Iodine

Iodine is a trace element, and your body uses it for one main job: making thyroid hormones. Thyroxine (T4) and triiodothyronine (T3) are built around iodine. They set the pace of metabolism and drive the growth of the brain and bones in a fetus and infant. That is how the NIH (the US National Institutes of Health) describes them.

An adult with enough iodine holds only about 15 to 20 mg, and 70% to 80% of it sits in the thyroid gland.

Few nutrients depend so much on where you live. Soil in mountain regions and in flood-prone river valleys holds little iodine, so the crops and animals raised there do too. The fix was salt.

Switzerland’s iodized salt program has run without a break since 1922. By 2020, 124 countries had laws requiring salt iodization, so that 88% of the world’s people use iodized salt. Iodine is also a nutrient with two edges: too little and too much can both upset the thyroid.

Here is what most people do not know. Sea salt, kosher salt and Himalayan salt are not usually iodized, and noniodized sea salt gives virtually no iodine, the NIH says.

In one analysis of 12 kinds of seaweed, nori held 16 µg (micrograms) of iodine per gram. Kelp granules sold as a salt substitute held 8,165 µg per gram. One gram of that kelp holds more than seven times the US upper limit (the most considered safe) of 1,100 µg a day.

Health benefits of iodine

• ★★★Slightly better thinking in children given iodineIn 8 trials in children, iodine slightly raised thinking scores (0.19 standard deviations) and cut hypothyroidism, an underactive thyroid (each from one study). One trial of 150 µg a day for 28 weeks in mildly deficient children raised the overall cognitive score 0.19 standard deviations.
• ★★★Little proof iodine pills help in pregnancyA Cochrane review found too little data to say if routine iodine pills in pregnancy help or harm. In a trial of mildly deficient women, 200 µg a day did not raise children’s IQ at age 5 to 6.
• ★★Iodized salt improved iodine levels in deficient areasIn a Cochrane review of 6 controlled studies, iodized salt improved iodine status in most of them. Goiter, a swollen thyroid, tended to fall, though not clearly in every study.
• ★★Possibly less blood loss in thyroid surgeryGiven by doctors before surgery for an overactive thyroid, iodine cut blood loss across 9 studies, though results varied widely. In one trial of 56 people, blood loss did not clearly differ.
• ★Eating kombu seaweed daily raised TSHHealthy Japanese adults ate 15 or 30 g of kombu a day (35 or 70 mg of iodine) for 7 to 10 days. In some, TSH (a thyroid-signaling hormone) rose above normal limits, and values returned to baseline 7 to 40 days after stopping.

Foods high in iodine

Iodine in food depends on the sea, the soil and what is added. Seaweed is the richest natural source, followed by fish and other seafood and eggs, the NIH says. Dairy products carry iodine from cattle feed supplements and iodine-based sanitizing agents, it adds.

EFSA lists marine products, eggs, milk and iodized salt as the richest sources, and in the UK milk is the main source in the diet. Most fruits and vegetables are poor sources.

Bread is a wild card: two slices made with an iodate dough conditioner give 296 µg, and two slices made without give 1 µg. About 20% of the white bread, whole-wheat bread and bun labels checked listed iodate.

We do not yet have an iodine ranking, because no food in our database has a measured iodine value. So the table below uses published measurements instead.

Amounts vary widely. Nonfat milk ranged from 38 to 160 µg per cup in 59 samples. Across 34 countries, milk iodine ranged from 5.5 to 49.9 µg per 100 g. It was higher in winter than summer, and higher in conventional than organic milk (42.1% lower in organic milk in the UK).

Seaweed varies the most: 16 to 2,984 µg per gram in commercial whole or sheet seaweeds. Kelp granules reach up to 8,165 µg per gram. Iodine also dissolves in cooking water. For scale, adults need 150 µg a day.

Salt matters too. Iodized table salt gives 78 µg in a quarter teaspoon, while noniodized sea salt, kosher salt and Himalayan salt give almost none. Most US salt intake comes from processed foods that use noniodized salt. See also sodium for the other side of salt.

