quartile verbal IQ OR 1.58 (95% CI 1.09 to 2.30); reading accuracy OR 1.69 (1.15 to 2.49); reading comprehension OR 1.54 (1.06 to 2.23).
Why this rating
Low because it is one observational study based on a single urine sample per woman, even though 21 other factors were adjusted for.
Form and dose studied
Measured iodine status, not a supplement.
Who was studied
Pregnant women in the UK, first trimester.
Limits
Observational; a single urine sample per woman; other explanations are possible.
Why researchers looked
Iodine is needed for fetal brain development, and the UK had been thought to have enough iodine.
Traditional use
Iodine is a nutrient the thyroid needs, not a traditional remedy. Its history as a medicine is not covered by the sources read.
Proposed mechanism Authoritative guidance
Not established. The authors point to thyroid hormone being needed for the unborn child's brain.
A proposed mechanism does not show that the effect happens in people.
See the Safety tab for pregnancy.
Iodine supplements in pregnancy and the child's brain development
Evidence againstTrials in mildly iodine-short pregnant women found no gain in the child's thinking or development from iodine supplements. Trials in severely deficient women have not been done, so this does not apply to severe deficiency.
3 studiesIodine tablets; the large trial used 200 micrograms a day (as studied…
Studies, doses and limits
What the trials found
A trial in 832 mildly iodine-deficient women in India and Thailand gave a daily iodine tablet or placebo until delivery; at age 5 to 6 there was no difference in IQ or executive function. A 2020 review (37 publications, 10 randomised trials) pooled 2 trials and found no effect on child cognitive, language or motor scores, and concluded there is not enough good-quality evidence to support current supplement recommendations in mild-to-moderate deficiency. A 2021 meta-analysis of 5 trials found no difference in cognitive, language or motor development in the first 2 years.
Results
No benefit for child development was found in mild deficiency. Results do not rule out a benefit in moderate or severe deficiency, which these trials did not test.
Effect sizes as reported
Gowachirapant 2017: verbal IQ difference -0.7 (95% CI -2.9 to 1.5); performance IQ -1.6 (-4.5 to 1.3). Dineva 2020: cognitive MD -0.18 (-1.22 to 0.87). Nazeri 2021: cognitive SMD 0.07 (-0.07 to 0.20).
Why this rating
Evidence is against a benefit in mild deficiency (consistent null results, one well-powered trial). It is low-quality for moderate deficiency, which has not been tested well.
Form and dose studied
Iodine tablets; the large trial used 200 micrograms a day (as studied). Forms of iodine are not given in the abstracts.
Who was studied
Pregnant women who were mildly iodine deficient (median urinary iodine 131 micrograms/L in the large trial).
Limits
Women started at about 11 weeks; only 313 of 832 children were analysed; few trials. The large trial was funded by two universities, a Swiss science fund and the Nestle Foundation.
Why researchers looked
Because low iodine in pregnancy is linked to lower child scores, researchers tested whether supplements fix it.
Traditional use
Iodine is a nutrient the thyroid needs, not a traditional remedy. Its history as a medicine is not covered by the sources read.
Proposed mechanism Authoritative guidance
Not established.
A proposed mechanism does not show that the effect happens in people.
Ask a doctor about iodine in pregnancy, as needs are higher; see the Safety tab.
Thyroid problems around pregnancy and after birth
Low certaintyA pooled analysis of trials found iodine supplements made postpartum overactive thyroid less likely, but the evidence is low quality. The same analysis found more digestive upset in pregnancy. Overall there were not enough data to judge routine supplements.
2 studiesIodine supplements, alone or with other vitamins and minerals.
Studies, doses and limits
What the trials found
A Cochrane review (2017) of 14 randomised trials (over 2,700 women) found lower postpartum hyperthyroidism with iodine (3 trials, 543 women, low quality) and more digestive intolerance in pregnancy (1 trial, 76 women, very low quality). It found no clear differences in hypothyroidism, preterm birth or thyroid antibodies. Nearly all trials were in mild or moderate deficiency.
Results
Mixed and uncertain. One possible benefit and one possible harm, both from few small trials.
Effect sizes as reported
Postpartum hyperthyroidism RR 0.32 (95% CI 0.11 to 0.91); digestive intolerance RR 15.33 (2.07 to 113.70); hypothyroidism in pregnancy RR 1.90 (0.57 to 6.38); preterm birth RR 0.71 (0.30 to 1.66).
Why this rating
Low: the Cochrane authors rate the main findings low or very low quality and conclude there are insufficient data. Two authors work for a nonprofit that runs salt iodisation programmes.
Form and dose studied
Iodine supplements, alone or with other vitamins and minerals.
Who was studied
Women before, during or after pregnancy, mostly in mild or moderate deficiency.
Limits
Few, small, low-quality trials. Not applicable to severe deficiency.
Why researchers looked
Needs for iodine rise in pregnancy and breastfeeding, so researchers tested supplements around pregnancy.
Traditional use
Iodine is a nutrient the thyroid needs, not a traditional remedy. Its history as a medicine is not covered by the sources read.
Proposed mechanism Authoritative guidance
Not established.
A proposed mechanism does not show that the effect happens in people.
Digestive upset was a possible effect; see the Safety tab.
Deaths in infants
Low certaintyIn one trial of infants, iodine supplements were linked to fewer deaths. This is one study, so it is not firm.
2 studiesA single large dose in infants (as studied in the review).
Studies, doses and limits
What the trials found
In the 2026 Cochrane review of children and teenagers, iodine reduced all-cause deaths in infants versus placebo in 1 study of 617 infants. In the 2017 Cochrane review, perinatal deaths were 34% lower with iodine, but the difference was not statistically significant and all the deaths were in one trial in a severely deficient setting.
Results
Fewer infant deaths in one study; not confirmed by others.
