Vitamins & Supplements · Mineral
Selenium
Selenium (Se) · also called Selenio, selenomethionine, sodium selenite, selenium yeast
Selenium is an essential trace mineral your body needs in small amounts for antioxidant enzymes and thyroid hormones. Health agencies say most people get enough from food. Supplements have not been shown to prevent cancer or heart disease, and in trials they slightly raised diabetes risk. The most important caution is that too much is harmful, and Brazil nuts can carry a lot.
General information, not medical advice. Last substantive update · Not medically reviewed. A named clinician has not signed off on this page. How this page is made
About Selenium
Selenium is an essential mineral found in Brazil nuts, fish, meat and eggs. Most people get enough from food. The studies below look at supplements, which gave benefits mainly in specific thyroid conditions and raised some risks in trials.
What it is
Selenium is a trace mineral. Your body builds it into antioxidant enzymes and uses it to handle thyroid hormones. The body needs only small amounts.
In food it is mostly selenomethionine. The amount in plant foods depends on the soil where they grew, so the same food can differ a lot between regions.
Pills may use selenomethionine, sodium selenite, sodium selenate or selenium-enriched yeast. Food sources and supplements are not the same thing.
Food sources
The NHS names Brazil nuts, fish, meat and eggs. MedlinePlus adds shellfish, red meat, grains, chicken, liver and garlic, and says plant levels depend on soil.
EFSA lists Brazil nuts as richest and says the amount varies hugely: roughly 100 to 2,000 micrograms in 5 or 6 nuts. It lists organ meats and tuna (1 to 3 micrograms per gram) and shrimp, crab and prawn (1 to 2).
Amounts per typical portion from USDA or the NIH Office of Dietary Supplements could not be read for this page.
What the research shows
Hashimoto thyroiditis: a 2024 review found lower TSH and thyroid antibodies, with moderate certainty, but no proof of symptom relief.
Cancer, heart disease and diabetes: large trials found no prevention. Some found small increases in diabetes and, in men with higher selenium, in high-grade prostate cancer.
Observational studies link higher blood selenium to fewer deaths. They cannot show that taking selenium helps.
At a glance
Traditional uses history, not proof
- Essential nutrient from food: Brazil nuts, fish, meat and eggs
- Added to multivitamins and mineral supplements
Studied for research
- Hashimoto thyroiditis lab markers, moderate certainty
- Graves eye disease, mixed
- Cancer prevention, against (high-certainty trials found no benefit)
- Heart disease prevention, against
- Type 2 diabetes, trials show a small increase in risk
- Death rates, very low certainty (observational)
- Sepsis in hospital, low
Main compounds
- Selenomethionine
- Sodium selenite
- Sodium selenate
- Selenium-enriched yeast
Common forms
- Food sources: Brazil nuts, fish, meat, eggs
- Selenomethionine tablets (SELECT)
- Selenium-enriched yeast (Danish trial)
- Intravenous selenium in hospital (sepsis trials)
Research links: 2018 Cochrane review: selenium for preventing cancer · 2024 review: selenium in Hashimoto thyroiditis · 2023 EFSA opinion: upper intake level for selenium · Research Library · Genes & Biomarkers Library
What it is and its history
- Accepted scientific name
- Selenium (chemical element Se, atomic number 34), an essential trace mineral
- Common names
- Selenium (English); Selenio (Spanish)
- Family or group
- Trace minerals
- Harvest and processing
- Plant selenium depends on soil. EFSA says almost 90% of selenium in plants is selenomethionine and that soil selenium is a major determinant of how much plant foods contain.
- Look-alikes and adulterants
- Name: Iodine; Note: Another trace mineral needed for the thyroid.; Name: Zinc; Note: Another essential trace mineral.
Traditional use and history history, not clinical proof
| Tradition or era | Documented use | Source |
|---|---|---|
Traditional use shows how it was used. It does not show that it works or is safe today.
What is inside, and how it may work
What it contains and how those compounds act in cells and animals. Laboratory and animal mechanisms are not proven effects in people.
