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Selenium

Evidence Fact Sheet

Selenium is an essential trace element built into ~25 selenoproteins (glutathione peroxidase, thioredoxin reductase, iodothyronine deiodinases) supporting antioxidant defense, thyroid hormone metabolism and immunity. Research doses 50-200 mcg/day; EFSA-authorized function claims; U-shaped dose-response with honest negatives in cancer and prostate trials.

Also known as: Se · L-Selenomethionine · Selenomethionine · Sodium selenite · Sodium selenate · Selenium yeast · Selenocysteine · Selênio · Sélénium

Overview

Selenium is an essential dietary trace element incorporated as the amino acid selenocysteine into roughly 25 selenoproteins, including the glutathione peroxidase (GPx) and thioredoxin reductase (TrxR) antioxidant enzymes and the iodothyronine deiodinases that convert thyroxine (T4) to active T3. Through these enzymes it underpins cellular antioxidant defense, thyroid hormone metabolism, immune function and spermatogenesis. Research and supplemental doses typically range 50-200 mcg/day, against a US RDA of 55 mcg/day and EFSA adequate intake of 70 mcg/day; tolerable upper intake levels are 400 mcg/day (US IOM) and 300 mcg/day (EFSA), reflecting a narrow U-shaped therapeutic window where both deficiency and excess carry risk. Regulatory status: EFSA authorizes six Article 13(1) function claims (immune, thyroid, hair, nails, spermatogenesis, oxidative-stress protection); the US FDA permits only Qualified Health Claims for cancer with a mandatory "limited and not conclusive" disclaimer; ANVISA and China permit it as an essential nutrient. This page reports research findings in studied populations and is not clinical guidance.

Mechanism of Action

Essential trace element incorporated into ~25 known selenoproteins as the 21st amino acid selenocysteine (Sec) · Cofactor for glutathione peroxidase (GPx) family enzymes that reduce hydrogen peroxide and lipid hydroperoxides — primary cellular antioxidant defense · Cofactor for thioredoxin reductase (TrxR) family regulating redox signaling and protein folding · Cofactor for iodothyronine deiodinases (DIO1/DIO2/DIO3) which catalyze peripheral conversion of thyroxine (T4) to active triiodothyronine (T3) · Selenoprotein P (SELENOP) is the primary plasma transport/storage form distributing selenium to brain, testis, and other priority tissues · Required for normal spermatogenesis via selenoprotein GPx4 in the mitochondrial capsule of sperm flagella

Body systems: Immune System · Endocrine & Metabolic

Evidence-Based Benefits

Each benefit below is anchored to a specific PubMed-indexed study. Effect sizes, sample sizes, and p-values are reported as published; no values are inferred. Honest negatives and null results are kept alongside the positive findings, and disease-research populations are described as such — Selenium is not characterized as a treatment for any disease.

Thyroid Autoimmunity (Hashimoto Thyroiditis)

Meta-analysis supported
  • SMD -0.96TPOAb · CI -1.36 to -0.56
  • 2358 participants29 cohorts pooled
  • SMD -0.21TSH (no THRT) · CI -0.43 to -0.02

In a meta-analysis of randomized trials in Hashimoto thyroiditis patients, selenium supplementation lowered thyroid peroxidase antibody (TPOAb) levels and modestly reduced TSH in patients not on thyroid hormone replacement. Adverse effects were comparable to control. This is the strongest signal for selenium and thyroid autoimmunity, framed as a marker change rather than disease cure.

Reported effect: TPOAb SMD -0.96 (CI -1.36 to -0.56; 29 cohorts; 2358 participants; I2=90%); TSH in patients without THRT SMD -0.21 (CI -0.43 to -0.02; 7 cohorts, 869 participants)

“TPOAb (SMD -0.96 [CI -1.36 to -0.56]; 29 cohorts; 2358 participants; I2 = 90%) ... selenium supplementation decreased TSH in patients without THRT (SMD -0.21 [confidence interval, CI -0.43 to -0.02]; 7 cohorts, 869 participants; I2 = 0%)”

Source: PMID 38243784 · Huwiler 2024 · Thyroid

Cancer Prevention

Null / no benefit Meta-analysis supported
  • RR 0.90any cancer · CI 0.70-1.17
  • N = 4765two RCTs

An honest negative: this Cochrane meta-analysis of randomized trials found no clear evidence that selenium supplementation reduces cancer risk, and the authors concluded there is no convincing evidence selenium prevents cancer in humans. The review also flagged concerns about a possible increased risk of type 2 diabetes from selenium supplements.

