Iodine: Thyroid, Food Sources, Deficiency and Excess
Iodine and the thyroid: food sources, official intake levels by country, deficiency, pregnancy, why too much also matters, and the very wide iodine range in kelp and seaweed products.
Content last updated . This page has not been medically reviewed.
What is Iodine good for?
What the body uses it for, and how strong the evidence is. A normal job in the body is not proof that extra supplements add benefit; each card says which.
Making thyroid hormones
Established roleIodine is a building block of thyroid hormones, which the thyroid gland makes.
Supplements: Supplements correct low intake; no good evidence of extra benefit when intake is adequate, and too much can harm the thyroid.
Normal thyroid gland size and function
Deficiency linkThe thyroid needs iodine to work normally. When intake is low the gland can enlarge, which is called goiter.
Supplements: Iodized salt prevents most simple goiters; extra iodine in people with enough does not add benefit.
Keeping metabolic rate healthy
Established roleThyroid hormones help keep the speed of the body's chemical reactions (metabolic rate) healthy, and iodine is needed to make them.
Supplements: Normal thyroid hormone production needs iodine. We found no source showing that extra iodine raises energy or speeds metabolism in people with enough iodine.
Growth and brain development in the unborn child and infant
Deficiency linkLow iodine in pregnancy and infancy can affect a child's growth and brain development. Needs are higher in pregnancy and breastfeeding.
Supplements: Cochrane found insufficient evidence on routine iodine supplements around pregnancy, mostly low-quality trials from mildly or moderately deficient areas.
Preventing endemic cretinism
Deficiency linkEnough iodine helps prevent cretinism, a severe developmental condition from deficiency that is very rare in the US.
Supplements: Prevention comes from adequate dietary iodine (for example iodized salt), not from extra supplements.
Children's growth, thinking and movement
Deficiency linkIn childhood, deficiency can reduce growth and thinking and motor function.
Supplements: Relevant to deficiency; no source we read shows benefit of extra iodine in children who have enough.
Helping cells turn food into energy
Established roleMedlinePlus lists helping cells convert food into energy among iodine's roles.
Supplements: Supplements correct low intake; no good evidence of extra benefit when intake is adequate, and too much can harm the thyroid.
Connected topics
Open a group to see how each topic connects and how strong the evidence is.
Health goals (2)
- Thyroid SupportEstablished role
Iodine is essential for normal thyroid function, but both too little and too much can cause problems, so ask a clinician before taking supplements or kelp.
- Metabolic HealthEstablished role
Iodine is needed for the thyroid hormones that help keep metabolic rate healthy. There is no evidence extra iodine improves metabolism when intake is adequate.
Body systems (4)
- Endocrine SystemEstablished role
Iodine is needed to make thyroid hormones, so it supports the endocrine (hormone) system.
- Hormone HealthEstablished role
Iodine is a building block of thyroid hormones. Both too little and too much can upset thyroid function.
- Metabolism & WeightEstablished role
Thyroid hormones, which need iodine, help keep metabolic rate healthy. No source shows extra iodine aids weight loss in people with enough iodine.
- Nervous SystemDeficiency link
Enough iodine in pregnancy and early life supports normal brain development; deficiency can impair it. This is about deficiency, not extra supplements.
Body parts (2)
Functional actions (1)
- Endocrine SupportEstablished role
Iodine is needed to make thyroid hormones, a core part of hormone (endocrine) function.
Root causes (1)
- Nutrient DeficiencyDeficiency link
Low iodine intake can cause goiter and underactive thyroid, most often where soil is iodine-poor. It is rare in the UK and US.
Habits (1)
- Balanced NutritionEstablished role
Most people can get enough iodine from a varied, balanced diet including dairy, eggs, fish and iodized salt. Strict vegans may need fortified foods or advice.
Biomarkers (1)
- TSHDeficiency link
Thyroid function is checked with blood tests such as TSH; both iodine deficiency and excess can change thyroid function. TSH does not measure iodine intake itself.
Conditions & symptoms (7)
- Endemic GoiterDeficiency link
Goiter that occurs in areas with iodine-poor soil, where deficiency is common.
- GoiterDeficiency link
Iodine deficiency is the most common cause of simple goiter, an enlarged thyroid. Iodized salt prevents most cases.
- Iodine Deficiency SyndromeDeficiency link
Low iodine causes a range of problems called iodine deficiency disorders, from goiter to impaired development.
- Endemic CretinismDeficiency link
Adequate iodine helps prevent cretinism, a severe developmental condition now very rare in the US.
- Hashimoto DiseaseDocumented risk
People with Hashimoto's or other autoimmune thyroid disease may be especially sensitive to the harmful effects of extra iodine, such as from kelp or supplements.