Effect sizes as reported
Infant deaths RR 0.32 (95% CI 0.14 to 0.73), 1 study, 617 infants. Perinatal deaths RR 0.66 (0.42 to 1.03).
Why this rating
The 2026 review rates it moderate, but it is a single study in a setting the abstract does not describe, and the 2017 review found a non-significant result, so low is stated here.
Form and dose studied
A single large dose in infants (as studied in the review).
Who was studied
Infants in iodine-deficient settings.
Limits
One trial per result; applies to deficient settings.
Why researchers looked
Severe iodine deficiency in pregnancy and infancy is linked to higher infant mortality.
Traditional use
Iodine is a nutrient the thyroid needs, not a traditional remedy. Its history as a medicine is not covered by the sources read.
Proposed mechanism Authoritative guidance
Not established.
A proposed mechanism does not show that the effect happens in people.
See the Safety tab for children's limits.
Birth size and baby development
Low certaintyCombining 5 trials, iodine supplements in pregnancy did not change birth weight, length or head size, or development in the first 2 years.
2 studiesIodine supplements in pregnancy.
Studies, doses and limits
What the trials found
A 2021 systematic review (14 trials, 5 pooled) found no difference in birth weight, length or head circumference, or in cognitive, language or motor development in the first 2 years. It found that supplements improved iodine status in mothers and babies and prevented the rise in thyroglobulin. The 2017 Cochrane review found no clear difference in low birthweight.
Results
No benefit found for birth size or early development.
Effect sizes as reported
Birth weight SMD -0.11 (95% CI -0.23 to 0.01); length -0.06 (-0.21 to 0.09); head circumference 0.26 (-0.35 to 0.88).
Why this rating
Low: few trials pooled and wide ranges; the finding is a lack of effect, not proof of no effect.
Form and dose studied
Iodine supplements in pregnancy.
Who was studied
Pregnant women, mostly mild to moderate deficiency.
Limits
Only 5 trials could be pooled.
Why researchers looked
Researchers asked whether better iodine in pregnancy leads to healthier or bigger babies.
Traditional use
Iodine is a nutrient the thyroid needs, not a traditional remedy. Its history as a medicine is not covered by the sources read.
Proposed mechanism Authoritative guidance
Not established.
A proposed mechanism does not show that the effect happens in people.
None specific.
Children's growth (weight)
Low certaintyIodine supplements made little or no difference to children's weight in trials.
1 studiesIodine alone or with iron.
Studies, doses and limits
What the trials found
In the 2026 Cochrane review, weight was not clearly different with iodine (3 studies, 658 children, low certainty). One trial of iodine plus iron was very uncertain for weight.
Results
Little or no difference in weight.
Effect sizes as reported
Weight MD -0.20 kg (95% CI -1.92 to 1.52), 3 studies, 658 children.
Why this rating
Low: the Cochrane authors lowered certainty for imprecision and differences between trials.
Form and dose studied
Iodine alone or with iron.
Who was studied
Children and teenagers.
Limits
Only weight was summarised in the abstract.
Why researchers looked
Thyroid hormone drives growth, so growth was a planned outcome.
Traditional use
Iodine is a nutrient the thyroid needs, not a traditional remedy. Its history as a medicine is not covered by the sources read.
Proposed mechanism Authoritative guidance
Not established.
A proposed mechanism does not show that the effect happens in people.
None specific.
Connections to the rest of health
How Iodine connects to the rest of our health knowledge graph. Every link says what kind of connection it is. A safety link (for example to the liver) is a warning, not a benefit. A study that merely mentions a topic never creates a benefit link.
Explore every connection: goals, body systems, organs, symptoms, genes
A. Wellness goals
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Thyroid Support | High certainty | Thyroid hormone, goitre and underactive thyroid from low iodine: The thyroid needs iodine to make its hormones. When intake is too low, the gland can swell (goitre) and can become underactive. Fixing low intake, for example with iodized salt, prevents most of this. Extra iodine in people who already get enough is not shown to help. | 11 12 10 14 13 | |
| Metabolic Health | High certainty | Thyroid hormone, goitre and underactive thyroid from low iodine: The thyroid needs iodine to make its hormones. When intake is too low, the gland can swell (goitre) and can become underactive. Fixing low intake, for example with iodized salt, prevents most of this. Extra iodine in people who already get enough is not shown to help. | 11 12 10 14 13 | |
| Thyroid Support | Moderate certainty | Children's thinking skills: In trials in children and teenagers, iodine supplements gave a slight rise in thinking-test scores. This came from one trial, in children who were likely short of iodine. | 10 11 | |
| Thyroid Support | Low certainty | Pregnancy iodine level and the child's brain development (observational): In a UK study, children of women with low iodine early in pregnancy were more likely to score in the lowest group for verbal IQ and reading at ages 8 and 9. This is a link, not proof that low iodine caused it. | 2 | |
| Thyroid Support | Evidence against | Iodine supplements in pregnancy and the child's brain development: Trials in mildly iodine-short pregnant women found no gain in the child's thinking or development from iodine supplements. Trials in severely deficient women have not been done, so this does not apply to severe deficiency. Birth size and baby development: Combining 5 trials, iodine supplements in pregnancy did not change birth weight, length or head size, or development in the first 2 years. | 5 3 8 6 | |
