Home Ingredients Selenium
AntioxidantImmune SupportThyroid Health

Selenium

Evidence Level
Moderate
2 Clinical Trials
4 Documented Benefits
3/5 Evidence Score

Selenium is an essential trace mineral built into about 25 selenoproteins, including antioxidant enzymes such as glutathione peroxidase and the enzymes involved in thyroid hormone metabolism. Most people in North America get well above the RDA of 55 mcg from foods such as seafood, meat, poultry and Brazil nuts. Supplements usually contain 55 to 400 mcg as selenomethionine, selenium yeast, selenite or selenate, all well absorbed. Benefit follows a U-shaped curve: in people with low status, supplements raised antioxidant enzyme activity and changed immune responses, and in autoimmune thyroiditis they lowered thyroid antibodies without changing thyroid hormone levels. In people who already get enough, 200 mcg a day did not prevent cancer in large trials and was linked to more type 2 diabetes. Keep total intake below 400 mcg a day.

Studied Dose RDA 55 mcg a day for adults; trials used 50 to 200 mcg a day; upper limit 400 mcg a day. In one 7.7-year trial, 200 mcg a day was linked to more type 2 diabetes.
Active Compound Selenomethionine, selenium-enriched yeast, sodium selenite or sodium selenate; the body absorbs up to about 90 percent from each.
Deficiency information View details

Selenium deficiency is very rare in the United States and Canada, where intakes even in low-selenium regions are well above the RDA. Keshan disease, a heart muscle disease, occurred in parts of China with very low soil selenium, and deficiency is also associated with Kashin-Beck disease, a joint disease in low-selenium and low-iodine areas. In North America, groups at higher risk include people on long-term kidney dialysis and people living with HIV.

Common symptoms

  • On its own, selenium deficiency rarely causes illness, but it produces biochemical changes that can predispose to illness under added stress
  • Keshan disease, a heart muscle disease, in severe deficiency in parts of China
  • Kashin-Beck disease, a type of osteoarthritis that starts in childhood, in low-selenium and low-iodine regions
  • Can worsen iodine deficiency, raising the risk of congenital hypothyroidism in infants

At-risk groups

  • People in countries whose diet comes mainly from vegetables grown in selenium-poor soils, such as parts of China
  • People undergoing long-term kidney dialysis
  • People living with HIV
  • Vegans in some European countries where average intakes are low
  • Note: Brazil nuts are very high in selenium (about 68 to 91 mcg per nut), so one or two can exceed the 55 mcg RDA
When to see a doctor: Selenium deficiency is very rare in North America, so testing matters mainly for people on long-term dialysis or living with HIV; a doctor can measure plasma or serum selenium. Selenium has a narrow safety margin: chronic intake above 400 mcg a day can cause hair loss, brittle nails, garlic breath and nervous system symptoms, so do not supplement above that without medical advice.
Explore related deficiencies →
Sources: NIH ODS: Selenium Fact Sheet for Health Professionals

Benefits

Antioxidant defense

Selenium is built into about 25 selenoproteins, including glutathione peroxidases and thioredoxin reductases, which help protect cells from oxidative damage. In UK adults with low selenium status, 50 or 100 mcg a day for 15 weeks raised glutathione peroxidase activity in white blood cells. A major review concludes that people whose status is already adequate should not expect benefit from extra selenium.

Supported by Trial 1, Trial 2

Thyroid hormone metabolism

Selenoproteins are needed for thyroid hormone metabolism, including production of active thyroid hormone. In women with autoimmune thyroiditis on levothyroxine, 200 mcg a day for 3 months lowered thyroid peroxidase antibodies while TSH, free T4 and free T3 did not change. A 412-patient, 12-month trial also lowered antibodies but did not improve quality of life more than placebo or change levothyroxine dose.

