Selenium and iodine supplements: how much you need, safe limits and thyroid cautions

What selenium and iodine do

Iodine is part of thyroid hormones. Without it, your thyroid can’t make them, and these hormones control metabolism, growth and a baby’s brain development. Selenium works alongside: it’s part of selenoproteins, which help make and process the same hormones and also protect cells from oxidative damage.

That’s why the two are often sold together. But they’re different stories. Iodine intake really is low for some groups, while selenium is usually adequate on a varied diet. And for both, the safe upper limit isn’t far above the daily need.

You’re more likely to fall short of iodine if:

• You’re vegan or eat little dairy, fish and eggs.
• You drink plant milk that isn’t fortified with iodine.
• You’re pregnant or breastfeeding, when the need is higher.

How much selenium and iodine you need

Reference values differ slightly between countries. In the UK, the NHS and Food Standards Scotland give the figures; across Europe, EFSA does. Upper limits count everything: food, salt, multivitamins and single supplements.

Europe has lowered its selenium limit. In 2023, EFSA set the upper level at 255 mcg a day for adults, including pregnant and breastfeeding women. The limit is based on hair loss, one of the first signs of too much selenium.

In practice, the 200 mcg selenium capsules common on iHerb can easily push you over 255 mcg once you add food and Brazil nuts. For daily use, 50–100 mcg makes more sense.

Selenium supplements: sources and forms

Brazil nuts are by far the richest source: one nut contains 68–91 mcg. Tuna, sardines, prawns, meat and eggs are good sources too. How much selenium plant foods contain depends heavily on the soil.

Supplements contain selenomethionine, selenium-enriched yeast, sodium selenite or sodium selenate. The body absorbs up to about 90% of the selenium from all of these, so the dose matters more than the form.

Signs of too much selenium (selenosis):

• garlic breath and a metallic taste are early warnings;
• hair loss and brittle or lost nails;
• in acute poisoning, severe stomach and nerve symptoms.

Selenium for the thyroid: what the evidence says

Selenium is often suggested for autoimmune thyroiditis (Hashimoto’s). NIH ODS summarises the research: in people treated with levothyroxine, selenium lowered thyroid peroxidase (TPO) antibodies. Even so, the reviewers concluded that the evidence doesn’t support selenium supplements for autoimmune thyroiditis. Lower antibody levels on a blood test don’t necessarily mean you’ll feel better.

Selenium hasn’t held up for preventing other diseases either. In the SELECT trial (35,533 men), 200 mcg a day didn’t lower prostate cancer risk. It found no effect on cardiovascular events either. Men who started with high selenium levels had a higher risk of aggressive prostate cancer on the supplement, so researchers advised men over 55 not to exceed recommended intakes.

Iodine supplements: dairy, iodised salt and kelp

The NHS lists cows’ milk and dairy, eggs, sea fish and shellfish as good sources of iodine. Iodised salt is a major source in many countries, so check the packet to see whether yours is. In the US, labels give about 45 mcg of iodine per gram. Sea salt without added iodine contains almost none.

Kelp is a poor way to get iodine. Commercial seaweeds contain anywhere from 16 to 2,984 mcg of iodine per gram. With a range that wide, kelp and seaweed supplements can’t give a reliable dose, and pregnancy is the worst time to risk too much. Many “iodine” products on iHerb are kelp-based, so read the ingredients: potassium iodide gives a predictable dose.

Germany’s Federal Institute for Risk Assessment (BfR) proposes capping iodine in supplements at 100 mcg a day for adults and 150 mcg in pregnancy and breastfeeding. That’s a sensible guide if you also eat dairy and fish.

Who should avoid iodine and selenium supplements

Too much iodine can upset your thyroid. It can cause an underactive thyroid and goitre, an overactive thyroid, or thyroiditis. People with autoimmune thyroid disease, and those with long-standing iodine deficiency, are the most sensitive.

The NHS advises getting healthcare advice before taking an iodine supplement. Don’t take iodine or selenium without your endocrinologist’s agreement if:

• You have Hashimoto’s thyroiditis or Graves’ disease.
• You have an overactive thyroid or thyroid nodules, especially in older age.
• You’re due to have radioactive iodine treatment. You’ll be on a low-iodine diet beforehand, and kelp products are off limits.
• You take antithyroid medicines such as carbimazole or methimazole. High-dose iodine adds to their effect.
• You take ACE inhibitors or potassium-sparing diuretics. Potassium iodide with them raises the risk of high blood potassium (hyperkalaemia).

Iodine needs rise in pregnancy and breastfeeding, but agree the dose with your midwife or GP. Our guide to pregnancy vitamins covers this period.

If you’re having chemotherapy with cisplatin, discuss selenium with your oncologist: the drug lowers selenium levels, but there isn’t enough evidence on whether supplements help.

Children should only take selenium or iodine on a paediatrician’s advice: doses depend on age and weight.

Run through this quick check before you order:

How to choose selenium and iodine on iHerb

1. Check your multivitamin first: it often contains both selenium and iodine.
2. For selenium, choose 50–100 mcg per capsule; 200 mcg products only on medical advice.
3. For iodine, choose potassium iodide at 100–150 mcg, not kelp or “sea vegetables”.
4. Add it all up: food, Brazil nuts, multivitamin and supplement should stay under 255 mcg of selenium and 600 mcg of iodine.
5. If you have a thyroid condition, buy only after talking to your endocrinologist.

Minerals that compete for absorption are covered in our vitamin compatibility chart. If you’re choosing supplements “for energy”, read our zinc guide and the rest of the energy and heart section first. For reading doses on the bottle, see our Supplement Facts guide.