| Thyroid Support | Low certainty | Thyroid problems around pregnancy and after birth: A pooled analysis of trials found iodine supplements made postpartum overactive thyroid less likely, but the evidence is low quality. The same analysis found more digestive upset in pregnancy. Overall there were not enough data to judge routine supplements. | 6 3 | |
| Womens Wellness | Low certainty | Thyroid problems around pregnancy and after birth: A pooled analysis of trials found iodine supplements made postpartum overactive thyroid less likely, but the evidence is low quality. The same analysis found more digestive upset in pregnancy. Overall there were not enough data to judge routine supplements. | 6 3 | |
| Preventive Wellness | Low certainty | Deaths in infants: In one trial of infants, iodine supplements were linked to fewer deaths. This is one study, so it is not firm. | 10 6 | |
| Preventive Wellness | Low certainty | Children's growth (weight): Iodine supplements made little or no difference to children's weight in trials. | 10 |
B. Body systems
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Endocrine System | Biological association | High certainty | Studied for: Thyroid hormone, goitre and underactive thyroid from low iodine Studied for: Thyroid problems around pregnancy and after birth | 11 12 10 14 13 6 3 8 |
| Nervous System | Biological association | Moderate certainty | Studied for: Children's thinking skills Studied for: Pregnancy iodine level and the child's brain development (observational) | 10 11 2 5 3 8 |
| Musculoskeletal System | Biological association | Low certainty | Studied for: Children's growth (weight) | 10 |
E. Health conditions
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Overactive thyroid | Adverse effect or risk | Documented risk | Excess iodine can cause overactive thyroid in susceptible people; raising iodine in deficient areas can transiently raise it. Excess iodine can cause overactive thyroid in susceptible people. Preparation or compound: High iodine intake |
4 12 9 |
| Underactive thyroid | Documented | Both deficiency and excess can cause underactive thyroid. Both too little and too much iodine can cause an underactive thyroid. Preparation or compound: Low or very high iodine intake |
12 9 | |
| Goiter | Well established | Iodine deficiency is a main cause of goitre; correcting it prevents most cases. Preparation or compound: Iodine intake |
12 11 |
I. Biomarkers
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Urinary iodine concentration (UIC) | Biological association | Moderate certainty | Measured in trials: rises with intake. A lab change is not the same as a health benefit. | 4 11 |
| TSH (thyroid-stimulating hormone) and free thyroxine | Biological association | Low certainty | Measured in trials: no consistent change. A lab change is not the same as a health benefit. | 3 12 |
| Thyroglobulin (a thyroid protein) | Biological association | Moderate certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 3 8 |
| Thyroid size (volume) | Biological association | Low certainty | Measured in trials: smaller increase. A lab change is not the same as a health benefit. | 3 |
K. Biological pathways
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Thyroid gland | Preliminary mechanism | Human | Takes up iodine to make hormones; enlarges (goitre) when iodine is low. | 12 14 |
| Urinary iodine concentration | Preliminary mechanism | Human | Main measure of population iodine intake. | 4 |
| Thyroglobulin | Preliminary mechanism | Human | Blood marker that falls with iodine supplementation in pregnancy. | 8 3 |
L. Active compounds
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Iodide / potassium iodide | Biological association | Human | Taken up by the thyroid and used to make thyroid hormones. | 11 14 |
| Thyroid hormones (thyroxine and related) | Biological association | Human | Made by the thyroid using iodine; iodine is a building block. | 14 9 |
Each connection is stored with the ingredient, the connected record, the kind of connection, the evidence status, the supporting studies, the preparation or compound involved, and a review date (last reviewed October 26, 2026).
Evidence at a glance: what changed in 1 lab markers
Tap a number to show only those markers. Counts are markers, not people.
The evidence
The research centre for Iodine. Every record here was checked against PubMed or the publisher's page; we never list a study we have not verified.
Evidence by outcome
Bars show how many studies we found for each outcome, not how well it works. People are added up across trials only (reviews are not added, to avoid double counting).
How we rate evidence
High certainty
Several well-run trials or reviews agree; further research is unlikely to change the picture much.
Moderate certainty
Good trials point the same way, but some doubt remains.
Low certainty
Small, short or inconsistent trials. The finding could change with better research.
Very low certainty
Very little or very weak human data. Treat as a possibility only.
Mixed findings
Trials or reviews disagree with each other.
Not enough evidence
Not enough human research to say either way.
Evidence against
Good research found no benefit, or found harm.
We take the reviewers' own risk-of-bias and GRADE statements where they exist, and otherwise judge from study size, design, consistency, directness and whether a product matches what is sold. Many publications do not make evidence high. Method note: We started from our existing iodine summary, then read MedlinePlus, NHS and a Linus Pauling Institute page and re-read PubMed abstracts for the trials and reviews cited. The NIH Office of Dietary Supplements page could not be opened. Only abstracts were read for studies. Items we could not verify are flagged in the research gaps.