Supported by Trial 1

Immune function

Selenoproteins help protect against infection. In 22 UK adults with low selenium, 50 or 100 mcg a day for 15 weeks boosted T cell responses and sped clearance of a live polio vaccine virus. In 117 adults aged 50 to 64 with marginal status, selenium as enriched yeast or onion before flu vaccination had both favourable and unfavourable effects on immune cells. Antibody responses did not change in either trial.

Supported by Trial 1

Results of large long-term supplementation trials

In SELECT, 35,533 men took 200 mcg a day of selenomethionine or placebo, and prostate cancer was not reduced (575 cases versus 529). A Cochrane review of trials at low risk of bias found no reduction in overall cancer (risk ratio 1.01), and some trials reported more type 2 diabetes with selenium. This is why extra selenium is not advised for people who already get enough.

Supported by Trial 2

Mechanism of action

1

Selenoprotein synthesis

Selenium is incorporated as selenocysteine into 25 human selenoproteins, which have antioxidant, anti-inflammatory and thyroid hormone functions.

2

Glutathione peroxidases

Glutathione peroxidases are selenoproteins that help protect cells from oxidative damage. Their activity rose when people with low selenium intake took supplements.

3

Thioredoxin reductases

Thioredoxin reductases are also selenoproteins. Selenoproteins as a group are involved in DNA synthesis, reproduction and protection from infection.

Clinical trials

1
Selenium and thyroid antibodies in autoimmune thyroiditis (3-month RCT)
PubMed

Randomized, blinded, placebo-controlled trial of 200 mcg a day sodium selenite or placebo for 3 months in women with autoimmune thyroiditis, all on levothyroxine. Primary outcome: thyroid peroxidase antibody concentration. (Gärtner et al. 2002, J Clin Endocrinol Metab)

70 women (36 selenium, 34 placebo), mean age 47.5, with autoimmune thyroiditis and high thyroid antibodies, all taking levothyroxine.

Mean antibody concentration fell to 63.6 percent of baseline on selenium versus 88 percent on placebo (p = 0.013), and antibodies normalised in 9 selenium patients versus 2 on placebo. TSH, free T4 and free T3 were unchanged in both groups, and thyroglobulin antibodies fell only in the placebo group. An antibody level is a laboratory marker, not a symptom.

2
SELECT: selenium and vitamin E in 35,533 men
PubMed

Randomized, placebo-controlled trial of 200 mcg a day L-selenomethionine, 400 IU a day vitamin E, both, or placebo, with planned follow-up of 7 to 12 years. Primary outcome: prostate cancer incidence. (Klein et al. 2011, JAMA)

35,533 men in the United States, Canada and Puerto Rico aged 55 or older (50 or older for Black men) with PSA of 4.0 ng/mL or less.

Prostate cancer occurred in 529 men on placebo, 575 on selenium (hazard ratio 1.09, 99% CI 0.93 to 1.27), 620 on vitamin E (hazard ratio 1.17, 99% CI 1.004 to 1.36) and 555 on both. Selenium did not lower prostate cancer risk, and vitamin E raised it.

Side effects and drug interactions

Common Potential side effects

Selenosis from chronically high intake: hair loss and brittle or lost nails are the most common signs, garlic breath and a metallic taste are early signs, and rash, nausea, diarrhea, fatigue, irritability and nervous system problems can also occur. The adult upper limit is 400 mcg a day.
Brazil nuts contain about 68 to 91 mcg of selenium per nut and could cause toxicity if eaten regularly in large amounts.
Type 2 diabetes: in a 7.7-year randomized trial in 1,202 people, 200 mcg a day produced 58 cases versus 39 on placebo (hazard ratio 1.55), with the highest risk in those who started with the highest blood selenium. A meta-analysis of 5 randomized trials put the increase at 11 percent (risk ratio 1.11), while a 2-year trial in older Danes with low status found no effect on HbA1c.

Important Drug interactions

Cisplatin: this chemotherapy drug can lower selenium levels. Small studies suggest selenium may reduce its toxicity, but a Cochrane review found the evidence insufficient. Discuss with the treating team.

