Quick answer

Iodine is a mineral the thyroid uses to make its hormones, which set the pace of metabolism. Adults need 150 mcg a day, pregnant women 220 mcg and breastfeeding women 290 mcg. Most people in the US get enough from food and iodized salt, and the upper limit for adults is 1,100 mcg a day.^1,2

Key facts

A quarter teaspoon of iodized salt provides about 78 mcg of iodine, roughly half an adult's daily need.²
About 28% to 29% of US adults take a supplement that contains iodine.²
Most adult multivitamins that contain iodine provide 150 mcg, as potassium iodide.³
Kelp supplements can hold far more iodine than expected, because the iodine in seaweed varies widely.^2,4
Iodine can interact with lithium and antithyroid medicines, and potassium iodide with ACE inhibitors and potassium-sparing diuretics.^1,5

What iodine does in the body

The thyroid gland traps iodine from the blood and builds it into two hormones, thyroxine (T4) and triiodothyronine (T3). These hormones govern protein building, enzyme activity and the rate at which the body uses energy, and they are needed for the skeleton and brain to develop before birth and in infancy.²

When iodine intake falls below about 100 mcg a day, the pituitary gland releases more thyroid-stimulating hormone (TSH), which drives the thyroid to trap more iodine and make more hormone.²

A healthy adult with enough iodine holds about 15 to 20 mg of it, 70% to 80% in the thyroid.²

What the evidence shows

Each claim below carries a grade and the amount studied. Iodine's job in the thyroid is well established. The sources read for this page show no added gain from extra iodine in people who already get enough, and too much can upset the thyroid.^2,6

Needed to make thyroid hormones, which regulate metabolism and are required for normal growth and development*^1,2

Moderate evidence

A long-established role, described by NIH: iodine is part of the thyroid hormones T4 and T3, which govern many of the body's chemical reactions and are needed for bone and brain development before birth and in infancy. Most people in the US already get enough from food and iodized salt, and the thyroid concentrates only the amount it needs for hormone production; most of the rest leaves in urine.

Amount studied: Recommended intake: 150 mcg a day for adults

Helps women who are pregnant, breastfeeding or planning a pregnancy meet their higher iodine needs*^1,2,6

Moderate evidence

Needs rise from 150 mcg a day to 220 mcg in pregnancy and 290 mcg while breastfeeding, because the mother makes about 50% more thyroid hormone and the baby depends on it early in pregnancy. US surveys suggest many pregnant women get somewhat less than they need, and the American Thyroid Association and the American Academy of Pediatrics recommend a daily supplement with 150 mcg. Trials have not shown that the supplement changes children's development: in two trials of 150 or 200 mcg a day, children's thinking, language and movement scores did not differ, and a Cochrane review found too little data to judge.

Amount studied: 150 mcg a day as potassium iodide, the amount the American Thyroid Association recommends

May improve some reasoning scores in children whose iodine intake is mildly low*^2,7

Early evidence

In one randomized trial, 184 children aged 10 to 13 in New Zealand with mildly low iodine took 150 mcg a day or a placebo for 28 weeks. The iodine group scored higher on 2 of 4 reasoning tests, and its overall score was 0.19 standard deviations above placebo. It is a single trial in children who were short of iodine, so the evidence is early and says nothing about people who already get enough.

Amount studied: 150 mcg a day for 28 weeks, in children aged 10 to 13

How much iodine you need

Adults and teens aged 14 and older need 150 mcg of iodine a day.^1,2

Iodine per day, from all sources^1,2
Who	Recommended	Upper limit
Children 1 to 8	90 mcg	200 to 300 mcg
Children 9 to 13	120 mcg	600 mcg
Teens 14 to 18	150 mcg	900 mcg
Adults 19 and older	150 mcg	1,100 mcg
Pregnancy, 19 and older	220 mcg	1,100 mcg
Breastfeeding, 19 and older	290 mcg	1,100 mcg

In FDA's Total Diet Study, intakes met or exceeded the average requirement in every age and sex group, before counting the salt people add at the table. The dosage guide shows how salt, food and supplements add up.²