Study library: every study we checked 16
| Study | Design | People | Product, dose, length | Result |
|---|---|---|---|---|
| Aakre 2021 Commercially available kelp and seaweed products - valuable iodine source or risk of excess intake? Food Nutr Res · PMID 33889064 · DOI |
Laboratory analysis n/a |
96 96 seaweed and kelp products bought on the Norwegian market (43 whole seaweeds, 39 foods containing seaweed, 14 supplements) |
Kelp and seaweed foods and supplements |
Iodine per portion ranged from 128 to 62,400 micrograms in whole seaweed, 30 to 25,300 micrograms in foods with seaweed, and 5 to 5,600 micrograms per daily dose in supplements. Oarweed, sugar kelp and kombu were highest. For 54 products one portion or dose would exceed the tolerable upper intake level. Labelling of species and iodine content was inadequate or inaccurate for several products. 54 of 96 products above the upper level in one portion (Not applicable) Methods, limits and fundingOutcomes: Iodine content by laboratory analysis; label accuracy Limitations: Norwegian market only; products change. Funding and conflicts: No competing interests declared. Read: abstract |
| Bath 2013 Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC). Lancet · PMID 23706508 · DOI |
Cohort study |
1040 First-trimester pregnant women in the UK (median urinary iodine 91.1 micrograms/L) and their children at ages 8 and 9 |
Iodine status (measured, not supplemented) Followed to age 8 to 9 · vs Women with iodine-to-creatinine ratio of 150 micrograms/g or more |
Children of women with a low ratio (under 150 micrograms/g) were more likely to be in the lowest quartile for verbal IQ, reading accuracy and reading comprehension. Scores worsened as the ratio fell. Lowest-quartile verbal IQ OR 1.58; reading accuracy OR 1.69; reading comprehension OR 1.54 (Verbal IQ 1.09 to 2.30; reading accuracy 1.15 to 2.49; comprehension 1.06 to 2.23) Methods, limits and fundingOutcomes: Child verbal IQ at age 8 and reading at age 9 Limitations: Observational: a link, not proof of cause. A single urine sample per woman. Adjusted for 21 factors but other causes remain possible. Funding and conflicts: Study states no funding. Read: abstract |
| Dineva 2020 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 · PMID 32320029 · DOI |
Systematic review meta analysis |
37 publications: 10 randomised trials, 4 non-randomised interventions, 23 observational studies; pregnant women with mild-to-moderate iodine deficiency | Iodine supplements (forms vary) Varies by trial (not given in the abstract) Pregnancy · vs Placebo or no supplement |
Most studies showed no effect on TSH or free thyroxine. Most randomised trials found lower maternal thyroglobulin, and 3 found a smaller rise in thyroid volume. Pooling 2 trials, there was no effect on child cognition, language or motor scores. The authors concluded there is not enough good-quality evidence to support current supplement recommendations in mildly to moderately deficient areas. Cognitive MD -0.18; language MD 1.28; motor MD 0.28 (Cognitive -1.22 to 0.87; language -0.28 to 2.83; motor -1.10 to 1.66) Methods, limits and fundingOutcomes: Maternal and infant TSH and free thyroxine, thyroglobulin, thyroid volume, child cognition, language and motor scores Limitations: Only 3 trials looked at child development and only 1 had enough people. Pooled results come from 2 trials. Funding and conflicts: Funding is not stated in the abstract. Read: abstract |
| Farebrother 2019 Excess iodine intake: sources, assessment, and effects on thyroid function. Ann N Y Acad Sci · PMID 30891786 · DOI |
Narrative review |
Review of excess iodine and thyroid function | Iodine from salt, water, milk, seaweed and supplements |
High intakes are well tolerated by most healthy people, but in some people excess may cause overactive or underactive thyroid, goitre or thyroid autoimmunity. People with existing thyroid disease or past iodine deficiency may react at intakes only slightly above need. The problems are usually mild and temporary, but iodine-induced overactive thyroid can be life-threatening in some people. Sources include over-iodised salt, water, milk, certain seaweeds and supplements. Median urinary iodine reflects a population total intake. No numbers in the abstract (Not applicable) Methods, limits and fundingOutcomes: Hyperthyroidism, hypothyroidism, goitre, thyroid autoimmunity; urinary iodine and thyroglobulin as markers Limitations: Narrative review. Funding and conflicts: Funding is not stated in the abstract. Read: abstract |
| Gowachirapant 2017 Effect of iodine supplementation in pregnant women on child neurodevelopment: a randomised, double-blind, placebo-controlled trial. Lancet Diabetes Endocrinol · PMID 29030199 · DOI |
Controlled trial |
832 Mildly iodine-deficient pregnant women in Bangalore (India) and Bangkok (Thailand); 313 children analysed at age 5 to 6 |
Iodine tablet (form not stated in the abstract) 200 micrograms a day (as studied) From early pregnancy (mean 10.7 weeks) until delivery · vs Placebo |
No difference between the iodine and placebo groups on any child outcome. Adverse events did not differ between groups. Verbal IQ difference -0.7; performance IQ -1.6; executive function score -0.9 (iodine minus placebo) (Verbal IQ -2.9 to 1.5; performance IQ -4.5 to 1.3; executive function -6.8 to 5.0) Methods, limits and fundingOutcomes: Child verbal IQ, performance IQ and executive function at age 5 to 6 Limitations: Women were only mildly deficient and started at about 11 weeks. Only 313 of 832 children were analysed. Not applicable to severe deficiency. Funding and conflicts: Funded by the Swiss National Science Foundation, the Nestle Foundation and two universities (Nestle Foundation is a food-industry foundation). Read: abstract |
| Harding 2017 Iodine supplementation for women during the preconception, pregnancy and postpartum period. Cochrane Database Syst Rev · PMID 28260263 · DOI |
Systematic review meta analysis |
2700 14 randomised trials (11 with data, over 2700 women) before, during or after pregnancy; nearly all in mild or moderate deficiency |
Iodine supplements, alone or with other nutrients Varies by trial Preconception, pregnancy or after birth · vs No iodine or placebo |
Not enough data to reach conclusions on routine supplements. Postpartum hyperthyroidism was less likely (low-quality evidence). Digestive upset in pregnancy was more likely (very low-quality evidence). No clear differences for the other outcomes. Postpartum hyperthyroidism RR 0.32; digestive intolerance RR 15.33; perinatal mortality RR 0.66 (not significant) (Postpartum hyperthyroidism 0.11 to 0.91; digestive intolerance 2.07 to 113.70; perinatal mortality 0.42 to 1.03) Methods, limits and fundingOutcomes: Postpartum hyperthyroidism, hypothyroidism, preterm birth, perinatal death, birthweight, digestive intolerance Limitations: Few, small, low-quality trials. The digestive upset finding is from one trial of 76 women. Funding and conflicts: Two authors work for a nonprofit that runs salt iodisation programmes; they recused themselves from one trial decision. Read: abstract |