Food	Amount per serving	Source
Bread, white, made with iodate dough conditioner, 2 slices	296 µg	1
Cod, baked, 3 oz (cod)	146 µg	1
Nori, dried, 2 tablespoons flaked (5 g)	116 µg	1
Oysters, cooked, 3 oz	93 µg	1
Yogurt, Greek, plain, nonfat, ¾ cup	87 µg	1
Milk, nonfat, 1 cup (milk)	84 µg (38 to 160 µg in 59 samples)	1
Iodized table salt, ¼ teaspoon	78 µg	1
Fish sticks, cooked, 3 oz	57 µg	1
Egg, hard-boiled, 1 large (eggs)	31 µg	1
Cheddar cheese, 1 oz	14 µg	1
Shrimp, cooked, 3 oz	13 µg	1
Tuna, canned in water, drained, 3 oz	7 µg	1
Soy beverage, 1 cup	3 µg	1
Chicken breast, roasted, 3 oz	1 µg	1
Sea salt, noniodized, ¼ teaspoon	0 µg	1
Dried seaweed, per gram (12 kinds in one analysis; amount per gram, not per serving)	16 to 8,165 µg	2

How iodine works

The basic chemistry is well known, but the details come from physiology studies, reviews and animal work, not from trials of extra iodine in people. Three ideas cover most of it.

The thyroid pulls iodide from the blood and builds hormones from it. Too little iodine makes the thyroid grow. Too much can switch it off or, in some people, on. Selenium and iron help the machinery.

Here is each one, and what has been seen in people.

The thyroid pulls iodide in and builds hormones

Iodine in food is mostly iodide. Iodate, the form in salt and bread, is reduced in the gut and absorbed as iodide too. Iodide is taken up quickly and almost completely in the stomach and first part of the small intestine, the NIH says. EFSA puts the absorption of inorganic iodide above 90%.

The thyroid then pulls iodide from the blood with a pump, the sodium/iodide symporter (NIS), the first step in making thyroid hormone. The same pump works in the salivary glands, the stomach lining and the lactating breast. There it puts iodide into milk for the nursing baby.

Too little iodine makes the thyroid grow

When iodine is short, the thyroid makes less hormone. The pituitary gland responds with more thyroid-stimulating hormone (TSH), which pushes the thyroid to take up more iodine. According to the NIH, TSH typically rises when intake falls below about 100 µg a day.

If intake falls to roughly 10 to 20 µg a day, hypothyroidism occurs despite the high TSH. The gland enlarges into a goiter, usually the earliest clinical sign. TSH can stay in the normal range when intake is low, so it is not a sensitive marker of deficiency.

In milder deficiency the thyroid can compensate and keep hormone levels normal, but at a price. Chronic stimulation raises the number of people with toxic nodular goiter and hyperthyroidism.

Too much iodine can switch it off, or on

A sudden large dose of iodine makes the thyroid stop using iodide to build hormone for a few days. This protective reaction is called the Wolff-Chaikoff effect. In rats, the thyroid then escapes by turning down its iodide pump, so hormone making resumes.

The escape fails in a few apparently normal people, in newborns and fetuses, and in people with autoimmune thyroiditis. It also fails in some treated Graves’ disease patients, and hypothyroidism follows. It is usually transient, and thyroid function returns to normal in 2 to 3 weeks after the iodide stops. Some patients need hormone replacement, and many need long-term follow-up because some go on to permanent hypothyroidism.

The opposite also happens. Thyroid nodules that make hormone on their own are common after long-standing deficiency. Where many people have them, extra iodine can raise the number of people with hyperthyroidism, usually for a limited time. The NIH notes iodine-induced hyperthyroidism can occur even when iodine is given to treat deficiency.

Selenium and iron help the machinery

Thyroid peroxidase, the enzyme that attaches iodine to thyroglobulin (the protein the hormones are built on), depends on iron. Iron-deficiency anemia blunts the effect of iodine supplements, and iron supplements improve it. Selenium is built into the deiodinase enzymes that handle thyroid hormone, and into antioxidant enzymes.

Adequate selenium supports hormone synthesis and protects the gland from damage by excess iodide. In regions with severe deficiency of both, iodine should be corrected before selenium is given, to avoid hypothyroidism.

