Selenium Health Benefits: Thyroid Support, Mercury, Cancer Claims, and Why More Is Not Better

Selenium is a trace mineral the body needs in small amounts to make selenoproteins, enzymes that protect cells from oxidative damage, help activate thyroid hormone, and support immune function. Popular articles list many benefits, from thyroid support and mercury detoxification to cancer prevention and heart health. Some of these claims have reasonable evidence; others have been tested in large trials and failed; and a few carry real risks when people take more selenium than they need.

The most important fact about selenium is its narrow range between too little and too much. People with low selenium status may benefit from more, while those who already get enough from their diet may be harmed by supplements. This article reviews what research shows about selenium’s main health roles, who is likely to benefit, and how to get the right amount safely.

What Selenium Does in the Body

Selenium is built into about 25 human selenoproteins. Glutathione peroxidases neutralize hydrogen peroxide and lipid peroxides, protecting cell membranes. Thioredoxin reductases help maintain the cell’s redox balance. Iodothyronine deiodinases convert the thyroid hormone thyroxine (T4) to its more active form, triiodothyronine (T3), and help regulate thyroid hormone levels. Selenoprotein P transports selenium in the blood to tissues such as the brain and testes. Selenium also plays roles in immune responses, sperm production, and DNA synthesis.

A 2012 review in The Lancet by Margaret Rayman of the University of Surrey summarized evidence linking selenium status to immune function, viral infections, reproduction, mood, thyroid function, cardiovascular disease, and cancer. Rayman emphasized the U-shaped relationship between selenium status and health: additional selenium may benefit people with low status, while people with adequate to high status might be harmed and should not take selenium supplements. This principle explains many of the mixed results in selenium research.

Who Is Low in Selenium?

Selenium content in food depends largely on soil selenium, which varies dramatically worldwide. Soils in parts of China, New Zealand, Scandinavia, parts of Europe, and Africa are low in selenium, and severe deficiency in some regions of China historically caused Keshan disease, a heart muscle disorder in children, and Kashin-Beck disease, a bone and joint disorder. Most people in the United States and Canada get adequate selenium because wheat and other crops grown on selenium-rich soils in the Great Plains supply it, along with meat, fish, and eggs. People at higher risk of low status include those living in low-selenium regions eating mostly local food, people on long-term dialysis, people with HIV, and people with severe malabsorption.

Selenium and the Thyroid

The thyroid gland contains more selenium per gram than any other organ, reflecting the role of selenoproteins in thyroid hormone production and in protecting the gland from hydrogen peroxide generated during hormone synthesis. This connection has made selenium a popular supplement for thyroid conditions.

Hashimoto’s thyroiditis. In this autoimmune condition, antibodies against thyroid peroxidase (TPO) gradually damage the thyroid. A 2010 systematic review and meta-analysis in Thyroid by Konstantinos Toulis and colleagues found that selenium supplementation was associated with a significant decrease in TPO antibody levels at three months and with improvements in mood or general well-being. However, the authors cautioned that improvements in thyroid function and structure had not been demonstrated and that such evidence would be needed before routine selenium supplementation could be recommended for Hashimoto’s. Lower antibody levels do not necessarily mean the disease is changing course.

Graves’ orbitopathy. A 2011 randomized, double-blind, placebo-controlled trial in the New England Journal of Medicine by Claudio Marcocci and colleagues of the European Group on Graves’ Orbitopathy assigned 159 patients with mild Graves’ eye disease to selenium (100 micrograms twice daily), the anti-inflammatory drug pentoxifylline, or placebo for six months. Selenium, but not pentoxifylline, improved quality of life, reduced eye involvement, and slowed progression compared with placebo, with benefits persisting at 12 months and no adverse events. The trial was conducted in Europe, where selenium status is lower than in the United States, so benefits may be smaller in selenium-replete populations. European guidelines have since recommended a six-month course of selenium for mild Graves’ orbitopathy in some patients.

