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Selenium supplement: do you need one, and how much is too much?

By the Life in Good Form editors. Updated 2026-10-15.

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Daily needs and why most people get enough

Selenium is an essential mineral used to build selenoproteins, including enzymes that protect cells and help make thyroid hormone. NIH lists an RDA of 55 mcg a day for adults, 60 mcg during pregnancy and 70 mcg while breastfeeding.

NIH also says most people in the United States consume adequate amounts. Seafood, meat and poultry are rich sources, and plant-food content varies with the soil where crops grow. People who eat a vegetarian or vegan diet may have lower intakes, and people who smoke tend to have lower selenium status.

Brazil nuts are the standout food. NIH lists 544 mcg in one ounce, about 6 to 8 nuts. That single serving is already above the adult upper limit, so Brazil nuts are a food to eat occasionally, not daily by the handful.

Selenium and the thyroid: what trials show

The thyroid holds more selenium than any other organ, and selenoproteins help convert the T4 hormone to active T3. That is why selenium is sold for thyroid health.

NIH summarizes the trials this way: selenium supplements can lower certain thyroid antibodies in people with chronic autoimmune thyroiditis (Hashimoto's), but other trials found no effect on thyroid function or quality of life. Results in pregnant women with thyroid autoimmunity are mixed, and NIH notes the American Thyroid Association recommends against selenium supplements for pregnant women with thyroid autoimmunity.

So selenium is not a thyroid treatment. If you have a thyroid condition, decide with your endocrinologist, and never use it to replace prescribed thyroid medicine.

Cancer and other claims

Selenium's antioxidant role led to cancer hopes that larger trials did not confirm. In the SELECT trial, 35,533 men aged 50 and older took 200 mcg a day of selenium as selenomethionine, with or without vitamin E. NIH reports the trial was stopped after 5.5 years because selenium did not lower prostate cancer risk, and follow-up after people stopped the supplements confirmed no benefit.

Treat broad claims about immunity, aging or detox the same way: selenium is necessary, but extra selenium has not been shown to add benefits for people who already get enough.

Forms and how to read the label

NIH lists selenomethionine, selenium-enriched yeast, sodium selenite and sodium selenate as common supplement forms, and says the body can absorb up to about 90% of the selenium from these forms. Form matters less than the amount.

As a label example, NOW Selenium 100 mcg lists 100 mcg of elemental selenium from L-selenomethionine per tablet, with directions of 1 tablet 1 to 2 times daily with food. NOW's caution says adults only, and that people living in areas of high soil selenium, such as the Dakotas, should consult a physician before use.

Check your multivitamin too. Many include selenium, and the totals add up quickly.

Signs of too much selenium

NIH sets the adult upper limit at 400 mcg a day from food and supplements. Chronically high intake causes selenosis. Early signs are a garlic odor on the breath and a metallic taste in the mouth. The most common signs are hair loss and brittle or lost nails, and others include skin rash, nausea, diarrhea, fatigue, irritability and nervous system problems. Organic and inorganic forms cause similar effects at high intakes.

Children have much lower limits, from 45 mcg for young infants to 280 mcg by ages 9 to 13. Keep adult tablets out of reach.

NIH notes that selenium may interact with some medicines; the chemotherapy drug cisplatin, for example, can affect selenium levels. If you are on cancer treatment, ask your oncology team before taking any antioxidant supplement.

How to choose a selenium supplement

• Ask whether you need it. A vegan diet, a diagnosed low level or a clinician's advice are reasons; general wellness usually is not.
• Choose the lowest amount that fits the advice, often 100 mcg or less.
• Count selenium in your multivitamin and how often you eat Brazil nuts.
• Look for a verifiable quality mark such as USP or NSF. FDA does not approve supplements for safety and effectiveness before they are sold.
• Stop and get advice if you notice hair or nail changes, a garlic breath odor or a metallic taste.

What to look for when you buy

Questions people ask

What is a selenium supplement good for?

It corrects low intake, which is uncommon in the United States. It may lower certain thyroid antibodies in autoimmune thyroiditis, but it has not been shown to improve thyroid function or prevent prostate cancer.

When should I take selenium?

Follow the label; one common label says to take it with food. Timing matters less than keeping your daily total under 400 mcg.

How many Brazil nuts equal a selenium supplement?

NIH lists 544 mcg of selenium in one ounce, about 6 to 8 nuts. That is above the adult upper limit, so a single nut or two goes a long way.

What are signs of too much selenium?

Garlic-smelling breath and a metallic taste come first; hair loss and brittle nails are the most common signs. Others are rash, nausea, diarrhea, fatigue and irritability.

Sources

• Selenium: Fact Sheet for Health Professionals (ods.od.nih.gov), NIH Office of Dietary Supplements
• Selenium 100 mcg Tablets (www.nowfoods.com/products/supplements/selenium-100-mcg-tablets), NOW Foods
• Information for Consumers on Using Dietary Supplements (fda.gov), U.S. Food and Drug Administration

Sources checked 2026-10-15.