Iodine

What it is

Iodine is a trace element found naturally in some foods, added to some types of salt and sold as a supplement. It is an essential part of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Supplements usually contain potassium iodide or sodium iodide; kelp and seaweed products also supply it, in amounts that vary widely.

What the research says

The thyroid needs iodine to make its hormones, which control growth and brain development. Severe deficiency is well established to cause goitre and harm children's growth and mental development. Whether supplement pills help people who are only mildly low is much less clear: a Cochrane review of pregnancy supplements rated the evidence low to very low, and a large placebo-controlled trial in mildly deficient pregnant women found no effect on the child's IQ at age 5 to 6. Too much iodine can also cause thyroid problems, and kelp supplements can contain far more than their labels suggest.

Evidence grade: Limited evidence.

What goes wrong

In cross-sectional studies, adults in populations with excess iodine (mainly from salt or water) had higher odds of overt and subclinical hypothyroidism. These are associations from low-quality studies. (Source 1)

• Meta-analysis, Low certainty.
• Size: 50 articles; 3 cross-sectional studies pooled for adults.
• Who: Adults in populations with excess vs adequate iodine.
• How long: Not applicable.
• Result: Overt hypothyroidism OR 2.78 (1.47 to 5.27); subclinical OR 2.03 (1.58 to 2.62)
• Funding: No specific funding.

Odds ratio of overt and subclinical hypothyroidism between excess and adequate populations were 2.78 (CI:1.47 to 5.27) and 2.03 (CI:1.58 to 2.62) in adults, respectively.

A 2024 review states that a large iodine load can cause an underactive or overactive thyroid in susceptible people, and lists iodine supplements, amiodarone and contrast dye among the sources. (Source 2)

• Systematic review, Certainty not rated.
• Size: Narrative review.
• Who: Susceptible individuals.
• How long: Not applicable.
• Result: Not applicable.
• Funding: Not stated.

Although excessive iodine intake or exposure is generally well tolerated, an acute iodine load may result in thyroid dysfunction (hypothyroidism or hyperthyroidism) in certain susceptible individuals

Interaction: the heart drug amiodarone delivers a large iodine load; each usual 200 mg dose supplies 75 mg of iodine. (Source 2)

• Systematic review, Certainty not rated.
• Size: Not applicable.
• Who: People taking amiodarone.
• How long: Not applicable.
• Result: 75 mg iodine per 200 mg dose.
• Funding: Not stated.

With each usual daily dose of 200 mg of amiodarone, individuals receive 75 mg of iodine, which like ICM, amounts to several hundredfold the recommended daily iodine intake.

Testing of 96 kelp and seaweed products found iodine per daily dose of supplements ranged from 5 to 5,600 micrograms, 54 products exceeded the upper limit per portion, and labelling was often inadequate or inaccurate. (Source 3)

• Survey study, Certainty not rated.
• Size: 96 products (14 supplements)
• Who: Products on the Norwegian market.
• How long: Not applicable.
• Result: Supplements 5 to 5,600 micrograms per daily dose.
• Funding: Not stated in the abstract.

The labelling of macroalgae species included, and declaration of iodine content, were inadequate or inaccurate for several products.

Interaction (NIH ODS position, 2024): potassium iodide taken with ACE inhibitors can increase the risk of high blood potassium, and high-dose iodine with antithyroid drugs can have an additive effect. (Source 4)

• Official position, Certainty not rated.
• Size: Not applicable.
• Who: People on ACE inhibitors or antithyroid medicines.
• How long: Not applicable.
• Result: Not applicable.
• Funding: Government.

Taking potassium iodide with ACE inhibitors can increase the risk of hyperkalemia

What the evidence supports

Iodised salt increased urinary iodine, and some but not all studies showed less goitre; adverse effects were not adequately studied. (Source 5)

• Systematic review, Low certainty.
• Size: 6 studies.
• Who: Mostly children, some adults, in iodine-deficient areas.
• How long: Varied.
• Result: Qualitative; urinary iodine rose in 5 of 6 studies.
• Funding: Not stated.

Some studies, but not all, also showed a reduction in the enlargement of the thyroid gland (goitre) that can accompany lack of iodine in the diet.

What the evidence does not support

The Cochrane authors said they were not confident in these results. (Source 6)

• Systematic review, Very low certainty.
• Size: 11 trials.
• Who: Women before, during or after pregnancy.
• How long: Varied.
• Result: Not applicable.
• Funding: Not stated.

The quality of the evidence was low to very low, mostly because few trials looked at each outcome or because of limitations in the study designs.

A placebo-controlled trial in mildly iodine-deficient pregnant women in India and Thailand found 200 micrograms of iodine a day had no effect on children's IQ or executive function at age 5 to 6. (Source 7)

• Randomized trial, Moderate certainty.
• Size: 832 women enrolled; 313 children analysed for IQ.
• Who: Mildly iodine-deficient pregnant women, Bangalore and Bangkok.
• How long: Pregnancy until delivery; children followed to age 5-6.
• Result: Verbal IQ difference -0.7 (95% CI -2.9 to 1.5; p=0.77); performance IQ -1.6 (-4.5 to 1.3; p=0.44)
• Funding: Mixed: Swiss National Science Foundation, Nestlé Foundation, Wageningen University and ETH Zurich.