| Leung 2012 Iodine-induced thyroid dysfunction. Curr Opin Endocrinol Diabetes Obes · PMID 22820214 · DOI |
Narrative review |
Review of iodine-induced thyroid problems | Iodine from many sources |
Excess iodine is generally well tolerated, but people with thyroid disease or other risk factors may develop thyroid dysfunction after acute or chronic exposure. Sources include iodine supplementation programmes, iodinated contrast studies, amiodarone, and excess seaweed consumption. No numbers in the abstract (Not applicable) Methods, limits and fundingOutcomes: Iodine-induced hypothyroidism and hyperthyroidism Limitations: Narrative review. Funding and conflicts: No conflicts of interest declared. Read: abstract |
| Nazeri 2021 Effects of iodine supplementation during pregnancy on pregnant women and their offspring: a systematic review and meta-analysis of trials over the past 3 decades. Eur J Endocrinol · PMID 33112293 · DOI |
Systematic review meta analysis |
14 trials in the review, 5 in the meta-analysis; pregnant women | Iodine supplements Varies by trial Pregnancy · vs Control groups |
Supplements improved iodine status in mothers and babies and prevented a rise in thyroglobulin during pregnancy. Other thyroid measures were inconclusive. No differences in birth size or in development in the first 2 years. Cognitive SMD 0.07; language 0.06; motor 0.07; birth weight -0.11 (Cognitive -0.07 to 0.20; language -0.22 to 0.35; motor -0.06 to 0.21; birth weight -0.23 to 0.01) Methods, limits and fundingOutcomes: Birth weight, length and head size; infant cognitive, language and motor development in the first 2 years Limitations: Only 5 of 14 trials could be pooled. Funding is not stated. Funding and conflicts: Funding is not stated in the abstract. Read: abstract |
| Sohn 2024 Risks of Iodine Excess. Endocr Rev · PMID 38870258 · DOI |
Narrative review |
Review of excess iodine from diet, supplements, amiodarone and contrast media | Iodine from many sources |
Excessive iodine intake is generally well tolerated. In susceptible people an acute iodine load can cause underactive thyroid (failure to escape the Wolff-Chaikoff effect) or overactive thyroid (Jod-Basedow phenomenon). Sources named: iodine-rich diet, fortified foods, amiodarone, supplements and iodinated contrast media. The authors say more data are needed to define safe upper limits, especially in high-risk groups. No numbers in the abstract (Not applicable) Methods, limits and fundingOutcomes: Thyroid dysfunction after an iodine load Limitations: Narrative review. Funding and conflicts: Funding is not stated in the abstract. Read: abstract |
| Velasco 2026 Iodine supplementation for preventing iodine deficiency disorders in children and adolescents. Cochrane Database Syst Rev · PMID 42720098 · DOI |
Systematic review meta analysis |
2528 8 randomised trials in 7 countries, 1982 to 2009; infants, children and adolescents |
Iodine alone, or with iron in one trial, oral or injected One trial used a daily dose for 28 weeks; the others used a single large dose (as studied in the trials) 3 to 22 months of follow-up · vs Placebo |
Slight gain in thinking scores (1 study, 166 children, rated high-certainty by the authors). Large fall in underactive thyroid (1 study, 264 children, moderate). Fewer infant deaths (1 study, 617, moderate). Little or no difference in weight (low). Goitre: very uncertain overall; after removing one high-risk-of-bias study a large fall (moderate). No trial reported side effects related to iodine. Cognitive SMD 0.19; hypothyroidism RR 0.03; infant deaths RR 0.32; weight MD -0.20 kg; goitre RR 0.68 (sensitivity analysis RR 0.31) (Cognitive 0.04 to 0.34; hypothyroidism 0.00 to 0.23; deaths 0.14 to 0.73; weight -1.92 to 1.52; goitre 0.15 to 3.07 (sensitivity 0.22 to 0.45)) Methods, limits and fundingOutcomes: Thinking (cognitive) scores, growth, underactive thyroid, goitre, deaths in infants, adverse events Limitations: Few trials, several outcomes rest on a single study, trials are old and mostly from iodine-deficient settings. Single large doses are not the same as daily intake. Funding and conflicts: No dedicated funding; the authors declare no conflicts. Read: abstract |
| Zimmermann 2009 Iodine deficiency. Endocr Rev · PMID 19460960 · DOI |
Narrative review |
Review of iodine deficiency worldwide | Iodine (diet and salt iodisation) |
Estimated 2 billion people have insufficient iodine intake. About 50% of Europe was mildly deficient. Deficiency in pregnancy and infancy may impair growth and brain development and raise infant mortality. Deficiency in childhood reduces growth, thinking and movement. Measures: urinary iodine, goitre, newborn TSH, blood thyroglobulin. Universal salt iodisation is the best strategy in most countries. The author states that assessing status in pregnancy is difficult. 2 billion people; about 50% of Europe mildly deficient (Not applicable) Methods, limits and fundingOutcomes: Iodine deficiency disorders, assessment methods, salt iodisation Limitations: 2009 review; intakes have changed since. Funding and conflicts: Funding is not stated in the abstract. Read: abstract |
| Zimmermann 2015 Iodine deficiency and thyroid disorders. Lancet Diabetes Endocrinol · PMID 25591468 · DOI |
Narrative review |
Review of populations and individuals with low, adequate and high iodine intake | Iodine (diet and salt iodisation) |
Severe deficiency causes goitre and underactive thyroid. In mild to moderate deficiency the thyroid compensates, at the price of more toxic nodular goitre and overactive thyroid across a population. Raising intake in a deficient population can cause a transient rise in overactive thyroid and a small rise in subclinical underactive thyroid and thyroid autoimmunity. Iodine intake does not change the risk of Graves disease or thyroid cancer overall. No numbers in the abstract (Not applicable) Methods, limits and fundingOutcomes: Goitre, hypothyroidism, hyperthyroidism, thyroid autoimmunity, thyroid cancer Limitations: Narrative review, not a systematic search per the abstract. Funding and conflicts: Funding is not stated in the abstract. Read: abstract |
| s-lpi Linus Pauling Institute, Oregon State University: Iodine (Micronutrient Information Center) · Source |
Health agency n/a |
Methods, limits and fundingRead: full-text | ||
| s-medlineplus MedlinePlus: Iodine in diet · Source |
Health agency n/a |
Methods, limits and fundingRead: full-text | ||
| s-nhs NHS: Vitamins and minerals - Iodine · Source |
Health agency n/a |
Methods, limits and fundingRead: full-text | ||
| s-ods NIH Office of Dietary Supplements: Iodine fact sheet (health professional) · Source |
Health agency n/a |
Methods, limits and fundingRead: not-read |
What we do not know yet
- No trial of iodine pills in adults who already get enough was found for energy, weight or mood.