Some foods contain goitrogens, which interfere with iodine uptake. Soy, cassava and cruciferous vegetables such as cabbage and broccoli are examples. The NIH says this matters mainly where iodine intake is marginal. For most people who eat a variety of foods, it says, reasonable amounts are not a concern.

What has been seen in people

The clearest results are about deficiency. In a severely deficient area of China, iodine in the first or second trimester cut infant neurologic abnormalities to 2%. With later treatment it was 9%. Across 37 studies, children in severely deficient areas scored 12.45 IQ points lower.

In mild deficiency the evidence is mixed. In school-age children iodine improved some thinking tests. But in pregnancy the largest trial found no gain in IQ at 5 to 6 years, and a Cochrane review found too little data.

On the other side, a five-year Chinese cohort linked more than adequate or excessive intake to more hypothyroidism. Kombu and seaweed raised TSH in trials. All of these are among the study cards above.

How much iodine do you need?

Adults need 150 µg of iodine a day. That is both the US recommended dietary allowance (ages 14 and up) and EFSA’s adequate intake (ages 18 and up).

Pregnancy raises the US allowance to 220 µg a day, and breastfeeding to 290 µg. EFSA sets 200 µg for both. The World Health Organization, UNICEF and the International Council for the Control of Iodine Deficiency Disorders recommend 250 µg for both, the NIH says.

For older adults the US allowance stays at 150 µg at every age, and EFSA’s value covers all adults from 18. EFSA’s number is not a measured requirement. It is the intake that matches a urinary iodine level of 100 µg/L, with absorption taken as 92%. Above that level, goiter was found in under 5% of European schoolchildren in almost all study areas.

Iodine deficiency: signs and who is at risk

Goiter, an enlarged thyroid, is usually the earliest clinical sign. If intake falls to roughly 10 to 20 µg a day, hypothyroidism occurs. In adults, mild to moderate deficiency can cause goiter, and impaired mental function and work productivity from hypothyroidism.

Severe deficiency in pregnancy can cause miscarriage, stillbirth and major developmental problems in the fetus. Chronic severe deficiency in the womb causes cretinism: intellectual disability, deaf-mutism, stiff muscles and stunted growth. Less severe deficiency in infants and children can mean a somewhat lower IQ.

The NIH calls iodine deficiency the most common cause of preventable intellectual disability in the world. These signs are not specific, and hypothyroidism has other causes.

Status is judged from urine. A median urinary iodine of 100 to 199 µg/L means intake is adequate in children and adults. In pregnancy the adequate range is 150 to 249 µg/L. Below 100 µg/L is insufficient, and deficiency is severe only below 20 µg/L.

A spot urine sample describes a population but cannot diagnose one person. TSH can stay normal when intake is low, so a doctor reads symptoms, an exam and thyroid tests together.

Pregnant women are the group where the gap shows most, and it differs by country. In US surveys the general population is iodine sufficient. But the median for pregnant women was 144 µg/L in 2007 to 2014, under the 150 µg/L line.

In 2001 to 2006, the median was 100 µg/L in pregnant women with no dairy in the past day. In dairy eaters it was 163 µg/L. In the UK cohort ALSPAC, the median in early pregnancy was 91.1 µg/L, classed as mild to moderate deficiency. In a salt-iodization study in Croatia, intake in pregnant women was borderline low even though the salt was adequately iodized.

Others at risk, the NIH says, include people who do not use iodized salt or who restrict salt. Vegans and people who eat little dairy, seafood or eggs are at risk too. So are people living where the soil is low in iodine, such as mountain regions and flood-prone river valleys.

A 2023 meta-analysis of 11 studies with 4,421 adults found vegans had the lowest iodine intake. None of the diet groups had a median urinary iodine in the optimal range. Unfortified plant milks do not fill the gap: in the UK their median was 7.3 µg/kg, against 438 µg/kg for winter cow’s milk.

Forms of iodine: absorption and effects

Iodine comes as small, soluble salts, so the form matters less for absorption than for stability, dose accuracy and what else comes with it. Iodide is absorbed almost completely, and iodate is turned into iodide in the gut before it is absorbed, the NIH says.

That is why the WHO recommends potassium iodate for salt: it is more stable in warm, damp or tropical climates. The FDA has approved potassium iodide and cuprous iodide for US salt. Supplements usually hold potassium iodide or sodium iodide, and many multivitamins contain 150 µg.

