Selenium

Also sold as: Se · L-selenomethionine · Sodium selenite

An essential trace mineral used in selenoproteins and thyroid hormone metabolism. Correcting deficiency differs from adding selenium when status is already adequate, where benefits disappear and excess can harm.

Longevity

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NERD_PROTOCOL // EVIDENCE_MATRIXper goal

Research

LongevityModerate · no benefit

Longevity: RCT meta-analyses do not establish lower all-cause mortality or cardiovascular risk in generally selenium-replete adults.

REALITY CHECK // Lower thyroid antibody numbers do not prove that Hashimoto is cured or that levothyroxine can be stopped. Cancer, cardiovascular, mortality and dementia prevention are not established in selenium-replete adults; the safety window is narrow and diabetes-risk signals make more an unsafe default.

Levels: how we grade evidence. Grey = evidence of no benefit; amber = evidence it makes this goal worse.

Ratings are self-reported by a self-selected group of users: how it felt, not proof that it works.

CHECK_YOUR_BOTTLE.EXEruns in your browser · nothing is sent or stored

Check your bottle

Compare what your label says with what was used in trials. This is a trial range, not a personal prescription.

Enter the amount from your label to see the result.

How it was studied

01 // STUDIED_DOSE_RANGE

Micrograms, not milligrams

50 micrograms = 0.05 mg; 200 micrograms = 0.2 mg. Examples explain labels, not trial targets.

02 // TIMING_&_MATRIX

Total intake and status

Count diet and all supplements; do not equate selenium yeast mass with elemental selenium.

03 // EVALUATION_WINDOW

3-6 months: markers

Antibody meta-analyses evaluate months; biomarker changes are not established symptom improvement.

04 // SAFETY_&_QUALITY_GATE

Selenosis gate

US dossier UL 0.4 mg/day total; revised EFSA number needs verification and is not filled speculatively.

UPPER LIMIT // The dossier reports US adult UL 400 micrograms/day (0.4 mg/day) from all sources. It flags the revised EFSA 2023 adult value near 255 micrograms/day for live confirmation; the obsolete 300 microgram figure must not be treated as current. EFSA numeric field remains null pending verification. Diet plus all supplements count; UL is not a target.

NERD'S NOTENerd Protocols · a clinical and sports dietitian based in the EU

Summary

Selenium reduces thyroid antibody titers in Hashimoto thyroiditis, but symptom relief, disease progression and reduced levothyroxine need remain unproven. Mild Graves orbitopathy has a specific benefit signal; adequate-status populations should not infer cancer, cardiovascular or dementia prevention.

Mechanism

Selenoproteins include glutathione peroxidases and deiodinases. Selenomethionine can enter body proteins nonspecifically and create tissue stores. These essential biochemical roles do not mean extra selenium improves outcomes above adequate status.

Typical studied dose

The supplied extraction does not give a reliable preparation-specific thyroid trial dose range; it discusses 200 micrograms/day in reader dosing questions. Calculator examples 0.05-0.2 mg (50-200 micrograms) are label-reading examples, not prescribed targets. Trial thresholds are unavailable rather than fabricated.

Upper limit

The dossier reports US adult UL 400 micrograms/day (0.4 mg/day) from all sources. It flags the revised EFSA 2023 adult value near 255 micrograms/day for live confirmation; the obsolete 300 microgram figure must not be treated as current. EFSA numeric field remains null pending verification. Diet plus all supplements count; UL is not a target.

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Popular stacks with Selenium

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Forms and label traps

Forms compared
Form on label	Active content	Studied range / day	Absorption & GI tolerance	Verdict
Pure L-selenomethionine, approximately 40% selenium Best for: Separating pure-compound mass from elemental label amount.	40% elemental / active	no comparable trial range	Organic form can be incorporated into body proteins; retention is not proof of superior clinical outcomes. Usually tolerated; symptoms of excess require assessment.	Caution CHECK PREPARATION Use elemental mode for an already-declared selenium dose. Approximate composition applies to pure SeMet, not capsule fill. Human trial range not supplied. PMID 22381456
Anhydrous sodium selenite, approximately 45% selenium Best for: Recognizing Na2SeO3 rather than sodium selenate.	45% elemental / active	no comparable trial range	Inorganic selenium has different metabolism; human outcome superiority is not established. Variable tolerance; no guaranteed side-effect advantage.	Caution CHECK PREPARATION Approximate composition for the specified compound only; source mistranslates the salt name. Do not apply to hydrates or fillers. PMID 21181292 PMID 31707637
Selenium-enriched yeast, assay required Best for: Reading the declared elemental selenium assay.	Unstandardized: active content unknown	no comparable trial range	Mixed organic selenium species; relative rat PK is not human efficacy. Consider yeast allergy or intolerance.	Caution CHECK PREPARATION No fixed percent or yeast-mass conversion. PMID 31707637

Common label traps

TRAP_01 // Selenium cures Hashimoto
Antibody reductions do not establish clinical cure, symptom benefit or a reason to stop thyroid medicine.
TRAP_02 // More protects against cancer
Cochrane finds no established prevention benefit in replete populations; extra exposure may harm.
TRAP_03 // Brazil nuts are an exact safe dose
Selenium content varies widely; count total intake rather than assuming a fixed amount per nut.