Daily iodine supplementation in mildly iodine-deficient pregnant women had no effect on child neurodevelopment at age 5–6 years.

Where the evidence is mixed

In pregnancy, iodine supplements were linked to less hyperthyroidism after birth but more nausea or vomiting, with low to very low certainty evidence. (Source 6)

• Systematic review, Low certainty.
• Size: Over 2,700 women in 11 trials.
• Who: Women before, during or after pregnancy.
• How long: Varied.
• Result: Outcomes mostly from 1 to 3 trials each.
• Funding: Not stated in the plain language summary.

Women who received iodine supplements were less likely to develop the unwanted effect of hyperthyroidism (an overactive thyroid gland) after giving birth (three trials involving 543 women) but they were more likely to experience nausea or vomiting during pregnancy (one trial involving 76 women) when compared to those who did not receive iodine.

How much

• Reference intake: The US Food and Nutrition Board (Institute of Medicine, 2001) set an RDA of 150 micrograms/day for adults, rising in pregnancy, as reported by NIH ODS (2024). (Source 4)
• Upper limit: The Food and Nutrition Board set a Tolerable Upper Intake Level of 1,100 micrograms/day for adults aged 19 and over, as reported by NIH ODS (2024). (Source 4)
• Studied: 200 micrograms of iodine by mouth once a day from early pregnancy until delivery. (Source 7)

A common belief, and what the research shows

The belief: Kelp is a natural, reliable way to get the right amount of iodine.

What the research shows: Tested kelp and seaweed products varied enormously: 'As macroalgae-containing products are unreliable iodine sources, inclusion of such products in the diet may pose a risk of consuming excessive amounts of iodine.'

Questions and answers

What is it?

Iodine is a trace element. It is found naturally in some foods, added to some salt, and sold as a supplement. (Source 4)

What does it do in the body?

The thyroid uses iodine to make its hormones. The hormones T4 and T3 contain iodine as an essential part. (Source 4)

Is it good or bad for you?

Both, depending on the amount. Too little harms thyroid function and, when severe, children's growth and brain development. Too much, from salt, water, supplements or medicines, is associated with higher rates of hypothyroidism, and a sudden large dose can upset the thyroid in susceptible people. (Source 2)

How do you get more of it?

Studied ways of raising intake include iodised salt, supplements (tablets, capsules, drops or injections in pregnancy trials) and seaweed products. Seaweed-based products vary widely in iodine content. (Source 6)

If it is harmful, what reduces it?

Excess iodine usually comes from supplements, kelp or seaweed products, amiodarone, iodinated contrast dye, or poorly monitored salt or water. The literature points to identifying and limiting the source. Monitoring salt and drinking water iodine is recommended at population level. (Source 1)

Why might someone be low in it or missing it?

Low intake is the usual cause, especially in regions with iodine-poor soil and no iodised salt. The NIH fact sheet says vegans, people with some food allergies or lactose intolerance, and others who eat little or none of the main iodine-containing foods may not get enough. Pregnancy and breastfeeding raise needs. (Source 4)

Which whole foods contain it or feed it?

Seaweed such as kelp, nori, kombu and wakame is among the richest sources, though amounts vary greatly. Fish, other seafood and eggs are also good sources, along with dairy and iodised salt. (Source 4)

What happens if you do not have it?

Iodine deficiency causes goitre (an enlarged thyroid) and low thyroid hormone at any age. In pregnancy and early childhood, severe deficiency can permanently impair physical growth and mental development. Less is known about mild deficiency. (Source 6)

How can you test for it?

Urinary iodine concentration, blood thyroglobulin, newborn TSH and thyroid size are used to judge the iodine status of populations. A single spot urine sample is not reliable for one person, because iodine intake varies from day to day. Thyroid blood tests show thyroid function, not iodine intake directly. (Source 2)

References

1. PLOS ONE. Effect of excess iodine intake on thyroid diseases in different populations: A systematic review and meta-analyses including observational studies. 2017. PMID 28282437, DOI 10.1371/journal.pone.0173722. Read the source
2. Endocrine Reviews. Risks of Iodine Excess. 2024. DOI 10.1210/endrev/bnae019. Read the source
3. Food & Nutrition Research. Commercially available kelp and seaweed products - valuable iodine source or risk of excess intake?. 2021. PMID 33889064, DOI 10.29219/fnr.v65.7584. Read the source
4. NIH Office of Dietary Supplements. Iodine - Health Professional Fact Sheet (updated November 5, 2024). 2024. Read the source
5. Cochrane. Iodised salt for preventing iodine deficiency disorders (plain language summary, CD003204). 2002. DOI 10.1002/14651858.CD003204. Read the source
6. Cochrane. Iodine supplementation for women during the preconception, pregnancy and postpartum period (plain language summary). 2017. PMID 28260263, DOI 10.1002/14651858.CD011761.pub2. Read the source
7. The Lancet Diabetes & Endocrinology. Effect of iodine supplementation in pregnant women on child neurodevelopment: a randomised, double-blind, placebo-controlled trial. 2017. PMID 29030199, DOI 10.1016/S2213-8587(17)30332-7. Read the source