Quick answer

Most documented problems with iodine come from taking too much. Long-term intakes above the adult upper limit of 1,100 mcg a day can upset thyroid function, and kelp supplements can hold far more iodine than expected: in one analysis, they ranged from 5 to 5,600 mcg per daily dose.^1,2

Key facts

The adult upper limit is 1,100 mcg a day, and 900 mcg for teens aged 14 to 18.¹
Potassium iodide tablets for radiation emergencies contain 65 or 130 mg, hundreds of times a supplement's dose.³
Iodine taken with lithium may lower thyroid function.⁴
Iodide can change the results of thyroid function tests.⁵

What too much iodine does

Very large doses, usually several grams, can cause burning of the mouth, throat and stomach, fever, stomach pain, nausea, vomiting, diarrhea, a weak pulse and coma.^1,6

People with autoimmune thyroid disease or with a history of iodine deficiency may have problems at intakes that are safe for most people.¹

Responses vary from person to person. In some people excess iodine blocks thyroid hormone production, which raises TSH and can enlarge the gland, while in others it makes the gland overactive. That is why the same high intake can push thyroid tests in either direction.¹

Kelp and seaweed supplements

Kelp supplements are made from seaweed, and the iodine in seaweed varies enormously: from 16 to 2,984 mcg per gram in seaweeds sold whole or as sheets.^1,6

In 96 seaweed and kelp products sold in Norway, supplements held 5 to 5,600 mcg of iodine per daily dose, labels were often inaccurate, and 54 products would exceed the upper limit in a single portion or dose.²

That makes kelp an unpredictable source of iodine. If you use a seaweed product, look for a stated iodine amount in micrograms and count it alongside your multivitamin and salt.^1,2

Side effects at drug doses

At the drug doses used in radiation emergencies, potassium iodide can cause swelling of the salivary glands, nausea, vomiting, diarrhea, stomach ache, fever, headache, a metallic taste and allergic reactions.^3,7

Long use of high doses can cause iodism, with a burning mouth or throat, a metallic taste, sore teeth and gums, head cold symptoms, irritated eyes and acne-like skin changes. These usually fade soon after stopping.^3,5

These come from the labels of potassium iodide medicines, taken at 65 to 130 mg a day in a radiation emergency, or more as a prescription drug. A supplement with 150 mcg holds hundreds of times less, but the lists show what high, sustained iodine intake can do.^3,5

Iodine allergy and seafood

A seafood or shellfish allergy does not necessarily mean a person is allergic to iodine.³

The potassium iodide labels tell anyone who has had an allergic reaction to iodine not to take them, and list the signs to watch for: hives or rash, swelling of the face, lips, tongue or throat, fever with joint pain, and trouble breathing, speaking or swallowing.⁷

Medicines iodine can affect

Iodide can add to the effects of lithium and of antithyroid medicines such as methimazole, leaving too little thyroid hormone. Potassium iodide also adds potassium, so with ACE inhibitors or potassium-sparing diuretics it can push blood potassium too high.^4,5,6

Staying within the limit

NIH notes that for most people, food and supplements together are unlikely to pass the 1,100 mcg upper limit. The exceptions are concentrated products: iodine-only supplements, some of which exceed the limit on their own, and seaweed products whose amounts are uncertain.^1,2

Questions people ask

Is kelp iodine safe?

Kelp is an unreliable source: the iodine in seaweed varies enormously, labels are often inaccurate, and in one analysis more than half of seaweed and kelp products would pass the adult upper limit in one portion or dose. Ask your doctor before using kelp, especially if you have a thyroid condition or are pregnant.^1,2

Is iodine safe if I have a thyroid condition?

Ask your doctor first. People with autoimmune thyroid disease may react to amounts that are fine for others, and the potassium iodide labels tell people with thyroid disease to check with a doctor and warn that iodide can change thyroid test results.^1,5,7

Can I take potassium iodide tablets every day?

Potassium iodide tablets sold for radiation emergencies are a drug, to be taken only when public officials say so and no longer than you are told. The potassium iodide in a multivitamin is a much smaller amount, usually about 150 mcg.^7,8

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