The honest bottom line: we found no trial that compared iodide with iodate, kelp, molecular iodine or Lugol’s solution for any health outcome. The Cochrane review of iodized salt included a comparison of potassium iodide with potassium iodate in salt, but its abstract gives no result for it.

What the evidence does show is that seaweed and kelp deliver a very uneven dose, and that labels are not always right. In a US test of 47 prenatal vitamins, 20 of the 25 that listed iodine held more than 20% less than the label said.

The trials in the benefits section used potassium iodide (pregnancy trials at 150 to 300 µg) and a daily 150 µg tablet (New Zealand children). Others used a one-off dose of iodized oil (Albania), molecular iodine (breast pain) and Lugol’s solution (thyroid surgery).

Form	Share of the mineral by weight	What studies show	Source
Potassium iodide (KI)	Not stated	The usual supplement form, with sodium iodide, and the iodine source in all of the iodine-containing top-selling US adult multivitamins and 73.5% of the iodine-containing prenatal ones. Absorbed almost completely: 96.4% in one man, and EFSA puts inorganic iodide above 90%. In 22 women the bioavailability was 59% for KI and 33% for the iodine in a seaweed capsule. The American Thyroid Association advises 150 µg as KI for women planning pregnancy, pregnant or breastfeeding, the NIH says.	1, 2, 3, 4, 6
Sodium iodide	Not stated	The NIH lists it with potassium iodide as the usual supplement form. A course of sodium iodide given to 5,000 schoolgirls in Akron, Ohio in 1916 to 1920 was reported as completely effective against goiter, in the first controlled prevention experiment.	1, 17
Potassium iodate	Not stated	Used to iodize salt and as a bread dough conditioner. It is reduced to iodide in the gut and absorbed as iodide. We found no trial comparing it with iodide for health outcomes.	1
Iodized salt	45 µg of iodine per gram of salt on US labels	Improved iodine status in most controlled trials, with no harm reported. At about 25 mg per kilogram it supplied enough iodine to every group in a study of three sites, though pregnancy was borderline low, and large swings in salt iodine raised the risk of both low and high intakes (33.6% of Philippine samples were low and 18% high).	1, 11, 12
Kelp and seaweed (food or capsules)	Varies: 16 to 8,165 µg per gram	Absorbed well in 18 volunteers (61.5% to 101%), but 33% against 59% for KI in another study. The dose is hard to control: 15 g of kombu a day raised TSH, 44 habitual seaweed eaters had a median urinary iodine of 1,200 µg/L, and 23.5% of the iodine-containing prenatal vitamins used kelp. We found no published laboratory measurement of kelp capsules.	3, 4, 5, 6, 19, 20
Molecular iodine (I2)	Not stated	One trial of 111 women gave 1.5, 3 or 6 mg a day for breast pain and found relief at 3 and 6 mg. We found no trial that compared it with iodide, or that tested it as a general supplement.	9
Iodized oil	Not stated	Given as a large oral dose (400 mg of iodine) in the Albanian school trial, which raised urinary iodine to 172 µg/L after 24 weeks. Most trials in the 2026 Cochrane review of children used single large doses. We found no trial comparing it with daily iodide.	10, 13
Lugol’s solution	Not stated	A mixture of potassium iodide and iodine, used in hospitals before thyroid surgery for Graves’ disease. A meta-analysis found less blood loss, but a randomized trial of 56 patients found none, with lower thyroid hormones. It is a medicine, not a supplement.	14, 15, 16
Potassium iodide tablets for radiation emergencies	65 or 130 mg tablets	Doses are hundreds of times the daily need, taken to block the thyroid from absorbing radioactive iodine. See the safety section for the FDA doses. They are not for daily nutrition.	7

Labels vary as much as the forms. Of the 89 top-selling US adult multivitamins in one survey, 74.2% contained iodine, with a median of 150 µg. Of 59 prenatal ones, 57.6% did, with a range of 25 to 290 µg. In Switzerland, 7 of 23 prenatal and 12 of 21 adult multivitamins contained no iodine.