Selenium and Mercury

Selenium binds tightly to mercury, and researchers have long studied whether selenium protects against mercury toxicity, including methylmercury from fish. Many ocean fish that contain mercury also contain selenium, which may partly offset mercury’s effects. A 2012 study in Environmental Science and Technology by Yu-Feng Li and colleagues in China gave 103 residents of a mercury-mining area either 100 micrograms of organic selenium daily as selenium-enriched yeast or non-selenium yeast for three months. Selenium supplementation increased urinary mercury excretion and reduced markers of oxidative damage. These findings apply to people with substantial mercury exposure and low selenium status; they do not mean selenium supplements detoxify the body in general or that they replace reducing mercury exposure. Choosing low-mercury fish remains the best strategy for most people.

Selenium and Cancer: Large Trials Disappointed

Early observational studies and a 1996 trial suggested selenium might lower cancer risk, especially prostate cancer, prompting widespread supplement use. Larger trials did not confirm these hopes. The Selenium and Vitamin E Cancer Prevention Trial (SELECT), reported in 2009 in JAMA by Scott Lippman and colleagues, randomly assigned 35,533 relatively healthy men to 200 micrograms of selenium daily, vitamin E, both, or placebo. Selenium did not prevent prostate cancer or other cancers, and the trial was stopped early. There was a statistically nonsignificant increase in type 2 diabetes among men taking selenium alone.

A 2018 Cochrane review by Marco Vinceti and colleagues analyzed randomized trials and found that selenium supplementation did not reduce overall cancer incidence (risk ratio 0.99, five trials with 21,860 participants) or cancer mortality. The reviewers concluded that the evidence did not support selenium supplements for cancer prevention, and noted signals of harm in some trials, including an increased risk of high-grade prostate cancer in SELECT among men with high baseline selenium and an increased risk of type 2 diabetes.

Selenium and Type 2 Diabetes

Although animal studies suggested selenium might improve glucose metabolism, human trials point the other way. A 2007 analysis in the Annals of Internal Medicine by Saverio Stranges and colleagues examined 1,202 participants without diabetes in the Nutritional Prevention of Cancer trial, conducted in low-selenium areas of the eastern United States, who took 200 micrograms of selenium daily or placebo. Over an average of 7.7 years, type 2 diabetes developed in 58 people taking selenium and 39 taking placebo, a 55 percent increase in risk (hazard ratio 1.55). Risk was highest among people with the highest baseline selenium levels. The authors concluded that selenium supplementation does not seem to prevent type 2 diabetes and may increase risk. This finding is a key reason experts advise against selenium supplements for people with adequate intake.

Food Sources of Selenium

According to the NIH Office of Dietary Supplements, the Recommended Dietary Allowance for selenium is 55 micrograms per day for adults, 60 micrograms during pregnancy, and 70 micrograms during breastfeeding. The Tolerable Upper Intake Level for adults is 400 micrograms per day. Seafood, organ meats, muscle meats, eggs, dairy, grains, and legumes all contribute selenium, with amounts varying by soil and animal feed.

Brazil Nuts: A Concentrated Natural Source

Brazil nuts are by far the richest common food source of selenium. A 2008 randomized trial in the American Journal of Clinical Nutrition by Christine Thomson and colleagues in New Zealand, a low-selenium country, compared two Brazil nuts daily with a 100 microgram selenomethionine supplement or placebo for 12 weeks. Two Brazil nuts a day raised blood selenium and glutathione peroxidase activity as effectively as the supplement, and the authors suggested that including Brazil nuts could avoid the need for fortification or supplements. However, selenium content varies enormously between nuts depending on where they were grown; U.S. reference data list 544 micrograms in a one-ounce serving of six to eight nuts, or roughly 70 to 90 micrograms per nut on average. Eating a handful daily could exceed the 400 microgram upper limit. One or two Brazil nuts a few times a week is a reasonable way to boost intake for people concerned about low status.

Too Much Selenium: Real Risks

Selenium toxicity, called selenosis, causes garlic breath, a metallic taste, hair loss, brittle or discolored nails, skin rashes, fatigue, irritability, gastrointestinal problems, and nerve damage. A 2010 report in the Archives of Internal Medicine by Jennifer MacFarquhar and colleagues at the CDC investigated a 2008 outbreak in which a liquid dietary supplement contained 200 times the labeled concentration of selenium. The investigation identified 201 cases in 10 states. Common symptoms included diarrhea, fatigue, hair loss, joint pain, nail discoloration or brittleness, and nausea, and symptoms such as fingernail loss, fatigue, and hair loss persisted 90 days or longer in many patients. The authors noted that the outbreak might have been prevented if manufacturers had been held to pharmaceutical manufacturing standards. The episode illustrates both the narrow safety margin for selenium and the quality risks of unregulated supplements.