Reported effect: RR 0.90 (95% CI 0.70 to 1.17) for any cancer, two studies, N = 4765

“no clear evidence that selenium supplementation reduced the risk of any cancer (risk ratio (RR) 0.90, 95% CI 0.70 to 1.17, two studies, N = 4765) ... To date, no convincing evidence suggests that selenium supplements can prevent cancer in humans.”

Source: PMID 24683040 · Vinceti 2014 · Cochrane Database Syst Rev

Prostate Cancer Prevention (SELECT Trial)

Null / no benefit RCT supported
  • HR 1.04prostate cancer · 99% CI 0.87-1.24
  • 35,533 menrandomized

An honest negative from one of the largest prevention trials ever run. In the SELECT randomized trial of relatively healthy, selenium-replete US men, selenium (alone or with vitamin E) did not prevent prostate cancer. This is the landmark trial cautioning against indiscriminate selenium supplementation in already-replete populations.

Reported effect: Hazard ratio for prostate cancer 1.04 (99% CI 0.87-1.24; n=432) for selenium; 35,533 men randomized

“Hazard ratios (99% confidence intervals [CIs]) for prostate cancer were 1.13 (99% CI, 0.95-1.35; n = 473) for vitamin E, 1.04 (99% CI, 0.87-1.24; n = 432) for selenium ... Selenium or vitamin E, alone or in combination at the doses and formulations used, did not prevent prostate cancer in this population of relatively healthy men.”

Source: PMID 19066370 · Lippman 2009 · JAMA

Cardiovascular Mortality (Selenium + CoQ10, Low-Selenium Elderly)

RCT supported
  • 5.9% vs 12.6%CV mortality · active vs placebo
  • P=0.015443 participants · 5.2 yr

In the KiSel-10 randomized trial of elderly Swedes with low baseline selenium, combined selenium plus coenzyme Q10 for several years was associated with significantly lower cardiovascular mortality versus placebo. The effect is specific to a selenium-low population and uses a selenium+CoQ10 combination, not selenium alone.

Reported effect: Cardiovascular mortality 5.9% (active) vs 12.6% (placebo), P=0.015; 443 participants; 5.2-year follow-up

“During a follow up time of 5.2 years a significant reduction of cardiovascular mortality was found in the active treatment group vs. the placebo group (5.9% vs. 12.6%; P=0.015).”

Source: PMID 22626835 · Alehagen 2013 · Int J Cardiol

Type 2 Diabetes Risk Signal

Null / no benefit Meta-analysis supported
  • OR 2.03observational · CI 1.51-2.72
  • OR 1.18RCTs · CI 0.95-1.47 (ns)

A nuanced honest negative on safety. Observational studies link higher selenium to greater odds of type 2 diabetes, but randomized trials did not show a significantly higher diabetes risk in those given selenium versus placebo. The discordance is a key caution: epidemiological association is not confirmed by trial evidence, and supplementing replete individuals is not benign.

Reported effect: Observational summary OR 2.03 (95% CI 1.51-2.72); RCT OR 1.18 (95% CI 0.95-1.47, not significant)

“a summary odds ratio (OR) (95% confidence interval (CI)) of 2.03 (1.51-2.72) ... among randomized clinical trials (RCTs) of Se, a higher risk of T2D was not observed for those who received Se compared to a placebo (OR = 1.18, 95% CI 0.95-1.47).”

Source: PMID 30563119 · Kohler 2018 · Nutrients

Selenium Status & Neurodegeneration (Alzheimer's)

Meta-analysis supported
  • SMD -0.41AD vs controls · CI -0.64 to -0.17
  • p < 0.001AD only · others null

A meta-analysis of selenium status across neurodegenerative diseases found significantly lower selenium concentrations in Alzheimer's disease patients versus healthy controls, but no significant relationship for Parkinson's, multiple sclerosis, ALS or Huntington's. This is an association in observational data, not evidence that supplementation treats dementia.

Reported effect: Alzheimer's disease vs controls SMD -0.41 (95% CI -0.64 to -0.17, p < 0.001); no significant association for the other four diseases

“A statistically significant relationship was only found for AD patients (SMD = -0.41, 95% CI (-0.64, -0.17), p < 0.001), especially for erythrocytes. ... no significant relationship was observed in the analysis of the other four diseases.”

