Selenium.

What the nutrient does

Selenium is part of proteins involved in antioxidant defence and thyroid hormone metabolism.2

Food, deficiency and safety

Seafood, meat, eggs and grains can provide selenium. Brazil nuts can contain high and variable amounts depending on where they are grown. Too much selenium from supplements or frequent consumption of high-selenium foods can cause toxicity, including gastrointestinal symptoms and hair or nail changes.

Selenium deficiency is uncommon in the United States, according to NIH. People may be at greater risk of inadequate selenium if they depend on food grown in selenium-poor regions, undergo kidney dialysis or live with HIV. NIH also notes that vegetarian or vegan diets may provide less selenium and that people who smoke tend to have lower selenium status; neither observation alone establishes deficiency. Risk varies between individuals; a risk factor or low level alone does not establish that supplements will help. Discuss diet and medical circumstances with a clinician.

The chemotherapy medicine cisplatin can lower selenium levels in blood and hair; NIH says the clinical significance of those changes is not known. Evidence is insufficient to establish that selenium supplements reduce chemotherapy side effects. If you are receiving cancer treatment, discuss supplements with your cancer-care team before starting them; do not change prescribed treatment on your own.

Upper limits differ by authority and age. For adults, US guidance gives 400 micrograms a day, while the European Food Safety Authority set 255 micrograms a day in 2023. These limits consider intake from food, drinks and supplements; they are not targets or personal safe doses. A prescribed treatment may need separate medical supervision. Because the selenium content of individual Brazil nuts varies, a simple nut count cannot establish a reliable intake.

Too much selenium over time can also cause a metallic taste, garlic-like breath or skin changes, but these symptoms have other possible causes. If you think you or a child may have taken too much of a supplement, contact a local poison information service or urgent medical service promptly, even if there are no symptoms. Severe breathing difficulty or collapse needs emergency help. Keep the product container for the care team.238

Does selenium help Hashimoto’s thyroiditis?

Hashimoto’s thyroiditis is an autoimmune condition in which the immune system affects the thyroid; some people develop an underactive thyroid and need thyroid hormone treatment. A 2024 review of 35 studies found changes in some blood markers with selenium, including lower thyroid antibodies on average, but results varied greatly across the antibody studies. A marker changing does not automatically mean people feel or function better. In a separate Danish placebo-controlled trial, 412 adults with autoimmune hypothyroidism already taking levothyroxine were assigned selenium-enriched yeast or placebo for 12 months. Both groups reported improved quality of life, with no significant difference between them, although antibody levels were lower in the selenium group. About four in five completed treatment. These findings do not settle the effect in people with different thyroid conditions, selenium status or treatments. The trial received public, university and charitable funding; Pharma Nord supplied the tablets. Excess selenium can be harmful. Do not replace prescribed thyroid medicine with a supplement; discuss specific products and total intake with a clinician. For another thyroid-related nutrient, see iodine and thyroid health.672

Read how iron, calcium and biotin can affect thyroid medicine or tests.

For pregnancy, the 2026 American Thyroid Association guideline conditionally recommends against offering selenium supplements to pregnant people with normal thyroid function who have thyroid peroxidase and/or thyroglobulin antibodies. This guidance concerns a specific group and does not mean dietary selenium is unnecessary. Discuss thyroid results and supplements with the clinician caring for the pregnancy.1

Did selenium supplements prevent prostate cancer?

The SELECT trial randomly assigned 35,533 men to selenium, vitamin E, both supplements or placebo to study prostate cancer prevention. The trial did not find that selenium alone or with vitamin E prevented prostate cancer. In later follow-up, the vitamin-E-only group had a higher prostate cancer risk than the placebo group. That result concerns the specific supplements and people studied; it does not show that selenium-rich food causes cancer, or that all antioxidant supplements have the same effects. The trial was publicly funded and followed participants after they stopped taking the study supplements. For the vitamin E result in more detail, see vitamin E and cancer evidence.

In the later SELECT analysis, the selenium-only and combined groups had numerically more prostate cancer diagnoses than placebo, but those differences were not statistically significant. That uncertainty does not prove a protective effect or establish that selenium prevents the risk observed with vitamin E alone.45

What a supplement claim needs to show

A role in antioxidant enzymes does not establish that extra selenium prevents cancer, treats thyroid disease or improves health in everyone. Read the supplement label guide and the interaction guide for practical context.2