Supplement Forms and Multivitamins

Selenium supplements come as selenomethionine, selenium-enriched yeast, sodium selenite, and sodium selenate. Selenomethionine and selenium yeast are well absorbed and can be incorporated into body proteins, while selenite and selenate are inorganic forms. Many multivitamins contain 50 to 70 micrograms of selenium, which, combined with diet, usually provides plenty. People taking a multivitamin plus a separate selenium supplement, eating Brazil nuts regularly, and eating selenium-rich foods could exceed the upper limit. Checking all supplement labels for selenium helps avoid unintended high intake.

Other Claimed Benefits

Heart health. Low selenium status has been associated with higher cardiovascular risk in some observational studies, and severe deficiency causes Keshan disease. However, trials in populations with adequate selenium, including SELECT, have not shown that supplements prevent heart disease.

Immune function. Selenium deficiency impairs immune responses and can allow some viruses to become more virulent, as shown in animal studies. Correcting deficiency supports immunity, but there is little evidence that extra selenium boosts immunity in well-nourished people.

Fertility. Selenium is needed for normal sperm development, and some small trials in men with infertility have reported improvements in sperm motility with selenium, often combined with other antioxidants. Evidence is limited and of mixed quality.

Mood and cognition. Some studies have linked low selenium to poorer mood and cognitive decline, and the 2010 Hashimoto’s meta-analysis noted improved well-being. Robust evidence that supplements improve mood or prevent dementia in people with adequate status is lacking.

Antioxidant protection. Selenium-containing enzymes are central to antioxidant defense, but more selenium beyond what is needed to maximize these enzymes does not add benefit and may cause harm.

Selenium in Pregnancy and Children

Selenium needs rise slightly during pregnancy and breastfeeding, to 60 and 70 micrograms daily respectively. Most prenatal vitamins contain selenium, and a varied diet usually meets needs in selenium-replete regions. Children need smaller amounts, and upper limits for children are much lower than for adults, ranging from 45 micrograms for infants to 280 micrograms for children aged 9 to 13 and 400 micrograms for teens aged 14 to 18. Parents should be cautious about giving children Brazil nuts in quantity or adult supplements containing selenium.

Testing Selenium Status

Blood or plasma selenium can be measured, and glutathione peroxidase activity or selenoprotein P levels provide additional information about functional status. Testing is not routine for healthy people but may be useful for those at risk of deficiency, people with symptoms of possible toxicity, or patients with thyroid disease considering supplements. Interpreting results requires considering regional norms, since typical levels differ substantially between countries.

Selenium and Iodine Work Together

Thyroid health depends on both iodine, the building block of thyroid hormones, and selenium, which supports the enzymes that activate hormones and protect the gland. In regions where both nutrients are deficient, correcting iodine deficiency without addressing selenium may not fully restore thyroid health, and some researchers have suggested that combined deficiency contributes to certain thyroid disorders. In the United States, iodized salt, dairy, and seafood supply iodine, though intake has declined in some groups. People considering supplements for thyroid health should discuss both nutrients with their clinician rather than self-treating, since excess iodine can also cause thyroid problems.

A Lesson From Finland

Finland has naturally low soil selenium, and dietary intake was low in the 1970s. In 1984, the Finnish government began adding selenium to agricultural fertilizers, which raised selenium levels in crops, animal products, and the population’s blood within a few years. The program is often cited as an example of improving a nutrient’s status across a population through agriculture rather than supplements. It also shows that selenium status depends heavily on where food is grown, which is why recommendations for supplements vary by country.

Questions to Ask Your Clinician

Is my selenium intake likely adequate given my diet and where I live? Would a selenium supplement help my thyroid condition, and for how long? Does my multivitamin already contain selenium? Are there risks given my diabetes risk or other conditions? Should my selenium level be tested? Clear answers help avoid both deficiency and excess.