- Moderately or severely iodine-deficient pregnant women have few well-designed trials.
- Allergy, kidney and liver safety, and heavy metals in seaweed were not researched.
- The NIH Office of Dietary Supplements fact sheet and the original National Academies report could not be read; limits come from a university page.
- Interactions with levothyroxine were not found; warfarin and lithium rest on one secondary source.
How new research is added
New studies are found by a saved PubMed search and placed in a review queue before the page changes.
Lab markers
What changed in blood or saliva tests during trials. A changed number is not the same as feeling or being healthier, and we give no "normal ranges".
Hormonal 1
TSH (thyroid-stimulating hormone) and free thyroxine
No consistent change in trials
TSH is the pituitary signal that tells the thyroid to work. Free thyroxine is the main thyroid hormone in the blood.
Low certaintyResults and what it means
Why it was measured
They show whether the thyroid is under- or over-active.
Statistical vs. clinical meaning
Supplements did not move these tests in mildly deficient pregnant women.
Safety implication
Both too little and too much iodine can change thyroid tests.
| Study | Result | People | Product |
|---|---|---|---|
| 3 | Most studies showed no effect of iodine supplements on maternal or infant TSH and free thyroxine | Mildly to moderately iodine-deficient pregnant women (10 randomised trials in the review) | Iodine supplements in pregnancy |
Other 3
Urinary iodine concentration (UIC)
Rises with intake in trials
The amount of iodine in a urine sample. It is the usual way to judge a group's iodine intake.
Moderate certaintyResults and what it means
Why it was measured
Most iodine taken in leaves in urine, so it reflects recent intake.
Statistical vs. clinical meaning
Useful for groups, less reliable for one person from one sample.
Safety implication
A single sample varies from day to day; ask a clinician how to read it.
| Study | Result | People | Product |
|---|---|---|---|
| 4 | Median urinary iodine of a population reflects total iodine intake from all sources and can identify populations with excess intake | Population surveys (review) | Not applicable |
| 11 | Listed with goitre, newborn TSH and blood thyroglobulin as ways to assess iodine status; assessing status in pregnancy is difficult | Review of deficiency | Not applicable |
Thyroglobulin (a thyroid protein)
Lowered in trials
A protein made in the thyroid that leaks into the blood. It tends to rise when iodine is low.
Moderate certaintyResults and what it means
Why it was measured
It is used as a marker of iodine status.
Statistical vs. clinical meaning
Shows supplements improve iodine status; it does not by itself show a health benefit.
Safety implication
None specific.
| Study | Result | People | Product |
|---|---|---|---|
| 3 | Most randomised trials found iodine supplements lowered maternal thyroglobulin | Mildly to moderately iodine-deficient pregnant women | Iodine supplements in pregnancy |
| 8 | Iodine supplements prevented the rise in thyroglobulin during pregnancy (more consistent than other thyroid measures) | Pregnant women in 14 trials | Iodine supplements in pregnancy |
Thyroid size (volume)
Smaller increase in trials
The size of the thyroid gland measured by ultrasound. It grows when iodine is low.
Low certaintyResults and what it means
Why it was measured
Enlargement is an early sign of deficiency.
Statistical vs. clinical meaning
Fits the idea that supplements ease strain on the thyroid when iodine is short.
Safety implication
None specific.
| Study | Result | People | Product |
|---|---|---|---|
| 3 | In 3 randomised trials iodine prevented or diminished the rise in maternal thyroid volume during pregnancy | Mildly to moderately iodine-deficient pregnant women | Iodine supplements in pregnancy |
Safety
What studies and official sources say about side effects, medicines and who should check first. Each point shows what kind of evidence it comes from.
Main points to know 4
- Too much iodine can upset the thyroid. Excess iodine is well tolerated by most healthy people. In some people it can cause an overactive or underactive thyroid, goitre or thyroid autoimmunity. People with thyroid disease, or who were once short of iodine, may react at intakes only slightly above need. The problems are usually mild and temporary, but iodine-induced overactive thyroid can be life-threatening in some people. Narrative review 4 9 7
- Kelp and seaweed supplements vary hugely in iodine. In a lab analysis of 96 seaweed products on the Norwegian market, iodine per portion ranged from 30 to 62,400 micrograms. For 54 products, one portion or daily dose would exceed the tolerable upper intake level. Labels on several products were inadequate or inaccurate. Supplements ranged from 5 to 5,600 micrograms per daily dose. Laboratory analysis 1
- Thyroid medicines and amiodarone. Amiodarone (a heart rhythm medicine) contains large amounts of iodine and can affect the thyroid. Antithyroid drugs and long-term lithium may raise the chance of underactive thyroid when combined with extra iodine. Talk to the prescriber or pharmacist before adding iodine or kelp. Authoritative guidance 13 9
- Pregnancy and breastfeeding need more iodine, but extra pills are not proven. Official recommended amounts are higher in pregnancy (220 micrograms) and breastfeeding (290 micrograms) than for other adults (150 micrograms), by MedlinePlus. Trials of supplements in mildly deficient pregnancy did not show a child brain benefit, and a Cochrane review found more digestive upset in one small trial. Ask a doctor or midwife before supplementing. Authoritative guidance 14 6 5
Side effects 3
| Effect | How often | Severity and recovery | Evidence |
|---|---|---|---|
| Underactive or overactive thyroid, goitre, or thyroid autoimmunity after high intake Susceptible groups include people with thyroid disease and people previously iodine deficient. In populations, raising intake in a deficient area can transiently raise overactive thyroid (Zimmermann 2015). | Not given as a rate in the abstracts read. | usually mild and temporary; iodine-induced overactive thyroid can be life-threatening in some people Described as usually transient in the review. | Narrative review 4 9 7 |
| Digestive upset in pregnancy with iodine supplements Very low-quality evidence from one small trial. | 1 trial of 76 women; RR 15.33 (95% CI 2.07 to 113.70) | not described Not described. | Controlled trial 6 |
| Possible drop in thyroid function with very high intake (MedlinePlus) MedlinePlus says high doses taken with anti-thyroid medicines may cause underactive thyroid. | MedlinePlus says iodine poisoning is rare in the US. | not described Not described. | Authoritative guidance 14 |
Liver
No liver injury from iodine was found in the sources read. A liver-safety database (LiverTox) was not searched.