Other outcomes studied

Outside the five leaderboard goals. Same evidence levels as above.

hashimoto_antibodies Moderate · benefit
Meta-analysis: TPOAb SMD -0.46 (-0.74 to -0.18) at 3 months and -0.80 (-1.38 to -0.21) at 6 months; TSH SMD -0.21 (-0.43 to -0.02) without levothyroxine. These are laboratory markers.
PMID 40898469 PMID 38243784 PMID 27702392
hashimoto_clinical_outcomes No established evidence
Clinical efficacy, quality of life, progression and reduced medication need are insufficiently documented despite antibody effects. This is uncertainty, not proof that every trial was negative.
PMID 27683225 PMID 23744563
mild_graves_orbitopathy Moderate · benefit
Meta-analyses agree on Clinical Activity Score improvement: MD -1.27 (-1.68 to -0.85) and -1.05 (-1.61 to -0.48). QoL differs: RR 2.54 (1.69-3.81) versus nonsignificant 1.72 (0.43-6.92). Applies to mild disease, especially borderline selenium status.
PMID 39138905 PMID 42355878
keshan_deficiency Strong · benefit
Deficiency correction prevents endemic cardiomyopathy in severely deficient Chinese populations; not an extra-dose effect in replete adults.
PMID 29627894
cancer_prevention Moderate · no benefit
Cancer-prevention efficacy is unsupported in selenium-replete adults. SELECT must not be read as assigning its vitamin-E prostate-cancer harm specifically to selenium.
PMID 29376219 PMID 21990298
male_fertility Limited · benefit
Mixed-formula studies show some semen-parameter signals; selenium-specific effects and pregnancy/live-birth benefit are uncertain.
PMID 31160241
pregnancy_thyroid_autoimmunity Limited · benefit
Limited pregnancy RCT evidence in thyroid-antibody-positive women is not a basis for a self-treatment protocol.
PMID 31129812
viral_immunity No established evidence
Status associations and mechanisms do not establish that selenium supplementation prevents or treats infection.
PMID 34739678 PMID 32334392
diabetes_risk Limited · makes it worse
Adequate/high-status supplementation has a type 2 diabetes harm signal; the relationship is context-dependent, not a proven universal outcome.
PMID 22381456 PMID 23358371
dementia_prevention Moderate · no benefit
PREADViSE did not establish dementia prevention with selenium with or without vitamin E in older men.
PMID 28319243

Reader questions

Is 200 micrograms automatically safe?

No. That is 0.2 mg before food and other supplements. Baseline status, cumulative exposure and duration matter; the UL is a ceiling, not an ideal dose.

Which form is best?

SeMet, selenite and enriched yeast differ chemically; human head-to-head outcome evidence is sparse. Rat absorption percentages do not prove a superior thyroid outcome.

Will it fix fatigue despite levothyroxine?

Lower antibodies do not establish better energy or allow stopping medication. Persistent fatigue needs individual assessment.

What are overdose signals?

Hair/nail changes, garlic breath, metallic taste, GI or neurologic symptoms warrant stopping supplemental exposure and assessment.

Stack interactions

Caution Iodine / thyroid treatment [CAUTION // CONTEXT]
Iodine deficiency changes the safety context of selenium repletion. Review both status and thyroid treatment; no proven levothyroxine absorption interaction.
PMID 28315909 PMID 22381456
Caution Diabetes treatment [CAUTION // CONTEXT]
Excess selenium in adequate/high-status users raises diabetes-risk concerns; this is not a claim that it improves glucose control.
PMID 22381456 PMID 23358371
Caution High-dose vitamin C with sodium selenite [CAUTION // CONTEXT]
The dossier describes chemical reduction and possible lower availability. Clinical magnitude is not established; do not generalize to SeMet.

Red flags: interactions and who should avoid it

Interactions

Iodine and selenium status interact in thyroid physiology; correcting selenium alone in iodine deficiency may worsen thyroid dysfunction. No direct levothyroxine absorption interaction is established here. Adequate/high-status selenium may worsen diabetes risk; glucose monitoring matters with diabetes treatment. High-dose vitamin C may reduce inorganic selenite availability; clinical magnitude is uncertain.

Avoid or ask a doctor first if

Avoid routine excess if selenium status is adequate/high, especially in pregnancy or breastfeeding without an individual indication. Stop and seek assessment for hair/nail loss or brittleness, garlic breath, metallic taste, GI or neurologic symptoms. Accidental large overdoses can be serious. Brazil-nut content varies greatly and cannot be treated as an exact dose.