A Balanced View

Selenium is an essential nutrient with genuine roles in thyroid function, antioxidant defense, and immunity, and correcting deficiency matters. Specific uses, such as short-term supplementation for mild Graves’ orbitopathy, have trial support. But many popular claims, including cancer prevention, have been tested and failed, and supplementation in people with adequate status may raise diabetes risk. For most people in selenium-replete regions, a varied diet provides enough, and the safest approach is to avoid high-dose supplements unless a clinician recommends them.

Practical Takeaways

Eat a varied diet including seafood, eggs, meat or legumes, and whole grains. If you enjoy Brazil nuts, eat one or two a few times a week rather than a handful daily. Check multivitamin labels before adding a separate selenium supplement. Avoid exceeding 400 micrograms daily from all sources. Consider supplements only for specific conditions or confirmed low status, under medical guidance. Choose third-party tested products if you supplement.

How Much Selenium Americans Get

National survey data summarized by the NIH Office of Dietary Supplements indicate that most Americans consume more than the RDA from food alone, and deficiency is rare in the general U.S. population. Bread, grains, meat, poultry, fish, and eggs supply most dietary selenium. This is why routine supplementation is generally unnecessary in the United States and why trials in American populations, which started from adequate selenium status, often found no benefit or possible harm. People in low-selenium regions of Europe, China, or New Zealand may have different needs.

Selenium, Soil, and Animal Feed

The selenium in plant foods reflects the soil they grow in, and the selenium in meat, milk, and eggs reflects animal feed. In the United States, livestock feed is commonly supplemented with selenium to prevent deficiency diseases in animals, which also raises selenium in animal foods. Plant-based eaters in low-selenium regions may need to pay closer attention to selenium, choosing foods such as Brazil nuts, legumes, whole grains from selenium-rich regions, or fortified products.

Medication Interactions

Selenium supplements can interact with some treatments. The chemotherapy drug cisplatin can lower selenium levels, and research on whether selenium supplements protect against cisplatin side effects has been inconclusive; patients receiving cancer treatment should not take supplements without consulting their oncology team. Selenium has antiplatelet effects at high doses in some studies, which could matter for people taking blood thinners. Anyone taking prescription medications should review supplements with a pharmacist or clinician.

Hair Loss and Selenium

Selenium is sometimes promoted for hair growth, but hair loss is actually a classic sign of selenium excess. While severe deficiency can affect hair and nails, there is no good evidence that selenium supplements improve hair growth in people with adequate status, and high doses can cause the very hair loss people hope to prevent.

Selenium for Vegetarians and Vegans

People who avoid animal foods can still meet selenium needs, but intake depends more heavily on where plant foods were grown. Whole grains, legumes, mushrooms, seeds, and Brazil nuts contribute selenium. In the United States, grains from selenium-rich regions usually provide adequate amounts. Vegans in low-selenium countries may benefit from including a Brazil nut a few times a week or a multivitamin containing selenium. Because plant selenium varies so much, a single food list cannot guarantee adequate intake everywhere.

Reading Supplement Labels

Selenium is listed in micrograms on supplement labels, sometimes with the chemical form. Check the total selenium across all products you take, including multivitamins, thyroid support formulas, hair and nail products, and antioxidant blends, which often include selenium. Products providing 200 micrograms or more daily, the dose used in trials linked to increased diabetes risk, deserve particular caution for people without a specific medical reason to take them. Third-party testing verifies that products contain what labels state, which matters given the 2008 outbreak caused by a mislabeled supplement.

Why Results Differ Between Studies

Selenium research often seems contradictory because results depend on baseline status. Trials in low-selenium populations, such as parts of China or Europe, are more likely to find benefits, while trials in selenium-replete populations, such as the United States, often find none or detect harm. The chemical form, dose, duration, and outcome measured also matter. Understanding the U-shaped relationship helps make sense of headlines: a study showing benefit in one country may not apply to people elsewhere who already get enough. When reading about selenium, always ask where the participants lived and what their selenium levels were before the study began, since that context often determines whether the results are relevant to you.

Common Myths

“Selenium supplements prevent cancer.” Large trials and a Cochrane review found no cancer prevention benefit.