- HyperthyroidismDocumented risk
Excess iodine can trigger an overactive thyroid in some people, particularly those with existing thyroid disease or past deficiency.
- HypothyroidismDeficiency link
Months of low iodine can cause underactive thyroid, but in the UK and US the usual cause is autoimmune (Hashimoto's disease), and too much iodine can also cause or worsen it.
Documented risks and interactions (1)
- Too much iodine can disturb the thyroid. High intake over long periods can change how the thyroid works, causing an overactive or underactive thyroid, goiter or thyroid autoimmunity in some people.
Safety first. Both too little and too much iodine can affect the thyroid. People with thyroid disease, people taking thyroid medicines, and anyone pregnant or breastfeeding should ask a clinician or pharmacist before taking iodine or kelp. [1][2] Thyroid symptoms need blood tests, not self-treatment.
Overview
Iodine is a trace mineral the body uses to make thyroid hormones, which help keep cells and the metabolic rate healthy. [3] MedlinePlus says humans need iodine for normal thyroid function and for production of thyroid hormones. [1] Evidence labels used on this page: established role / deficiency prevention, mixed, insufficient, marketing claim, Proper evidence not found.Food sources
The NHS (UK) lists cows' milk and dairy products, eggs, sea fish and shellfish, and adds that cereals and grains contain iodine in amounts that depend on soil iodine. [3] MedlinePlus lists iodized salt as the main food source, with seafood, dairy products and kelp also rich. [1]| Food (US examples) | Amount of iodine | Source |
|---|---|---|
| Iodized salt, one quarter teaspoon | 78 micrograms (mcg) | MedlinePlus, reviewed 2025 [1] |
| Cod, 3 ounces | 146 mcg | MedlinePlus, reviewed 2025 [1] |
Seaweed varies far more; see below. For a longer list see Foods high in iodine and iodized salt and public health. The NHS says strict vegans who eat no fish, eggs or dairy may want iodine-fortified foods (some plant-based drinks are fortified) or a supplement, and to seek advice from a healthcare professional. [3] See Supplements in vegetarian and vegan diets.
Official intake levels
Amounts are micrograms of iodine a day. Bodies differ and none is a target.| Body and year | Group | Level |
|---|---|---|
| US Dietary Reference Intakes, via MedlinePlus (reviewed 2025) | Children 1 to 8; 9 to 13 | RDA 90 mcg; 120 mcg [1] |
| US Dietary Reference Intakes, via MedlinePlus | Age 14 and older | RDA 150 mcg [1] |
| US Dietary Reference Intakes, via MedlinePlus | Pregnancy; breastfeeding | RDA 220 mcg; 290 mcg [1] |
| US Dietary Reference Intakes, via MedlinePlus | Infants 0 to 6 months; 7 to 12 months | Adequate Intake 110 mcg; 130 mcg [1] |
| NHS (UK), page reviewed 2020 | Adults | 140 mcg [3] |
| NHS (UK) | Supplements | 0.5 mg (500 mcg) or less a day unlikely to cause harm [3] |
| EFSA (EU), 2025 overview | Adults | Tolerable upper intake level 600 mcg from all dietary sources [4] |
| EFSA (EU) | Children 1 to 3; 4 to 6; 7 to 10 years | Tolerable upper intake level 200; 250; 300 mcg [4] |
| Health Canada dietary reference intake tables | Adults | Upper level 1,100 mcg [5] |
An RDA is the level that meets the needs of nearly all healthy people; an Adequate Intake is a best estimate where evidence is thinner. [1] EFSA defines its upper level as the maximum level of total chronic daily intake not expected to pose a risk of adverse health effects. [4] Some people react to intakes only slightly above need, so it is not a safe amount for everyone. [6] The NIH Office of Dietary Supplements pages were blocked to us, so US figures come from MedlinePlus.