Featured in these supplement guides

Immune Support→Inflammation→Thyroid Support→

More Antioxidant Ingredients

Antioxidant
Green Tea
Green tea comes from the leaves of Camellia sinensis and is rich in catechins, especially…
Antioxidant
Lutein
Lutein is a yellow carotenoid that collects in the retina, where, together with zeaxanthin…
Antioxidant
Curcumin/Turmeric
Curcumin, the main bioactive compound in turmeric (Curcuma longa), is usually taken as a s…
Antioxidant
NAC (N-Acetylcysteine)
N-acetylcysteine (NAC) is a modified form of the amino acid cysteine. The body uses cystei…

Explore Other Ingredients

Immune Support
Zinc
Zinc is an essential trace mineral that hundreds of enzymes need, and it plays a role in i…
Thyroid Health
Iodine
Iodine is an essential trace mineral that the thyroid uses to make thyroid hormones, which…
Bone Health
Vitamin D
Vitamin D is a fat-soluble vitamin the body turns into a hormone that controls calcium abs…

Related articles from our blog

📝 Article
Selenium: Benefits, the Narrow Safe Range, and Who Needs It
Selenium is a mineral with an unusual personality. You need only a tiny amount, but that tiny amount is genuinely essent…
June 25, 2026
📝 Article
The 5 Best Immune-Support Multivitamins of 2026
Immune function is supported by nutrients from food and, when the diet leaves gaps, a well-formulated daily multivitamin…
September 21, 2026
📝 Article
The 7 Best Prostate Supplements of 2026
Walk the men's-health aisle and every prostate bottle promises comfortable nights and a healthy prostate. The honest rea…
September 18, 2026

Frequently asked questions about Selenium

How much selenium should I take?

The RDA is 55 mcg a day for adults. Multivitamins often contain about 55 mcg, and selenium-only supplements typically 100 to 400 mcg. Selenium has a narrow safe range, so keep total intake from all sources below 400 mcg a day.

What is selenium good for?

Selenium is a trace mineral essential for thyroid hormone metabolism and for antioxidant enzymes. It supports thyroid, immune, and antioxidant function. In people who already get enough, extra selenium has not been shown to add anything: 200 mcg a day did not prevent cancer in a 35,533-man trial and has been linked to a higher rate of type 2 diabetes. Many people get enough from food, especially Brazil nuts.

Can I get enough selenium from Brazil nuts?

Yes. Brazil nuts contain about 68 to 91 mcg of selenium each, so one or two can supply a full day's requirement, and eating large amounts every day could lead to too much. You may not need a separate supplement.

Can you take too much selenium?

Yes. Selenium toxicity (selenosis) can occur above about 400 mcg per day and may cause hair loss, brittle nails, garlic breath, and nerve issues. Stay within the recommended range and avoid stacking multiple high-selenium sources.

What is Selenium?

Selenium is an essential trace mineral built into about 25 selenoproteins, including antioxidant enzymes such as glutathione peroxidase and the enzymes involved in thyroid hormone metabolism. Most people in North America get well above the RDA of 55 mcg from foods such as seafood, meat, poultry and Brazil nuts.

What is Selenium used for?

Selenium is researched primarily for Antioxidant, Immune Support, and Thyroid Health. Selenium is built into about 25 selenoproteins, including glutathione peroxidases and thioredoxin reductases, which help protect cells from oxidative damage.

What are the signs of Selenium deficiency?

Selenium deficiency is very rare in the United States and Canada, where intakes even in low-selenium regions are well above the RDA. Keshan disease, a heart muscle disease, occurred in parts of China with very low soil selenium, and deficiency is also associated with Kashin-Beck disease, a joint disease in low-selenium…

What is the recommended dosage of Selenium?

The clinically studied dose is RDA 55 mcg a day for adults; trials used 50 to 200 mcg a day; upper limit 400 mcg a day. In one 7.7-year trial, 200 mcg a day was linked to more type 2 diabetes. Always follow the product label and check with a healthcare provider for personal advice.