Main compounds
| Compound | Family | Where it is concentrated | Activity studied | Level of evidence |
|---|---|---|---|---|
| Selenomethionine | organic selenium amino acid | Main form in plant foods and the form used in the SELECT trial | The usual dietary form; EFSA says almost 90% of selenium in plants is selenomethionine. 2 7 | Human data |
| Sodium selenite and sodium selenate | inorganic selenium salts | Inorganic supplement forms named in the existing nutrient summary; not tested here against other forms. 2 | Human data | |
| Selenium-enriched yeast | yeast grown on selenium | Used in the Danish PRECISE trial; content varies by product. 11 | Human data |
Mechanisms, receptors and pathways
| Target or pathway | Gene or receptor | What was found | Level |
|---|---|---|---|
| Selenoproteins (antioxidant enzymes) | Not specified in the sources read | Selenium is built into antioxidant enzymes that help prevent cell damage (MedlinePlus, NHS). 14 15 | Human data |
| Thyroid hormone handling | Not specified in the sources read | Selenium is needed for thyroid hormone production. Trials in Hashimoto thyroiditis show lower TSH and TPO antibodies. 3 | Human data |
Absorption, metabolism and elimination
- Absorption
- Not found in the sources read. MedlinePlus says severe gut disorders such as Crohn disease may impair absorption.
- Distribution
- EFSA says following absorption selenium compounds are mostly transported to the liver.
- Metabolism
- EFSA says the liver is the principal site of selenium metabolism.
- Elimination
- Not found.
- Bioavailability
- Not established across forms in the sources read.
Isolated compounds versus whole extracts
Foods give selenium mostly as selenomethionine along with other nutrients. Supplements deliver single forms. The trial results on this page are for supplements; food intake is judged separately by agencies. EFSA says selenium from the natural content of foods does not raise concern.
Nutritional composition
Food sources: the NHS names brazil nuts, fish, meat and eggs; MedlinePlus names brazil nuts, fish, shellfish, red meat, grains, eggs, chicken, liver and garlic, and says plant selenium depends on the soil. EFSA lists kidney (1 to 3 micrograms per gram), tuna (1 to 3) and shrimp, crab and prawn (1 to 2) as rich, and Brazil nuts as richest with a very wide range (5 to 72 micrograms per gram dry weight, up to 400 micrograms in one nut). EFSA says main contributors in Europe are milk and dairy, meat, grains and fish. Amounts per typical portion from USDA or the NIH sheet were not available (USDA API rate-limited, NIH sheet blocked), so per-portion values are not given.
2 moderate certainty1 mixed findings1 low certainty1 very low certainty1 not enough evidence3 evidence against
What was looked at
The 9 benefits and uses checked for Selenium, each with how strong the evidence is. Tap one to open it.
- Meeting the body's need (preventing deficiency)Moderate certainty
- Hashimoto thyroiditis (lab markers)Moderate certainty
- Graves' eye disease (orbitopathy)Mixed findings
- Preventing cancerEvidence against
- Preventing heart diseaseEvidence against
- Type 2 diabetes and blood sugarEvidence against
- Living longer (selenium status and deaths)Very low certainty
- Sepsis in hospital (intravenous selenium)Low certainty
- Immune function, fertility and other claimsNot enough evidence
Lab markers tracked in the studies
Thyroid peroxidase antibodies (TPOAb)TSH (thyroid-stimulating hormone)Clinical Activity Score (Graves' orbitopathy)HbA1c (average blood sugar over about 3 months)Blood selenium (status)
What is Selenium good for?
The main studied uses and how strong the human evidence is for each. Most positive findings apply to people with low intake or specific thyroid conditions.
Benefits, one by one
Each card is a research question, not a promise. Tap a goal to filter. Open a card to see the studies, doses and limits.
Meeting the body's need (preventing deficiency)
Moderate certaintySelenium is an essential mineral. The body uses it to make antioxidant enzymes and thyroid hormones. Health agencies describe clear problems when intake is very low, such as Keshan disease (a heart muscle condition) in parts of China, and say deficiency is rare in the US and UK. For people who eat enough, no source we read shows that extra selenium adds benefit.
2 studiesFood sources (selenomethionine mostly) and supplements are different …
Studies, doses and limits
What the trials found
We read two health-agency pages (NHS, MedlinePlus). We did not re-read the original Chinese supplementation reports behind the Keshan disease statement.
Results
MedlinePlus says Keshan disease caused many childhood deaths in China until the link to selenium was found and supplements were given. It also links deficiency to Kashin-Beck disease (joint and bone disease). It says deficiency can happen with long intravenous feeding or with severe gut disease such as Crohn disease. The NHS says people who eat meat, fish or nuts should get enough from food.
Why this rating
Moderate. Agency statements are consistent, but we read summaries, not the original deficiency studies.
Form and dose studied
Food sources (selenomethionine mostly) and supplements are different routes; the agencies say a varied diet usually covers needs.