Source: PMID 37686737 · Zhou 2023 · Nutrients

Gestational Diabetes (Selenium Status)

Meta-analysis supported
  • SMD -1.29GDM vs control · CI -1.60 to -0.97
  • 27 studies1588 GDM · 2450 controls

A meta-analysis found serum selenium was significantly lower in women with gestational diabetes than in healthy pregnant women. This is an observed status difference, not evidence that selenium supplementation prevents gestational diabetes; the effect varied by region.

Reported effect: Selenium SMD -1.29 (95% CI -1.60 to -0.97, p < 0.00001) lower in GDM; 27 studies, 1588 GDM patients and 2450 controls

“Selenium level was significantly lower in women with GDM than those without GDM (SMD = -1.29; 95% CI: -1.60 to -0.97, p < 0.00001). ... Twenty-seven studies involving 1588 patients with GDM and 2450 healthy pregnant women contributed to this meta-analysis.”

Source: PMID 35080114 · Xu 2022 · Diabetes Metab Res Rev

Male Fertility / Semen Parameters (Antioxidant Combination)

Meta-analysis supported
  • MD 5.05motility · CI 2.77-7.34
  • 16.2% vs 11.2%pregnancy · P=.05 (ns)

A meta-analysis of carnitine, coenzyme Q10 and selenium in idiopathic male infertility found small improvements in sperm motility and concentration, but no significant difference in pregnancy rate. Selenium effects were not separable from the other antioxidants, and the authors emphasize the differences were small.

Reported effect: Sperm motility mean difference 5.05 (95% CI 2.77-7.34, P<.0001); pregnancy rate 16.2% (treatment) vs 11.2% (placebo), P=.05 (not significant)

“Pregnancy rate in the treatment group (69/426, 16.2%) was not different from the placebo (45/401, 11.2%) (P = .05) ... Treatment group showed higher motility [mean difference 5.05, 95% CI (2.77, 7.34), P =<.0001]”

Source: PMID 34871624 · Sharma 2022 · Urology

Multiple Sclerosis Supplementation

Null / no benefit Meta-analysis supported
  • no evidenceselenium alone

An honest negative on the evidence gap. This meta-analysis of selenium in multiple sclerosis found no evidence regarding supplementation with selenium alone and concluded there is a lack of evidence on the safety and efficacy of selenium supplements in MS patients, recommending caution and future trials.

Effect size: this study reports the direction of the finding but does not state a specific numeric effect size, so none is given here rather than estimated.

“there is no evidence regarding supplementation with selenium alone ... There is a lack of evidence regarding the safety and efficacy of selenium supplements in MS patients.”

Source: PMID 38155333 · Rahmani 2024 · Biol Trace Elem Res

Dosage (research context · not a recommendation)

US IOM RDA: 55 mcg/day adults (60 mcg/day pregnancy, 70 mcg/day lactation). EFSA NDA Panel AI: 70 mcg/day adults (EFSA Journal 2014;12(10):3846). EFSA NRV (labelling): 55 mcg. Common supplemental doses: 50-200 mcg/day; the NPC trial used 200 mcg/day Se-yeast. US IOM Tolerable Upper Intake Level (UL): 400 mcg/day adults. EFSA UL (SCF 2000, reaffirmed NDA 2023): 300 mcg/day adults. U-shaped dose-response: both deficiency and excess associated with adverse outcomes (Rayman 2012 Lancet review).