Deficiency
MedlinePlus says many months of low iodine may cause goiter (an enlarged thyroid) or hypothyroidism, most often where soil is iodine-poor, and that lack of iodine is common in pregnant women and older children. [1] The NHS describes deficiency as rare in the UK. [7] A 2009 review estimated about 2 billion people worldwide have insufficient intake, and said assessing iodine status in pregnancy is difficult. [8] No one should diagnose deficiency from symptoms; thyroid blood tests are ordered by a clinician. See iodine and the thyroid and Thyroid problems and fatigue.Evidence by use
| Question | What the source found | Strength |
|---|---|---|
| Preventing goiter and deficiency-related hypothyroidism | Iodized table salt prevents most simple goiters. [9] | Established role |
| Supplements in pregnancy and breastfeeding | A 2017 Cochrane review found insufficient data on routine supplementation before, during or after pregnancy. [10] See iodine in pregnancy. | Insufficient |
| Iodine for general energy or "metabolism" in people with enough iodine | Proper evidence not found. We read NHS, MedlinePlus and NIDDK pages and searched PubMed for trials of iodine supplements in adults with normal thyroid function and found none. | Marketing claim |
Too much iodine
The NHS says taking high doses for long periods could change the way the thyroid works, with symptoms such as weight gain. [3] MedlinePlus says very high intake can reduce thyroid function, and that high doses with anti-thyroid medicines can add up and cause hypothyroidism. [1] A 2019 review says excess iodine may precipitate hyperthyroidism, hypothyroidism, goiter or thyroid autoimmunity in some people, and that people with earlier thyroid disease may be more susceptible. [6] NIDDK says some cough syrups and heart medicines may contain a lot of iodine. [11]Kelp and seaweed products
Iodine in seaweed varies enormously. A US analysis of 12 commercially available seaweeds found 16 micrograms per gram in nori and over 8,165 micrograms per gram in one processed kelp product; the authors said some seaweed dishes may exceed 1,100 micrograms a day. [12] A kelp product can therefore supply far more than a day's reference intake. See Seaweed, sea moss and kelp and kelp and seaweed supplements.Medicines, life stages and conditions
- Medicines. Iodine can add to anti-thyroid medicines. [1] Amiodarone and iodinated contrast media are iodine sources in health care. [13] Ask a pharmacist; see medication interactions.
- Pregnancy and breastfeeding. Needs rise; the sources we read advise asking a clinician about prenatal products. [14] See prenatal and postnatal vitamins.
- Children. The sources we read give reference intakes for children but no supplement advice; ask a pediatrician.
- Kidney or liver conditions, and before surgery. Not addressed in the sources we read; that is not proof of safety. See supplements before surgery and existing conditions.
Labels and quality
Check whether a label states micrograms of iodine per serving, and add up multivitamins and kelp products; see reading supplement labels and quality testing. We did not find a source comparing iodine forms for health outcomes: Proper evidence not found.Frequently asked questions
Do I need an iodine supplement?
The NHS says most people can get all the iodine they need from a varied and balanced diet, and to seek advice before using a supplement. [3]
Is kelp a safe way to get iodine?
Iodine in kelp products varies hugely, and one product in a US analysis had over 8,000 micrograms per gram. [12]
Can too much iodine harm the thyroid?
Yes, in some people. The NHS and MedlinePlus both say high intake can change thyroid function. [3][1]
Does iodine matter in pregnancy?
Needs are higher (US RDA 220 micrograms), but trial evidence on supplements is unsettled, so ask a clinician. [1][10]
What are good food sources of iodine?
Iodized salt, seafood, dairy products and eggs. [1][3]
References
- MedlinePlus (US National Library of Medicine). Iodine in diet (review date 21 January 2025). Link. Read 2026-10-23.
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK, US). Hypothyroidism (underactive thyroid) (last reviewed March 2021). Link. Read 2026-10-23.
- NHS (UK). Vitamins and minerals: Iodine (page last reviewed 3 August 2020). Link. Read 2026-10-23.
- European Food Safety Authority. Overview on Tolerable Upper Intake Levels as derived by the SCF and the EFSA NDA Panel, version 11 (August 2025). Link. Read 2026-10-23.
- Health Canada. Dietary reference intakes: reference values for elements (tables). Link. Read 2026-10-23.
- Excess iodine intake: sources, assessment, and effects on thyroid function. Ann N Y Acad Sci. 2019. Link. Read 2026-10-23.
- NHS (UK). Underactive thyroid (hypothyroidism) (page last reviewed 28 April 2025). Link. Read 2026-10-23.
- Zimmermann MB. Iodine deficiency. Endocr Rev. 2009. Link. Read 2026-10-23.
- MedlinePlus (US National Library of Medicine). Simple goiter (review date 25 January 2026). Link. Read 2026-10-23.
- Harding KB et al. Iodine supplementation for women during the preconception, pregnancy and postpartum period. Cochrane Database Syst Rev. 2017. Link. Read 2026-10-23.
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK, US). Hyperthyroidism (overactive thyroid) (last reviewed August 2021). Link. Read 2026-10-23.
- Teas J et al. Variability of iodine content in common commercially available edible seaweeds. Thyroid. 2004. Link. Read 2026-10-23.
- Risks of Iodine Excess. Endocr Rev. 2024. Link. Read 2026-10-23.
- MotherToBaby (Organization of Teratology Information Specialists). Iodine fact sheet (April 2025). Link. Read 2026-10-23.