Is Selenium safe, and does it have side effects?

For most healthy adults, Selenium is well tolerated at studied doses. Reported effects can include: Selenosis from chronically high intake: hair loss and brittle or lost nails are the most common signs, garlic breath and a metallic taste are early signs, and rash, nausea, diarrhea, fatigue, irritability and nervous system problems can also occur. It may also interact with some medications. Selenium is not right for everyone, so check with a healthcare provider first if you are pregnant or breastfeeding, have a medical condition, or take prescription medication.

Does Selenium interact with any medications?

Possible interactions include: Cisplatin: this chemotherapy drug can lower selenium levels. Small studies suggest selenium may reduce its toxicity, but a Cochrane review found the evidence insufficient. Discuss with the treating team. If you take prescription medication, check with a pharmacist or doctor before using it.

How strong is the scientific evidence for Selenium?

NutraSmarts rates the evidence for Selenium as Moderate (3 out of 5). It is backed by 2 clinical trials and 12 cited references summarized on this page. A higher rating reflects more, larger, and better-designed human studies.

References(12 citations)

Evidence ratings on NutraSmarts are based on the totality of human clinical research, with emphasis on randomized controlled trials, meta-analyses, and systematic reviews. The references below directly support claims made throughout this page.

  1. Toulis KA, Anastasilakis AD, Tzellos TG, Goulis DG, Kouvelas D. Selenium supplementation in the treatment of Hashimoto's thyroiditis: a systematic review and a meta-analysis. Thyroid. 2010;20(10):1163-73. doi: 10.1089/thy.2009.0351.PubMedUsed to support: Thyroid antibody effect, clinical benefit unproven. This systematic review and meta-analysis of randomized, placebo-controlled trials in Hashimoto's patients already on levothyroxine found that 3 months of selenium significantly lowered thyroid peroxidase antibody titres (weighted mean difference -271.09, 95% CI -421.98 to -120.19) and raised the chance of reporting better mood or well-being (risk ratio 2.79, 95% CI 1.21 to 6.47). Levothyroxine requirement and thyroid ultrasound appearance were unchanged or underreported, and the authors state that an improvement in thyroid function and morphology should be demonstrated before routine selenium is recommended. The population is patients with an autoimmune thyroid disease, not healthy adults.
  2. Klein EA, Thompson IM Jr, Tangen CM, Crowley JJ, Lucia MS, Goodman PJ, Minasian LM, Ford LG, Parnes HL, Gaziano JM, Karp DD, Lieber MM, Walther PJ, Klotz L, Parsons JK, Chin JL, Darke AK, Lippman SM, Goodman GE, Meyskens FL Jr, Baker LH. Vitamin E and the risk of prostate cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2011;306(14):1549-56. doi: 10.1001/jama.2011.1437.PubMedUsed to support: Cancer prevention not supported. In 35,533 men randomized to 200 mcg a day L-selenomethionine, 400 IU a day vitamin E, both, or placebo, prostate cancer occurred in 575 men on selenium versus 529 on placebo (hazard ratio 1.09, 99% CI 0.93 to 1.27), while vitamin E significantly increased risk (620 cases, hazard ratio 1.17). The paper is titled for the vitamin E result and is cited here for its selenium arm.
  3. Stranges S, Marshall JR, Natarajan R, Donahue RP, Trevisan M, Combs GF, Cappuccio FP, Ceriello A, Reid ME. Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Ann Intern Med. 2007;147(4):217-23. doi: 10.7326/0003-4819-147-4-200708210-00175.PubMedUsed to support: Harm signal. In this secondary analysis of a randomized, double-blind, placebo-controlled trial in 1202 people from low-selenium areas of the eastern United States, 200 mcg/day of selenium over a mean 7.7 years produced 58 cases of type 2 diabetes versus 39 on placebo (12.6 vs 8.4 per 1000 person-years; hazard ratio 1.55, 95% CI 1.03 to 2.33). Risk rose across baseline plasma selenium tertiles, reaching a hazard ratio of 2.70 (95% CI 1.30 to 5.61) in the highest tertile. The authors conclude selenium does not prevent type 2 diabetes and may increase risk of it.