Regulatory Status · 4 Markets

US · FDA
United States (FDA): Selenium has NO authorized Significant Scientific Agreement (SSA) health claim under 21 CFR Part 101 Subpart E. The only FDA-permitted disease-risk claims for selenium are Qualified Health Claims (QHCs) under 21 CFR 101.93, allowed through FDA letters of enforcement discretion. The original 2003 FDA decision letters (Feb 21 and Apr 28, 2003, Wellness Lifestyles/Wellness International petition) granted QHC status to the statement: "Some scientific evidence suggests that consumption of selenium may reduce the risk of certain forms of cancer. However, FDA has determined that this evidence is limited and not conclusive." A subsequent 2009 FDA decision (FDA-2008-Q-0323) added site-specific QHCs for bladder, prostate, and thyroid cancers; following the May 27, 2010 federal court ruling (Alliance for Natural Health v. Sebelius) some disclaimers were shortened. Selenium is also permitted as a nutrient content claim ingredient under 21 CFR 101.54 ("good source" / "excellent source of selenium") when meeting %DV thresholds. All disease-related claims are restricted to dietary supplement labels and must carry the FDA-mandated qualifier. Structure/function claims under DSHEA (21 USC 343(r)(6)) are separately permitted with the standard disclaimer ("This statement has not been evaluated by the FDA. This product is not intended to diagnose, treat, cure, or prevent any disease.") and FDA notification within 30 days of marketing.
EU · EFSA
European Union (EFSA / Reg 432/2012): Six authorized Article 13(1) function health claims for selenium under Commission Regulation (EU) No 432/2012 (CELEX:32012R0432), Annex — immune function, thyroid function, maintenance of normal hair, maintenance of normal nails, normal spermatogenesis, and protection of cells from oxidative stress. All six share the standard condition: the claim may be used only for food which is at least a source of selenium per the SOURCE OF [VITAMIN/MINERAL] reference in the Annex to Regulation (EC) No 1924/2006 (>=15% NRV per 100 g/mL or per portion; NRV = 55 mcg per Regulation (EU) No 1169/2011 Annex XIII). EFSA NDA Panel underlying opinions: EFSA Journal 2009;7(9):1220 (immune, thyroid, hair, nails, oxidative stress) and EFSA Journal 2010;8(10):1727 (spermatogenesis). Tolerable Upper Intake Level (UL): 300 mcg/day for adults (SCF 2000, reaffirmed by EFSA NDA 2023). No Article 14 disease-risk-reduction claims and no Article 14 children development claims have been authorised for selenium.
CN · China
Approved in China as an essential mineral / health-food nutrient raw material, permitted as a nutrient fortifier under GB 14880-2012, with health-food registrations covering immunity and antioxidant functions. Nutrient-supplement filing supports 'selenium supplementation'; specific SAMR health-function claims require the registration pathway.
BR · ANVISA
Brasil (ANVISA): Selênio é reconhecido como nutriente essencial e está listado como constituinte autorizado de suplementos alimentares na Instrução Normativa IN 28/2018 (Anexos III/IV — limites mínimos e máximos por dose diária). As alegações funcionais autorizadas (Anexo V) seguem o padrão verbatim oficial "O selênio auxilia ..." e cobrem famílias consistentes com o escopo EFSA: função do sistema imune, função da tireoide, manutenção de cabelos e unhas, espermatogênese e proteção das células contra os radicais livres (antioxidante). RDC 243/2018 (composição/categorização de suplementos), RDC 459/2020 (regras de transição) e Decreto-Lei 986/1969 (rotulagem base) também aplicáveis.

Authorized Claims

ANVISA — “O selênio é um antioxidante que auxilia na proteção dos danos causados pelos radicais livres.” (ANVISA IN 28/2018 Anexo V)

Safety

Selenium exhibits a narrow therapeutic window. Chronic intakes >400 mcg/day risk selenosis: brittle/loss of hair and nails, garlic-like breath, gastrointestinal disturbances, peripheral neuropathy, and in severe cases hepatic and renal toxicity. The SELECT trial (Lippman 2009 JAMA) found NO benefit of 200 mcg/day selenomethionine for prostate cancer prevention in selenium-replete US men and signaled possible increased risk of type 2 diabetes in baseline-selenium-replete subgroups (Stranges 2007 Ann Intern Med). The NPC trial (Clark 1996 JAMA) primary skin-cancer endpoint was NEGATIVE; secondary total-cancer-mortality signal was hypothesis-generating only and was not confirmed in SELECT. Bioavailability and biological behavior differ markedly by form: Se-yeast and L-selenomethionine incorporate non-specifically into body proteins; sodium selenite/selenate enter the regulated selenoprotein pool more directly. Pregnancy/lactation: stay within RDA. Patients with autoimmune thyroid disease should consult a clinician before high-dose supplementation. Co-administration with high-dose vitamin C may reduce selenite absorption (use selenomethionine form or temporal separation).

Goals: longevity-stack · reproductive-health

Lifestyles: senior-60-plus

References

PubMed-indexed citations anchoring the benefit findings above. Effect sizes are reported as published.