Is iodine safe? Supplements, dose and interactions

Iodine from ordinary food is not a concern for most people. For adults, the US tolerable upper intake level (UL) is 1,100 µg a day from food and supplements, also in pregnancy and breastfeeding. The UL is the most a day unlikely to cause harm. It is 900 µg at ages 14 to 18 and 600 µg at ages 9 to 13.

The NIH says the levels do not apply to people receiving iodine as medical treatment. The EU figure is lower. In 2002 the Scientific Committee on Food set 600 µg a day for adults, including pregnant and breastfeeding women, about half the US value.

It came from short studies of 10 to 32 people, in which about 1,700 to 1,800 µg raised the TSH response. An uncertainty factor of 3 was then applied. EFSA’s 2014 opinion on iodine reference values restates that number rather than setting a new one.

• Thyroid effects. Too much iodine can cause goiter, high TSH, hypothyroidism, hyperthyroidism and thyroiditis, and the NIH links excessive intake to thyroid papillary cancer. In China, intake above the adequate range went with more subclinical hypothyroidism over 5 years. Some people, such as those with autoimmune thyroid disease, can react at intakes that are safe for others. Acute iodine poisoning is rare and comes from doses of many grams.
• Seaweed and kelp. A single gram of kelp can exceed the upper limit, and labels often omit iodine. Kombu at 15 g a day raised TSH in healthy adults, and regular seaweed eaters had a median urinary iodine of 1,200 µg/L. Treat kelp tablets and heavy seaweed intake with caution, especially with a thyroid condition.
• Medicines. Iodine can add to antithyroid drugs such as methimazole and cause hypothyroidism. Potassium iodide with ACE inhibitors or potassium-sparing diuretics (spironolactone, amiloride) can raise blood potassium. Amiodarone, a heart-rhythm drug, is very rich in iodine: a 100 mg tablet holds 250 times the recommended daily intake. Of people taking it, 14% to 18% develop overt thyroid dysfunction. Iodinated contrast dye used in scans is another source of excess, according to a 2024 review.
• Pregnancy and breastfeeding. Safe upper limits in pregnancy have not been well defined. The American Thyroid Association and the American Academy of Pediatrics advise 150 µg a day as a supplement, the NIH says. But a Spanish survey linked doses of 200 µg or more to a higher chance of raised TSH. A Cochrane review found more digestive upset with supplements, though from one small trial. Check your prenatal vitamin and ask your doctor, particularly with thyroid disease.
• Radiation emergencies. Potassium iodide tablets only protect the thyroid from radioactive iodine. They do nothing for other radioactive material or for outside radiation, and they back up evacuation, sheltering and food control. FDA guidance from 2001 lists 130 mg for adults and for pregnant or breastfeeding women. It lists 65 mg for ages over 3 to 12 and for adolescents (130 mg if over 70 kg). It lists 32 mg for over 1 month to 3 years and 16 mg from birth to 1 month. The dose is taken once a day, because protection lasts about 24 hours. Adults over 40 are advised to take it only if the projected thyroid dose is above 500 cGy. State and local emergency plans decide when it is used. People with known iodine sensitivity should avoid it. People with a multinodular goiter, Graves’ disease or autoimmune thyroiditis need caution if it continues beyond a few days.

How to get more iodine

• Check your salt. Iodized table salt gives 78 µg in a quarter teaspoon, more than half the daily need. But sea, kosher and Himalayan salt usually have none, and processed foods mostly use noniodized salt. The label will say if the salt is iodized.
• Eat fish, dairy and eggs. Baked cod gives 146 µg in 3 ounces and nonfat Greek yogurt 87 µg in ¾ cup. Nonfat milk gives 84 µg in a cup and a hard-boiled egg 31 µg. Plant milks are usually not fortified: only 3 of 47 UK products were. So vegans may need iodized salt, a fortified food or a supplement.
• Keep seaweed occasional and supplements measured. Five grams of nori gives 116 µg, but kelp can give far more than the limit. If you take a multivitamin, look for one that states about 150 µg of iodine, and ask your doctor before taking more.

History of iodine

The story starts with seaweed ash. In 1811 Courtois noticed a violet vapor rising from burning seaweed ash, and Gay-Lussac soon identified it as a new element, iodine. In 1813 the Swiss physician Coindet guessed that the old seaweed remedy for goiter worked because of its iodine, and treated patients with it.