How often. No frequency was found.
Monitoring. No agency monitoring advice was found.
This is a safety connection. It is not evidence that Iodine supports liver health.
Medicines and supplements 6
| With | Why it matters | Importance | Evidence | What to do |
|---|---|---|---|---|
| Amiodarone (heart rhythm medicine) prescription | Amiodarone contains large amounts of iodine and can change thyroid function. | A 2024 review names amiodarone as a source of iodine excess that can cause thyroid dysfunction in susceptible people. | Narrative review Moderate certainty 9 7 13 | Tell the prescriber about any iodine or kelp product. |
| Antithyroid drugs (propylthiouracil, methimazole, carbimazole) prescription | Not described in the source. | The Linus Pauling Institute page says these medicines may raise the risk of underactive thyroid when combined with extra iodine. MedlinePlus says high doses with anti-thyroid medicines may cause underactive thyroid. | Authoritative guidance Low certainty 13 14 | Ask the prescriber or pharmacist before taking iodine or kelp. |
| Lithium (long-term) prescription | Not described in the source. | The Linus Pauling Institute page says long-term lithium may raise the risk of underactive thyroid with extra iodine. Primary studies were not read. | Authoritative guidance Low certainty 13 | Ask the prescriber or pharmacist. |
| Warfarin (pharmacologic doses of potassium iodide) prescription | Not described in the source. | The Linus Pauling Institute page says pharmacologic doses of potassium iodide may decrease the blood-thinning effect of warfarin. Primary studies were not read. | Authoritative guidance Very low certainty 13 | Ask the prescriber or pharmacist. |
| Iodinated contrast dye used in scans medical procedure | The dye carries a large iodine load. | A 2024 review calls contrast media a common source of iodine excess in healthcare and reviews screening for dye-induced thyroid problems. | Narrative review Moderate certainty 9 | Tell the care team about thyroid disease before a scan with contrast. |
| Levothyroxine (thyroid hormone tablets) and other supplements prescription | Not found. | No source read addressed iodine with levothyroxine. | None found Not enough evidence | Ask the prescriber or pharmacist. |
Who should check first 4
| Situation | Advice | Evidence |
|---|---|---|
| Thyroid disease (overactive, underactive, nodules, autoimmune), or past iodine deficiency | Ask a doctor before using iodine, kelp or seaweed supplements. | Narrative review 4 9 |
| Taking amiodarone, antithyroid drugs or lithium | Ask the prescriber before adding iodine or kelp. | Authoritative guidance 13 |
| Pregnancy, breastfeeding or planning pregnancy | Ask a doctor or midwife; needs are higher, but supplements were not shown to help the child's brain in mild deficiency. | Controlled trial 5 3 14 |
| Children | Official upper limits are much lower for children than adults (200 micrograms for ages 1 to 3 and 300 for ages 4 to 8, by the National Academies via the Linus Pauling Institute). Ask a pediatrician. | Authoritative guidance 13 |
Overdose, product quality and regulators
Summary. Excess iodine is mostly a thyroid issue. MedlinePlus says iodine poisoning is rare in the US. No lethal-dose figure was read.
Animal. Not reviewed.
High dose or long term signals. Long-term high intakes could change how the thyroid works (NHS). The 2024 review says more data are needed to define safe upper limits, especially in high-risk groups.
Overdose response. Not found.
Genotoxicity. Not researched.
The main quality issue found is the huge range of iodine in kelp and seaweed products and inaccurate labels. Heavy metals and arsenic in seaweed were not researched here.
Iodine content and labels of kelp and seaweed products. Per portion: 128 to 62,400 micrograms in whole seaweed, 30 to 25,300 in seaweed foods, 5 to 5,600 per daily dose in supplements. Several labels inadequate or inaccurate. 1
| Body | Action | Date |
|---|---|---|
| US National Academies (Institute of Medicine), as reported by the Linus Pauling Institute (United States) | Tolerable upper intake level for iodine: 200 micrograms a day for ages 1 to 3, 300 for 4 to 8, 600 for 9 to 13, 900 for 14 to 18, and 1,100 for adults 19 and over. Not possible to establish for infants, whose intake should come from food and formula only. This is a limit, not a target. source | 2024-05 |
| MedlinePlus (US National Library of Medicine) (United States) | Recommended dietary amounts: 150 micrograms for ages 14 and older, 220 in pregnancy, 290 when breastfeeding; infants 110 (0 to 6 months) and 130 (7 to 12 months); children 90 (1 to 8 years), 120 (9 to 13). The page gives no upper limit. source | 2025-01-21 |
| NHS (United Kingdom) | Adults need 140 micrograms a day. Supplements of 0.5 mg (500 micrograms) or less a day are unlikely to cause harm; long-term high doses could change how the thyroid works. Strict vegans may want fortified foods or a supplement and should seek advice. source | 2020-08-03 |
When to get medical help
- Racing heart, tremor, sweating, weight loss, or a fast pulse after starting iodine or kelp (possible overactive thyroid): contact a doctor promptly
- A fast or irregular heartbeat with chest pain, fainting or severe weakness: call 911 or your local emergency number
- New neck swelling, trouble swallowing or breathing: seek urgent medical help
Stop using the product and contact a clinician or emergency service. In the US, Poison Control is 1-800-222-1222.