Who was studied
People with very low intake: long-term intravenous feeding, severe gut disease, or living where soils are very low in selenium.
Limits
No trial of deficiency correction was read for this page. Supplements help where intake is low; the sources do not show extra benefit above adequate intake.
Why researchers looked
Selenium is built into selenoproteins, which the body needs for thyroid hormone handling and antioxidant defence.
Proposed mechanism Authoritative guidance
Selenium is part of antioxidant enzymes (MedlinePlus). The exact steps were not tested in the pages read.
A proposed mechanism does not show that the effect happens in people.
Too much is also harmful (see Safety).
Hashimoto thyroiditis (lab markers)
Moderate certaintyIn people with Hashimoto thyroiditis, an autoimmune thyroid condition, a 2024 review of trials found selenium supplements lowered TSH a little and lowered thyroid antibody levels. These are lab markers. The review does not say symptoms improved. Results for antibodies varied a lot between studies.
1 studiesThe forms and amounts varied between the pooled studies. See the stud…
Studies, doses and limits
What the trials found
A 2024 systematic review and meta-analysis of randomised trials (35 studies; the TPO antibody result pools 29 cohorts, 2,358 people), certainty rated moderate by the reviewers.
Results
TSH fell in people not on thyroid hormone replacement (SMD -0.21, 95% CI -0.43 to -0.02). TPO antibodies fell (SMD -0.96, -1.36 to -0.56; heterogeneity 90%). Thyroid hormones (fT4, fT3), TGAb and thyroid volume did not change significantly. Adverse effects were similar to control.
Effect sizes as reported
TPOAb SMD -0.96; TSH SMD -0.21
Why this rating
Moderate by the reviewers' own GRADE, but it is for lab markers and antibody results differed a lot between studies.
Form and dose studied
The forms and amounts varied between the pooled studies. See the studies table.
Who was studied
Adults with Hashimoto thyroiditis, some on thyroid hormone replacement and some not.
Limits
Markers, not symptoms or long-term outcomes. Large differences between studies.
Why researchers looked
The thyroid holds more selenium than most organs, and selenium-containing enzymes handle thyroid hormones.
Proposed mechanism Hypothesis
Not tested in the review read. The usual idea is that selenium enzymes reduce oxidative stress in the thyroid.
A proposed mechanism does not show that the effect happens in people.
Adverse effects were similar to control in the review. Ask a doctor before adding selenium if you have a thyroid condition.
Graves' eye disease (orbitopathy)
Mixed findingsOne 2011 trial in 159 people with mild Graves' orbitopathy (eye disease linked to an overactive thyroid) found selenium improved quality of life and slowed progression compared with placebo. A 2026 review of five trials found a small improvement in one score but no clear change in quality of life or vision. The evidence is mixed.
2 studiesSelenium compared with placebo (and with pentoxifylline, a different …
Studies, doses and limits
What the trials found
One randomised, double-blind, placebo-controlled trial (n=159) and a 2026 meta-analysis of five trials (n=303).
Results
In the trial, selenium improved quality of life (P<0.001), reduced eye involvement (P=0.01) and slowed progression (P=0.01) at 6 months, with the same result at 12 months. In the 2026 meta-analysis, Clinical Activity Score fell by 1.05 (95% CI -1.61 to -0.48) but eyelid opening, quality of life (RR 1.72, 0.43 to 6.92) and visual function did not differ significantly.
Effect sizes as reported
Clinical Activity Score MD -1.05 (meta-analysis)
Why this rating
Mixed: a positive single trial, a cautious later review with few, different trials.
Form and dose studied
Selenium compared with placebo (and with pentoxifylline, a different drug) in the trial. See the studies table.
Who was studied
Adults with mild Graves' orbitopathy; not tested in severe disease.
Limits
Few trials. Trials in groups with different baseline selenium gave different results (the review calls for studies split by selenium status).
Why researchers looked
Oxygen free radicals are thought to play a part in this eye disease, so an antioxidant mineral was tested.
Proposed mechanism Hypothesis
The trial authors say free radicals and cytokines play a role. The trial itself did not test the mechanism.
A proposed mechanism does not show that the effect happens in people.
No adverse events with selenium in the 2011 trial. This is a medical condition: treatment decisions belong with a specialist.
Preventing cancer
Evidence againstLarge trials give a clear answer: selenium supplements did not lower the chance of getting cancer in healthy adults. A Cochrane review rated this high certainty. People with higher selenium in the blood had less cancer in observational studies, but that kind of study cannot show cause. In the biggest trial, men who already had higher selenium levels had more high-grade prostate cancer on selenium.
