Why does the thyroid need iodine?

Iodine is part of the thyroid hormones T4 and T3. Without enough, the thyroid can enlarge (goiter) and hormone levels can fall.

Can too much iodine harm the thyroid?

Yes. High intakes can cause goiter, high TSH and hypothyroidism, and can also trigger hyperthyroidism or thyroiditis. The NHS says 0.5 mg or less a day from supplements is unlikely to cause harm.

Is kelp a good way to get iodine?

Kelp is sold as an iodine source, but the iodine in seaweed varies widely, from 16 to 2,984 mcg per gram. Some iodine-only supplements are above the upper limit.

Do vegans need an iodine supplement?

Vegans and people who eat little dairy, seafood or eggs may not get enough iodine. The NHS suggests iodine-fortified foods or a supplement, with advice from a health professional.

mineral

Iodine: Thyroid Role, Daily Needs and Safety Limits

Also known as potassium iodide, sodium iodide, iodide, kelp, iodized salt

Short answer

Iodine is a trace mineral the thyroid uses to make the hormones T4 and T3 ^[1]. US adults need 150 mcg a day, 220 mcg in pregnancy and 290 mcg while breastfeeding ^[1]. Seafood, seaweed, dairy, eggs and iodized salt supply it ^[1]. Too much can also disturb the thyroid; the adult upper limit is 1,100 mcg a day ^[1].

Last reviewed 20 October 2026 · 2 sources

What is Iodine?

Iodine is a trace element that is an essential part of the thyroid hormones thyroxine (T4) and triiodothyronine (T3) ^[1]. These hormones regulate protein synthesis, enzyme activity and metabolic activity, and are needed for skeletal and nervous system development in fetuses and infants ^[1]. Without enough iodine, thyroid-stimulating hormone stays high and the thyroid can enlarge (goiter) ^[1].

Forms of Iodine you'll see on labels

Potassium iodide or sodium iodide
The usual forms in supplements ^[1]. A small study found potassium iodide is almost completely absorbed (96.4%) ^[1].
Kelp
A seaweed sold in supplements ^[1]. Iodine in seaweeds varies hugely, from 16 to 2,984 mcg per gram ^[1].
Iodized salt
US iodized salt is labeled 45 mcg iodine per gram; FDA allows potassium iodide and cuprous iodide, and WHO prefers potassium iodate for stability ^[1].

How much Iodine do adults need?

NIH lists these Recommended Dietary Allowances (RDA) for adults 19+ in micrograms (mcg). Reference values, not a personal recommendation.

Group	Amount per day
Men 19+	150 mcg
Women 19+	150 mcg
Pregnancy	220 mcg
Breastfeeding	290 mcg

Upper limit: 1,100 mcg/day for adults 19+, including pregnancy and breastfeeding, from food and supplements ^[1]

How Iodine appears on a label

Iodine is listed in mcg on Supplement Facts labels, and the Daily Value is 150 mcg ^[1]. Many multivitamins contain 150 mcg; some, but not all, prenatal vitamins contain iodine; and some iodine-only products are above the upper limit ^[1]. Food Nutrition Facts labels usually do not list iodine unless it was added, so they cannot show iodine in seafood or seaweed ^[1]. Bread made with iodate dough conditioners can provide a substantial amount of iodine without listing it on the Nutrition Facts panel ^[1].

When to take Iodine

No source we checked names a best time of day or says whether to take iodine with food.

What to keep apart from Iodine

Antithyroid medicines (for example methimazole)
High doses of iodine with these medicines can have an additive effect and could cause hypothyroidism ^[1]. Ask your doctor.
ACE inhibitors
Taking potassium iodide with these blood pressure medicines can raise the risk of high blood potassium ^[1]. Ask your pharmacist.
Potassium-sparing diuretics
Potassium iodide with medicines such as spironolactone or amiloride can raise the risk of high blood potassium ^[1]. Ask your pharmacist.
Goitrogen foods (soy, cassava, cabbage, broccoli, cauliflower)
They can interfere with iodine uptake, but NIH says this mainly matters for people with marginal iodine status in iodine-poor areas ^[1].

Foods that contain Iodine

Seaweed (nori, kelp, kombu, wakame)
Cod, tuna and other fish
Oysters and shrimp
Greek yogurt, milk and cheese
Eggs
Iodized table salt
Bread made with iodate dough conditioner
Beef liver

Who should ask a doctor or pharmacist first

Talk to a doctor first if you have thyroid disease, especially autoimmune thyroid disease, because some people react to iodine amounts that are safe for most ^[1]. Ask first if you take antithyroid medicines, ACE inhibitors or potassium-sparing diuretics ^[1], or if you are preparing for radioactive iodine treatment, which usually needs a low-iodine diet ^[1]. If you are pregnant, planning a pregnancy or breastfeeding, ask about iodine: the American Thyroid Association recommends 150 mcg/day of potassium iodide for these groups ^[1]. The NHS suggests vegans considering a supplement seek professional advice ^[2].

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Iodine: questions people ask

Iodine is part of the thyroid hormones T4 and T3 ^[1]. Without enough, the thyroid can enlarge (goiter) and hormone levels can fall ^[1].

NIH lists 150 mcg a day for adults, 220 mcg in pregnancy and 290 mcg while breastfeeding ^[1]. The UK NHS lists 140 mcg a day for adults ^[2].

Yes. High intakes can cause goiter, high TSH and hypothyroidism, and can also trigger hyperthyroidism or thyroiditis ^[1]. The NHS says 0.5 mg or less a day from supplements is unlikely to cause harm ^[2].

Needs rise in pregnancy, and the American Thyroid Association and American Academy of Pediatrics recommend a supplement with 150 mcg iodine for women who are planning, pregnant or breastfeeding ^[1]. Not all prenatal vitamins contain iodine, so check the label and ask your doctor ^[1].

Usually not. Specialty salts such as sea salt, kosher salt and Himalayan salt are not usually iodized, and noniodized sea salt provides almost no iodine ^[1].

Kelp is sold as an iodine source, but the iodine in seaweed varies widely, from 16 to 2,984 mcg per gram ^[1]. Some iodine-only supplements are above the upper limit ^[1].

Vegans and people who eat little dairy, seafood or eggs may not get enough iodine ^[1]. The NHS suggests iodine-fortified foods or a supplement, with advice from a health professional ^[2].

Almost always no. US food makers almost always use noniodized salt, and must list it as iodized if they use it ^[1].

Sources

Iodine - Health Professional Fact Sheet — NIH Office of Dietary Supplements
Vitamins and minerals: Iodine — NHS

Educational information, not medical advice. The amounts above are official reference values, not a recommendation for you. Check with your doctor or pharmacist before starting or changing supplements, especially if you take medication, are pregnant or have a health condition. Supin sells a subscription, not supplements. Last reviewed 20 October 2026.