“More selenium means more antioxidant protection.” Once selenoproteins are maximized, extra selenium adds no benefit and may cause harm.

“Everyone with thyroid problems should take selenium.” Evidence supports selenium mainly for mild Graves’ eye disease in specific settings; benefits for Hashimoto’s are uncertain.

“Selenium detoxes mercury from the body.” Selenium increased mercury excretion in heavily exposed people with low status, but reducing exposure is the main strategy.

“Natural sources can’t cause toxicity.” Large amounts of Brazil nuts can exceed safe limits.

Key Terms

Selenoproteins: Proteins containing selenium, including antioxidant and thyroid enzymes.

Glutathione peroxidase: A selenium-dependent antioxidant enzyme.

TPO antibodies: Antibodies against thyroid peroxidase, elevated in Hashimoto’s thyroiditis.

Selenosis: Selenium toxicity from excessive intake.

Selenomethionine: An organic form of selenium found in foods and supplements.

The Bottom Line

Selenium is essential for antioxidant enzymes, thyroid hormone activation, and immune function, and adults need about 55 micrograms daily. Evidence supports selenium for correcting deficiency and, in specific settings, for mild Graves’ eye disease; it lowers thyroid antibodies in Hashimoto’s but without proven effects on thyroid function. Selenium increased mercury excretion in heavily exposed people with low status. Large trials found no benefit for cancer prevention, and supplements may increase type 2 diabetes risk in people who already have adequate selenium. Most people in selenium-replete regions get enough from food. Brazil nuts are a potent natural source, best eaten in small amounts.

Frequently Asked Questions

What are the main health benefits of selenium?

It supports antioxidant defense, thyroid hormone metabolism, and immune function. Supplements help mainly when intake is low.

Should I take selenium for my thyroid?

It may help mild Graves’ eye disease and lowers antibodies in Hashimoto’s, but discuss it with your clinician first.

Does selenium prevent cancer?

No. Large trials, including SELECT with 35,533 men, found no cancer prevention benefit.

How many Brazil nuts should I eat?

One or two a few times a week is plenty for most people; a handful daily can exceed the safe upper limit.

Can selenium be harmful?

Yes. Excess causes hair and nail loss, nerve problems, and other symptoms, and supplements may raise diabetes risk.

References

1. Rayman MP. Selenium and human health. Lancet. 2012;379(9822):1256–1268. PMID 22381456
2. Toulis KA, Anastasilakis AD, Tzellos TG, et al. Selenium supplementation in the treatment of Hashimoto’s thyroiditis: a systematic review and a meta-analysis. Thyroid. 2010;20(10):1163–1173. PMID 20883174
3. Marcocci C, Kahaly GJ, Krassas GE, et al. Selenium and the course of mild Graves’ orbitopathy. New England Journal of Medicine. 2011;364(20):1920–1931. PMID 21591944
4. Li YF, Dong Z, Chen C, et al. Organic selenium supplementation increases mercury excretion and decreases oxidative damage in long-term mercury-exposed residents from Wanshan, China. Environmental Science and Technology. 2012;46(20):11313–11318. PMID 23033886
5. Lippman SM, Klein EA, Goodman PJ, et al. Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2009;301(1):39–51. PMID 19066370
6. Vinceti M, Filippini T, Del Giovane C, et al. Selenium for preventing cancer. Cochrane Database of Systematic Reviews. 2018;1:CD005195. PMID 29376219
7. Stranges S, Marshall JR, Natarajan R, et al. Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Annals of Internal Medicine. 2007;147(4):217–223. PMID 17620655
8. Thomson CD, Chisholm A, McLachlan SK, Campbell JM. Brazil nuts: an effective way to improve selenium status. American Journal of Clinical Nutrition. 2008;87(2):379–384. PMID 18258628
9. MacFarquhar JK, Broussard DL, Melstrom P, et al. Acute selenium toxicity associated with a dietary supplement. Archives of Internal Medicine. 2010;170(3):256–261. PMID 20142570
10. NIH Office of Dietary Supplements. Selenium Fact Sheet for Health Professionals. ods.od.nih.gov

Last updated: October 6, 2026