  1. PMID 38243784 · Huwiler 2024 · Thyroid — Thyroid Autoimmunity (Hashimoto Thyroiditis)
  2. PMID 24683040 · Vinceti 2014 · Cochrane Database Syst Rev — Cancer Prevention
  3. PMID 19066370 · Lippman 2009 · JAMA — Prostate Cancer Prevention (SELECT Trial)
  4. PMID 22626835 · Alehagen 2013 · Int J Cardiol — Cardiovascular Mortality (Selenium + CoQ10, Low-Selenium Elderly)
  5. PMID 30563119 · Kohler 2018 · Nutrients — Type 2 Diabetes Risk Signal
  6. PMID 37686737 · Zhou 2023 · Nutrients — Selenium Status & Neurodegeneration (Alzheimer's)
  7. PMID 35080114 · Xu 2022 · Diabetes Metab Res Rev — Gestational Diabetes (Selenium Status)
  8. PMID 34871624 · Sharma 2022 · Urology — Male Fertility / Semen Parameters (Antioxidant Combination)
  9. PMID 38155333 · Rahmani 2024 · Biol Trace Elem Res — Multiple Sclerosis Supplementation

Frequently Asked Questions

1. What does selenium actually do in the body?

Selenium is an essential trace element incorporated as selenocysteine into roughly 25 selenoproteins. These include the glutathione peroxidase and thioredoxin reductase antioxidant enzymes and the iodothyronine deiodinases that convert thyroxine (T4) into active T3, which is why selenium is central to both antioxidant defense and thyroid hormone metabolism. EFSA authorizes function claims for its roles in immune function, thyroid function, hair, nails, spermatogenesis and protection of cells from oxidative stress.

2. Does selenium prevent cancer?

The strongest evidence says no. A Cochrane meta-analysis (Vinceti 2014, PMID 24683040) found a risk ratio of 0.90 (95% CI 0.70-1.17) for any cancer across two trials (N=4765) and concluded there is no convincing evidence selenium supplements prevent cancer in humans. The large SELECT trial (Lippman 2009, PMID 19066370) found selenium did not reduce prostate cancer (hazard ratio 1.04, 99% CI 0.87-1.24) in 35,533 relatively healthy, selenium-replete men. US FDA permits only a Qualified Health Claim carrying a mandatory 'limited and not conclusive' disclaimer.

3. Is there a downside to taking too much selenium?

Yes. Selenium has a narrow U-shaped dose-response window: both deficiency and excess carry risk, with tolerable upper intake levels set at 400 mcg/day (US IOM) and 300 mcg/day (EFSA). On the safety side, observational studies linked higher selenium to greater odds of type 2 diabetes (summary OR 2.03, 95% CI 1.51-2.72 in Kohler 2018, PMID 30563119), although randomized trials did not confirm a significant increase (OR 1.18, 95% CI 0.95-1.47). The Cochrane cancer review also flagged a possible diabetes risk signal.

4. What is the evidence for selenium and thyroid autoimmunity?

In Hashimoto thyroiditis, a meta-analysis of randomized trials (Huwiler 2024, PMID 38243784) found selenium lowered thyroid peroxidase antibodies (SMD -0.96, CI -1.36 to -0.56; 2358 participants) and modestly reduced TSH in patients not on thyroid hormone replacement (SMD -0.21, CI -0.43 to -0.02), with adverse effects comparable to control. These are marker changes in studied patients, not evidence that selenium cures thyroid disease.

5. Does the cardiovascular finding mean everyone should take selenium for heart health?

No. The KiSel-10 trial (Alehagen 2013, PMID 22626835) showed lower cardiovascular mortality (5.9% vs 12.6%, P=0.015) in elderly Swedes who were low in selenium at baseline, and it used selenium combined with coenzyme Q10, not selenium alone. The benefit is specific to a selenium-deficient population; the SELECT and Cochrane data argue against indiscriminate supplementation in people who are already selenium-replete.

6. What about selenium for fertility, brain health, or pregnancy?

These are mostly associations or combination findings, not proof of treatment effect. A meta-analysis (Sharma 2022, PMID 34871624) found small improvements in sperm motility (mean difference 5.05) from a carnitine/CoQ10/selenium combination but no difference in pregnancy rate, and selenium could not be isolated. Lower selenium was associated with Alzheimer's disease (SMD -0.41) but not other neurodegenerative diseases (Zhou 2023, PMID 37686737), and with gestational diabetes (SMD -1.29; Xu 2022, PMID 35080114). For multiple sclerosis, a meta-analysis (Rahmani 2024, PMID 38155333) found no evidence on selenium alone.

Last evidence review: 2026-06-05

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