3 studiesSELECT used one form (selenomethionine); yeast and other forms were t…
Studies, doses and limits
What the trials found
A 2018 Cochrane review (10 randomised trials, 27,232 people; 70 cohort studies), the SELECT trial (35,533 men) and a later analysis of SELECT by baseline selenium status.
Results
Any cancer, low-risk-of-bias trials: RR 1.01 (95% CI 0.93 to 1.10). Prostate cancer RR 1.01 (0.90 to 1.14). SELECT: prostate cancer HR 1.04 (99% CI 0.87 to 1.24) with selenium alone. Later analysis: no effect with lower baseline selenium, and +91% high-grade prostate cancer with higher baseline selenium (P = .007).
Effect sizes as reported
Any cancer RR 1.01; prostate cancer HR 1.04
Why this rating
High-certainty trial evidence of no benefit (Cochrane GRADE); the observational link is very low certainty.
Form and dose studied
SELECT used one form (selenomethionine); yeast and other forms were tested in other trials.
Who was studied
Mostly men (88%), mostly relatively healthy and in countries with adequate selenium.
Limits
Does not rule out a benefit in groups with a specific gene type or very low selenium; the review says more research is needed.
Why researchers looked
Earlier studies suggested lower cancer in people with higher selenium, so large trials were run.
Proposed mechanism Hypothesis
Not tested in the studies read.
A proposed mechanism does not show that the effect happens in people.
Raised high-grade prostate cancer in men with higher selenium status; possible raised melanoma and type 2 diabetes signals in trials.
Preventing heart disease
Evidence againstA Cochrane review of 12 trials found selenium-only supplements did not lower deaths, heart disease deaths or heart disease events. Non-HDL cholesterol was lower in one trial. The authors say the evidence does not support selenium supplements to prevent heart disease.
1 studiesSelenium-only supplements versus placebo; see the studies table.
Studies, doses and limits
What the trials found
Cochrane review (2013): 12 trials, 19,715 people, mostly low risk of bias.
Results
All-cause death RR 0.97 (95% CI 0.88 to 1.08); heart disease death RR 0.97 (0.79 to 1.2); all heart disease events RR 1.03 (0.95 to 1.11). Total cholesterol fell but not significantly. Hair loss (RR 1.28) and mild dermatitis (RR 1.17) were higher with selenium in SELECT.
Effect sizes as reported
Heart disease events RR 1.03
Why this rating
No effect in the main trials; the review rates the evidence as limited.
Form and dose studied
Selenium-only supplements versus placebo; see the studies table.
Who was studied
General population and people at higher heart risk, mostly in the US.
Limits
Two large US trials dominate the result; none measured blood pressure over the longer term.
Why researchers looked
Selenium enzymes protect against oxidative stress, and observational studies had linked low selenium to heart disease.
Proposed mechanism Hypothesis
Not tested in the review read.
A proposed mechanism does not show that the effect happens in people.
Hair loss and mild dermatitis were more common on selenium.
Type 2 diabetes and blood sugar
Evidence againstSelenium has not been shown to prevent diabetes, and trials point the other way. In one long trial, more people on selenium developed type 2 diabetes. A 2018 review of trials found an 11% higher risk, borderline. A trial in older Danes found no lasting change in HbA1c.
3 studiesSelenium and selenium-enriched yeast; see the studies table.
Studies, doses and limits
What the trials found
Two randomised trials (n=1,202 and n=491) and a 2018 review of 5 trials and 50 observational studies.
Results
Stranges 2007: hazard ratio 1.55 (95% CI 1.03 to 2.33), highest in people with the highest starting blood selenium. 2018 review of trials: RR 1.11 (1.01 to 1.22). Danish trial: HbA1c did not differ from placebo after 2 years.
Effect sizes as reported
Type 2 diabetes RR 1.11 (trials)
Why this rating
Several trials point to a small increase in risk, not a benefit. Diabetes was a secondary outcome in the first trial.
Form and dose studied
Selenium and selenium-enriched yeast; see the studies table.
Who was studied
People in low-selenium areas of the US; older adults in Denmark.
Limits
Borderline trial effect; observational data can be confounded; the Danish result fits a possible U-shaped response.
Why researchers looked
Animal studies had suggested selenium might help blood sugar handling.
Proposed mechanism Hypothesis
Not tested in the studies read.
A proposed mechanism does not show that the effect happens in people.
This is a safety signal, not a benefit: see Safety.