The French chemist Chatin proposed in 1851 that lack of iodine caused goiter. In 1896 Baumann and Roos found iodine in the thyroid itself.

In the early 1900s, Swiss and American doctors showed that iodine prevents goiter and cretinism. Between 1916 and 1920, a course of sodium iodide given to 5,000 schoolgirls in Akron, Ohio, was reported as completely effective against goiter.

Michigan switched from plain to iodized salt in 1924. A survey four years later found no case of toxic goiter from it in three test counties. Switzerland has iodized its salt without a break since 1922. US salt makers have added iodine since the 1920s, though it is still voluntary.

The story continues. US urinary iodine fell by more than 50% between 1971 to 1974 and 1988 to 1994, as dairies and bakers used less iodine. It has been stable since.

After the Chernobyl accident in 1986, potassium iodide was widely used in Poland, and childhood thyroid cancer did not rise substantially there. In Belarus and Ukraine, where children were mildly iodine deficient and prophylaxis was not used, thyroid cancer in children rose sharply.

The EU listed its iodine health claims in 2012, and EFSA set an adequate intake of 150 µg in 2014.

Frequently asked questions about iodine

How much iodine do you need if you have thyroid problems?

There is no single answer. The NIH says people with autoimmune thyroid disease may react to intakes that are safe for others. Iodine can also add to antithyroid drugs such as methimazole and cause hypothyroidism. In a five-year Chinese cohort, more than adequate or excessive intake went with more subclinical hypothyroidism and autoimmune thyroiditis. Ask your endocrinologist before taking iodine, kelp or a high-dose supplement.

Do you get enough iodine from sea salt or Himalayan salt?

Usually not. The NIH says sea salt, kosher salt, Himalayan salt and fleur de sel are not usually iodized. Noniodized sea salt gives virtually no iodine, against 78 µg in a quarter teaspoon of iodized table salt. Most salt in US processed foods is noniodized too. Check the label for “iodized” or “provides iodide”.

Which foods are highest in iodine?

Seaweed is the richest, though it varies from 16 to 8,165 µg per gram. Among everyday foods in the NIH table, baked cod gives 146 µg in 3 ounces and oysters 93 µg. Nonfat Greek yogurt gives 87 µg in ¾ cup, nonfat milk 84 µg in a cup and a hard-boiled egg 31 µg. Bread made with iodate dough conditioner can give 296 µg in two slices. We do not have an iodine ranking yet.

Can you get too much iodine from seaweed?

Yes. In 44 Norwegian seaweed eaters the median urinary iodine was 1,200 µg/L. In healthy adults, 15 g of kombu a day raised TSH above the normal limit in some people. One gram of the richest kelp granules in one analysis held 8,165 µg, more than seven times the US upper limit of 1,100 µg. Nori, by contrast, held 16 µg per gram, and labels often do not state the iodine.

Do you need an iodine supplement in pregnancy?

The NIH says the American Thyroid Association and the American Academy of Pediatrics advise 150 µg a day. This covers women planning pregnancy, pregnant or breastfeeding, but the trial evidence is mixed. A Cochrane review found too little data. The largest trial in mildly deficient women (832 women) found no effect on children’s IQ at 5 to 6 years. Of the 59 best-selling US prenatal vitamins, only 34 contained iodine. Ask your doctor, especially if you have a thyroid condition.

Do vegans get enough iodine?

Often not. A 2023 meta-analysis of 11 studies with 4,421 adults found vegans had the lowest iodine intake. None of the diet groups had a median urinary iodine in the optimal range. Unfortified plant milks have little: a median of 7.3 µg/kg against 438 µg/kg for winter cow’s milk. Only 3 of 47 UK products were fortified. Options are iodized salt, iodine-fortified foods or a supplement.

What is potassium iodide for in a nuclear emergency?

It floods the thyroid with stable iodine so it takes up less radioactive iodine. It protects only the thyroid, not other organs, and it backs up evacuation, sheltering and food control, according to FDA guidance. The dose depends on age, for example 130 mg for adults, and state and local emergency plans decide when it is used. It is not a daily supplement.