General information only, not medical advice. Ask a clinician or pharmacist before use, especially with medicines, in pregnancy or with a health condition. See the Natural Remedy Safety guide.
Frequently asked questions
What is iodine?
What does iodine do in the body?
What foods have iodine?
What happens if you do not get enough iodine?
Do iodine supplements help during pregnancy?
Do iodine supplements help children think better?
A 2026 Cochrane review found a slight rise in thinking scores, from one study of 166 children. It does not show a benefit for children who already get enough.
Can you get too much iodine?
Is kelp a safe way to get iodine?
Kelp products vary hugely. In one lab study of 96 products, iodine ranged from 5 to 62,400 micrograms per portion or dose, and 54 products would exceed the upper limit in a single portion. Labels were not always accurate.
Does iodine interact with medicines?
Do vegans need to think about iodine?
NHS says people on a strict vegan diet that excludes fish, eggs and dairy may want fortified foods or a supplement, and should seek advice from a healthcare professional.
Does extra iodine boost metabolism, energy or weight loss?
Explore by health goal
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Editorial transparency
- Written by
- AlternativeMed editorial team. Research compiled with AI assistance, then checked against the sources listed.
- Medical review
- None. This page has not been reviewed by a physician or pharmacist.
- First published
- 2026-10-26
- Last substantive update
- 2026-10-26
- How we research
- We started from our existing iodine summary, then read MedlinePlus, NHS and a Linus Pauling Institute page and re-read PubMed abstracts for the trials and reviews cited. The NIH Office of Dietary Supplements page could not be opened. Only abstracts were read for studies. Items we could not verify are flagged in the research gaps.
- How we rate evidence
- See "How we rate evidence" in the Evidence tab.
- Independence
- No product is promoted on this page. No affiliate links are shown. One trial cited was funded in part by the Nestle Foundation; this is stated where it is used.
Sources 16
Each source was opened and checked. Where we read only the abstract, it says so. The small numbers in the text point here.
- Aakre I, Solli DD, Markhus MW, et al.. Commercially available kelp and seaweed products - valuable iodine source or risk of excess intake?. Food Nutr Res. 2021. PMID 33889064. DOI 10.29219/fnr.v65.7584. (abstract)
- Bath SC, Steer CD, Golding J, Emmett P, Rayman MP. Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC).. Lancet. 2013. PMID 23706508. DOI 10.1016/S0140-6736(13)60436-5. (abstract)
- Dineva M, Fishpool H, Rayman MP, Mendis J, Bath SC. 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. PMID 32320029. DOI 10.1093/ajcn/nqaa071. (abstract)
- Farebrother J, Zimmermann MB, Andersson M. Excess iodine intake: sources, assessment, and effects on thyroid function.. Ann N Y Acad Sci. 2019. PMID 30891786. DOI 10.1111/nyas.14041. (abstract)
- Gowachirapant S, Jaiswal N, Melse-Boonstra A, et al.. Effect of iodine supplementation in pregnant women on child neurodevelopment: a randomised, double-blind, placebo-controlled trial.. Lancet Diabetes Endocrinol. 2017. PMID 29030199. DOI 10.1016/S2213-8587(17)30332-7. (abstract)
- Harding KB, Pena-Rosas JP, Webster AC, et al.. Iodine supplementation for women during the preconception, pregnancy and postpartum period.. Cochrane Database Syst Rev. 2017. PMID 28260263. DOI 10.1002/14651858.CD011761.pub2. (abstract)
- Leung AM, Braverman LE. Iodine-induced thyroid dysfunction.. Curr Opin Endocrinol Diabetes Obes. 2012. PMID 22820214. DOI 10.1097/MED.0b013e3283565bb2. (abstract)
- Nazeri P, Shariat M, Azizi F. Effects of iodine supplementation during pregnancy on pregnant women and their offspring: a systematic review and meta-analysis of trials over the past 3 decades.. Eur J Endocrinol. 2021. PMID 33112293. DOI 10.1530/EJE-20-0927. (abstract)
- Sohn SY, Inoue K, Rhee CM, Leung AM. Risks of Iodine Excess.. Endocr Rev. 2024. PMID 38870258. DOI 10.1210/endrev/bnae019. (abstract)
- Velasco I, Rueda-Etxebarria M, Trak-Fellermeier MA, et al.. Iodine supplementation for preventing iodine deficiency disorders in children and adolescents.. Cochrane Database Syst Rev. 2026. PMID 42720098. DOI 10.1002/14651858.CD014475.pub2. (abstract)
- Zimmermann MB. Iodine deficiency.. Endocr Rev. 2009. PMID 19460960. DOI 10.1210/er.2009-0011. (abstract)
- Zimmermann MB, Boelaert K. Iodine deficiency and thyroid disorders.. Lancet Diabetes Endocrinol. 2015. PMID 25591468. DOI 10.1016/S2213-8587(14)70225-6. (abstract)
- . Linus Pauling Institute, Oregon State University: Iodine (Micronutrient Information Center). . . (full-text)
- . MedlinePlus: Iodine in diet. . . (full-text)
- . NHS: Vitamins and minerals - Iodine. . . (full-text)
- . NIH Office of Dietary Supplements: Iodine fact sheet (health professional). . . (not-read)
Where to buy
Check the label first. Whatever you buy, look for these:
- The amount of the active ingredient per serving, not just the front-label claim
- A third-party testing mark (USP, NSF or ConsumerLab)
- Other ingredients and allergens
- Ask a pharmacist about interactions with your medicines
Talk to a pharmacist or doctor before you start any supplement, especially if you take medicines. See the Safety tab for interactions.
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