Sources (PubMed, 22)

Clinical efficacy of selenium supplementation in patients with Hashimoto thyroiditis: A systematic review and meta-analysis.
PMID 40898469 · 2025 · Systematic review and meta-analysis
Time-specific thyroid antibody effects in Hashimoto.
Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials.
PMID 38243784 · 2024 · Systematic review and meta-analysis of randomized trials
Thyroid marker changes, including TSH without levothyroxine.
Selenium Supplementation Significantly Reduces Thyroid Autoantibody Levels in Patients with Chronic Autoimmune Thyroiditis: A Systematic Review and Meta-Analysis.
PMID 27702392 · 2016 · Systematic review and meta-analysis
Antibody reductions, not proof of clinical cure.
Insufficient documentation for clinical efficacy of selenium supplementation in chronic autoimmune thyroiditis, based on a systematic review and meta-analysis.
PMID 27683225 · 2017 · Systematic review and meta-analysis
Insufficient documentation of clinical efficacy despite marker changes.
Selenium supplementation for Hashimoto's thyroiditis.
PMID 23744563 · 2013 · Cochrane systematic review
Insufficient clinical outcome evidence in Hashimoto.
The efficacy and safety of selenium supplementation versus placebo in the treatment of Graves' orbitopathy: A systematic review and meta-analysis of randomised controlled trials.
PMID 39138905 · 2024 · Systematic review and meta-analysis of RCTs
Mild Graves orbitopathy activity and quality-of-life results.
Efficacy of Selenium Supplementation in Graves' Orbitopathy: A Systematic Review and Meta-Analysis of Randomized Controlled Trials with Trial Sequential Analysis.
PMID 42355878 · 2026 · Systematic review and meta-analysis with trial sequential analysis
Orbitopathy activity benefit but imprecise quality-of-life result.
Prevention of Keshan Disease by Selenium Supplementation: a Systematic Review and Meta-analysis.
PMID 29627894 · 2018 · Systematic review and meta-analysis
Keshan disease prevention in deficient endemic populations.
Selenium for preventing cancer.
PMID 29376219 · 2018 · Cochrane systematic review
Lack of established cancer-prevention efficacy.
Vitamin E and the risk of prostate cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT).
PMID 21990298 · 2011 · Randomized trial follow-up
SELECT results; paper centers on vitamin E risk and cannot establish selenium-specific harm by itself.
Selenium, antioxidants, cardiovascular disease, and all-cause mortality: a systematic review and meta-analysis of randomized controlled trials.
PMID 33053149 · 2020 · Systematic review and meta-analysis of RCTs
No established selenium-only cardiovascular or mortality benefit.
Selenium supplementation for the primary prevention of cardiovascular disease.
PMID 23440843 · 2013 · Cochrane systematic review
Primary cardiovascular prevention not established.
Influence of oral vitamin and mineral supplementation on male infertility: a meta-analysis and systematic review.
PMID 31160241 · 2019 · Systematic review and meta-analysis
Male-infertility supplement evidence with mixed preparations.
Selenium supplementation in the management of thyroid autoimmunity during pregnancy: results of the \"SERENA study\", a randomized, double-blind, placebo-controlled trial.
PMID 31129812 · 2019 · Randomized double-blind placebo-controlled trial
Pregnancy thyroid-autoimmunity study in a selected population.
COVID-19 and Selenium Deficiency: a Systematic Review.
PMID 34739678 · 2022 · Systematic review
Selenium-status associations in COVID-19, not established intervention efficacy.
Enhancing immunity in viral infections, with special emphasis on COVID-19: A review.
PMID 32334392 · 2020 · Review
Mechanistic immune context, not clinical antiviral proof.
Selenium and human health.
PMID 22381456 · 2012 · Narrative review
Essential biology, tissue retention, toxicity and status-dependent risk.
[Selenium supplementation trials for cancer prevention and the subsequent risk of type 2 diabetes mellitus: selenium and vitamin E cancer prevention trial and after].
PMID 23358371 · 2013 · Narrative review of trial analyses
Diabetes-risk concerns after cancer-prevention trials.
Association of Antioxidant Supplement Use and Dementia in the Prevention of Alzheimer's Disease by Vitamin E and Selenium Trial (PREADViSE).
PMID 28319243 · 2017 · Randomized trial-associated follow-up
No established dementia-prevention benefit in PREADViSE.
Chemical form of selenium affects its uptake, transport, and glutathione peroxidase activity in the human intestinal Caco-2 cell model.
PMID 21181292 · 2011 · In vitro intestinal-cell study
Chemical-form transport differences; not human outcome superiority.
Comparison of Bioavailability, Pharmacokinetics, and Biotransformation of Selenium-Enriched Yeast and Sodium Selenite in Rats Using Plasma Selenium and Selenomethionine.
PMID 31707637 · 2020 · Animal pharmacokinetic study
Rat yeast-versus-selenite comparison, not a conversion fraction or human efficacy proof.
A concise review of Hashimoto thyroiditis (HT) and the importance of iodine, selenium, vitamin D and gluten on the autoimmunity and dietary management of HT patients.Points that need more investigation.
PMID 28315909 · 2017 · Narrative review
Iodine/selenium and Hashimoto management uncertainties.

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