  4. Rayman MP. Selenium and human health. Lancet. 2012;379(9822):1256-68. doi: 10.1016/S0140-6736(11)61452-9.PubMedUsed to support: Review: selenium is incorporated into selenoproteins with antioxidant, anti-inflammatory and thyroid hormone functions; low status is linked to poor immune function; the link with health is U-shaped, so people with low status may benefit while those with adequate or high status should not take supplements and may face higher type 2 diabetes risk.
  5. Vinceti M, Filippini T, Del Giovane C, Dennert G, Zwahlen M, Brinkman M, Zeegers MP, Horneber M, D'Amico R, Crespi CM. Selenium for preventing cancer. Cochrane Database Syst Rev. 2018;1(1):CD005195. doi: 10.1002/14651858.CD005195.pub4.PubMedUsed to support: Selenium supplements do not prevent cancer. This Cochrane review pooled 10 randomized trials with 27,232 participants; in the primary analysis restricted to the trials at lowest risk of bias (3 trials, 19,475 participants) selenium did not reduce overall cancer incidence (risk ratio 1.01, 95% CI 0.93 to 1.10, high-certainty evidence), and there was no reduction for colorectal, lung, bladder, breast or prostate cancer. Pooling all the randomized trials gave the same answer (risk ratio 0.99, 95% CI 0.86 to 1.14). The trials at lowest risk of bias suggested an increase in melanoma risk, and the review also reports a slightly increased risk of type 2 diabetes with supplementation. This is a systematic review, not a single trial.
  6. Larsen C, Winther KH, Cramon PK, Rasmussen ÅK, Feldt-Rasmusssen U, Knudsen NJ, Bjorner JB, Schomburg L, Demircan K, Chillon TS, Gram J, Hansen SG, Brandt F, Nygaard B, Watt T, Hegedus L, Bonnema SJ. Selenium supplementation and placebo are equally effective in improving quality of life in patients with hypothyroidism. Eur Thyroid J. 2024;13(1):e230175. doi: 10.1530/ETJ-23-0175.PubMedUsed to support: The largest selenium trial in autoimmune thyroiditis was null on how patients felt. 412 patients with thyroid peroxidase antibodies of 100 IU/mL or more, all on levothyroxine, took 200 mcg/day selenium-enriched yeast or placebo for 12 months. Quality of life improved equally in both groups, with no difference on any ThyPRO-39 scale or on the composite score (P = 0.602). Antibody levels were lower on selenium (1995 vs 2344 kIU/L, P = 0.016) but this did not change levothyroxine dose or the free T3 to free T4 ratio.
  7. Broome CS, McArdle F, Kyle JA, Andrews F, Lowe NM, Hart CA, Arthur JR, Jackson MJ. An increase in selenium intake improves immune function and poliovirus handling in adults with marginal selenium status. Am J Clin Nutr. 2004;80(1):154-62. doi: 10.1093/ajcn/80.1.154.PubMedUsed to support: Small trial in people who started low. 22 UK adults with plasma selenium below 1.2 micromol/L took 50 or 100 mcg/day of sodium selenite or placebo for 15 weeks and received oral live polio vaccine. Selenium raised lymphocyte glutathione peroxidase activity, increased interferon gamma and other cytokines, brought forward the peak T cell response and increased T helper cells, and sped clearance of the vaccine virus from stool. Antibody responses were unaffected. The authors note these participants had a functional selenium deficit, so the result speaks to correcting low status rather than to supplementing people who already have enough.