Who may not get enough

NIH lists these groups as the most likely to fall short of iodine:¹

People who do not use iodized salt¹
Pregnant women, who need about 50% more iodine than other women¹
People who eat few or no dairy foods, seafood or eggs, including vegans¹
People in regions with iodine-poor soil who eat mostly local food¹

Soy, cassava and vegetables such as cabbage, broccoli and cauliflower contain goitrogens, which interfere with how the thyroid takes up iodine. For people who get enough iodine, reasonable amounts of these foods are not a concern.^1,2

Food sources

Fish such as cod and tuna, seaweed, shrimp and other seafood, dairy products, eggs and iodized salt provide iodine.^1,2

Three ounces of baked cod has about 146 mcg of iodine, three quarters of a cup of plain Greek yogurt 87 mcg, a cup of nonfat milk about 84 mcg and a large hard-boiled egg 31 mcg.²

Food labels usually list iodine only when it has been added, so foods that are naturally rich in it, such as seaweed and fish, may not show it.^1,2

Supplement forms

Supplements usually contain iodine as potassium iodide or sodium iodide, and many multivitamins include it. In a small study, people absorbed 96.4% of potassium iodide.^1,2

Iodine-only supplements are also sold, and many contain high doses, sometimes above the upper limit. Kelp products vary the most, and the side effects guide explains why.²

Too much iodine, and medicines

High iodine intakes can cause some of the same problems as too little, including goiter, raised TSH and an underactive thyroid, and can also cause an overactive thyroid, thyroid inflammation and thyroid papillary cancer.^1,2

The upper limit for iodine from all sources is 1,100 mcg a day for adults and 900 mcg for teens aged 14 to 18. It does not apply to people taking iodine as a treatment under a doctor's care.^1,2

Ask your pharmacist or doctor first

High doses of iodine taken with antithyroid medicines such as methimazole can add to their effect and leave the body with too little thyroid hormone. The prescription potassium iodide label says iodide may strengthen these medicines' effects on the thyroid.^1,2,5

On the medicine's label

Ask your pharmacist or doctor first

Iodide taken with lithium may lower thyroid function. The lithium label lists iodide preparations as an interaction and asks prescribers to watch for an underactive thyroid, and the potassium iodide label gives the same warning.^5,8

On the medicine's label

Ask your pharmacist or doctor first

Potassium iodide taken with ACE inhibitors such as lisinopril can raise blood potassium to an unsafe level. NIH and the potassium iodide label both list this interaction.^1,2,5

On the medicine's label

Ask your pharmacist or doctor first

Potassium iodide taken with potassium-sparing diuretics such as spironolactone or amiloride can raise blood potassium too high. NIH and the potassium iodide label both list this interaction.^1,2,5

On the medicine's label

People preparing for radioactive iodine treatment are usually told to follow a low iodine diet, under 50 mcg a day, for 1 to 2 weeks beforehand. That means stopping kelp and multivitamins that contain iodine.¹

Ask first if you have a thyroid condition or take thyroid medicine
Ask first if you take any of the medicines above
Ask first if you are pregnant or breastfeeding and want more than a prenatal vitamin provides
Do not take iodine if you have had an allergic reaction to it⁹

Questions people ask

What does an iodine supplement do?

It supplies the raw material for thyroid hormone. If you already get enough from food and iodized salt, the thyroid takes only what it needs and most of the rest leaves in urine. The groups for whom experts recommend a supplement are women who are pregnant, breastfeeding or planning a pregnancy.^1,2

Do men need iodine supplements?

Men need the same 150 mcg a day as women who are not pregnant, and most get it from dairy, seafood, eggs, bread made with iodate and iodized salt. A man who avoids those foods and skips iodized salt may fall short.^1,2

Can you get too much iodine from kelp?

Yes. The iodine in seaweed varies enormously, and in one analysis of seaweed and kelp products, more than half would exceed the 1,100 mcg adult upper limit in a single portion or dose.^2,4

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Sources

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.