  8. Vinceti M, Filippini T, Rothman KJ. Selenium exposure and the risk of type 2 diabetes: a systematic review and meta-analysis. Eur J Epidemiol. 2018;33(9):789-810. doi: 10.1007/s10654-018-0422-8.PubMedUsed to support: Pooled evidence points the same way as the single trial that first raised the alarm, though the effect is smaller. Across 5 randomized trials, 200 mcg/day of selenium raised type 2 diabetes incidence by 11 percent versus placebo (risk ratio 1.11, 95% CI 1.01 to 1.22), with a larger relative increase in women than in men. The review also retrieved 50 observational studies, and among those reporting multiple exposure levels the relation was roughly linear across the range, with a risk ratio of 3.6 (95% CI 1.4 to 9.4) at a serum selenium of 140 micrograms per litre compared with under 45. The authors describe the relative increase as small but potentially important because both diabetes and selenium exposure are common.
  9. Stranges S, Rayman MP, Winther KH, Guallar E, Cold S, Pastor-Barriuso R. Effect of selenium supplementation on changes in HbA1c: Results from a multiple-dose, randomized controlled trial. Diabetes Obes Metab. 2019;21(3):541-549. doi: 10.1111/dom.13549.PubMedUsed to support: No glycaemic benefit at any dose. In the Danish PRECISE trial, 491 volunteers aged 60 to 74 with low selenium status were randomized to 100, 200 or 300 mcg/day of selenium-enriched yeast or placebo yeast. HbA1c fell in every group over 2 years with no difference between selenium and placebo, and the authors report the findings as consistent with a U-shaped response of glucose metabolism to selenium.
  10. Gärtner R, Gasnier BC, Dietrich JW, Krebs B, Angstwurm MW. Selenium supplementation in patients with autoimmune thyroiditis decreases thyroid peroxidase antibodies concentrations. J Clin Endocrinol Metab. 2002;87(4):1687-91. doi: 10.1210/jcem.87.4.8421.PubMedUsed to support: The trial behind the thyroid claim. 70 women with autoimmune thyroiditis, all on levothyroxine, received 200 mcg/day of sodium selenite or placebo for 3 months. Mean thyroid peroxidase antibody concentration fell to 63.6 percent of baseline on selenium versus 88 percent on placebo (P = 0.013), and antibodies normalised in 9 selenium patients versus 2 on placebo, with normalised ultrasound echogenicity in those patients. Mean TSH, free T4 and free T3 were unchanged in both groups, so thyroid function itself did not change. Thyroglobulin antibodies fell significantly in the placebo group and were unchanged on selenium.
  11. Ivory K, Prieto E, Spinks C, Armah CN, Goldson AJ, Dainty JR, Nicoletti C. Selenium supplementation has beneficial and detrimental effects on immunity to influenza vaccine in older adults. Clin Nutr. 2017;36(2):407-415. doi: 10.1016/j.clnu.2015.12.003.PubMedUsed to support: Randomized, double-blind trial in 117 healthy adults aged 50 to 64 with marginal selenium status: selenium yeast (50 to 200 mcg a day) or selenium-enriched onion for 12 weeks, with flu vaccination at week 10, raised some cellular immune responses (such as T cell proliferation) but lowered granzyme B in CD8 cells at 200 mcg; mucosal antibody responses were unchanged.
  12. Office of Dietary Supplements, National Institutes of Health. Selenium: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements. 2025;Updated September 4, 2025..SourceUsed to support: Not PubMed-indexed (US government fact sheet). Selenium is part of 25 selenoproteins with roles in thyroid hormone metabolism, DNA synthesis, reproduction and protection from oxidative damage and infection; RDA 55 mcg and adult upper limit 400 mcg; supplement forms absorbed up to about 90 percent; Brazil nuts 68 to 91 mcg per nut; deficiency very rare in the US and Canada, with Keshan and Kashin-Beck disease in low-selenium regions; risk groups (selenium-poor regions, dialysis, HIV); signs of selenosis; and the cisplatin interaction.