Living longer (selenium status and deaths)
Very low certaintyIn 20 cohort studies, people with higher blood selenium were less likely to die during follow-up. This is an observational link: it cannot show that taking selenium helps, and it does not test supplements.
1 studiesBlood selenium reflects food and supplement intake and health.
Studies, doses and limits
What the trials found
A 2025 systematic review and meta-analysis of 20 cohort studies.
Results
Per standard deviation higher selenium marker: all-cause death RR 0.87 (95% CI 0.83 to 0.90), heart disease death RR 0.89 (0.84 to 0.94), cancer death RR 0.85 (0.78 to 0.94).
Effect sizes as reported
All-cause death RR 0.87 per SD
Why this rating
Observational only; trials in deficient groups are still needed (the authors say so).
Form and dose studied
Blood selenium reflects food and supplement intake and health.
Who was studied
General populations, countries with low or adequate selenium supply.
Limits
Confounding and reverse causation are possible.
Why researchers looked
Cohort studies compare outcomes in people with different selenium levels.
Proposed mechanism Hypothesis
Not tested.
A proposed mechanism does not show that the effect happens in people.
Randomised trials of supplements did not lower deaths (see heart disease above).
Sepsis in hospital (intravenous selenium)
Low certaintyIn people very ill with sepsis, intravenous selenium did not clearly lower deaths in a review of 13 trials. Some secondary outcomes such as ICU stay looked better. The authors did not suggest it for routine use. This is a hospital treatment, not a pill for healthy people.
1 studiesIntravenous selenium given by hospital teams.
Studies, doses and limits
What the trials found
A 2019 meta-analysis of 13 randomised trials.
Results
Death at day 28 RR 0.94 (95% CI 0.82 to 1.06); at 90 days RR 0.73 (0.36 to 1.47). Fewer ventilator-associated pneumonias (RR 0.61, 0.42 to 0.89).
Effect sizes as reported
Day-28 death RR 0.94
Why this rating
The main outcome showed no clear effect; some secondary outcomes were favourable but not the key ones.
Form and dose studied
Intravenous selenium given by hospital teams.
Who was studied
People with sepsis in intensive care.
Limits
Different amounts and timing across trials; hospital use only.
Why researchers looked
Blood selenium falls in critical illness.
Proposed mechanism Hypothesis
Not tested in the review read.
A proposed mechanism does not show that the effect happens in people.
Medical decision for the intensive care team.
Immune function, fertility and other claims
Not enough evidenceThe NHS and MedlinePlus say selenium helps the immune system and reproduction. We read no trial showing that extra selenium improves these in people with enough already. Evidence for these uses is insufficient here.
2 studiesNot applicable
Studies, doses and limits
What the trials found
No trials were read for these uses; only agency statements.
Results
No numbers. The agencies describe roles, not supplement benefits.
Why this rating
No trial evidence read.
Limits
Evidence was not searched for these uses beyond agency pages.
Why researchers looked
These are normal roles of selenium in the body.
Proposed mechanism Hypothesis
Not tested.
A proposed mechanism does not show that the effect happens in people.
Too much selenium can cause hair loss and nail changes.
Connections to the rest of health
How Selenium connects to the rest of our health knowledge graph. Every link says what kind of connection it is. A safety link (for example to the liver) is a warning, not a benefit. A study that merely mentions a topic never creates a benefit link.
Explore every connection: goals, body systems, organs, symptoms, genes
A. Wellness goals
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Immune Support | Moderate certainty | Meeting the body's need (preventing deficiency): Selenium is an essential mineral. The body uses it to make antioxidant enzymes and thyroid hormones. Health agencies describe clear problems when intake is very low, such as Keshan disease (a heart muscle condition) in parts of China, and say deficiency is rare in the US and UK. For people who eat enough, no source we read shows that extra selenium adds benefit. | 15 14 | |
| Thyroid Support | Moderate certainty | Meeting the body's need (preventing deficiency): Selenium is an essential mineral. The body uses it to make antioxidant enzymes and thyroid hormones. Health agencies describe clear problems when intake is very low, such as Keshan disease (a heart muscle condition) in parts of China, and say deficiency is rare in the US and UK. For people who eat enough, no source we read shows that extra selenium adds benefit. | 15 14 | |
| Thyroid Support | Moderate certainty | Hashimoto thyroiditis (lab markers): In people with Hashimoto thyroiditis, an autoimmune thyroid condition, a 2024 review of trials found selenium supplements lowered TSH a little and lowered thyroid antibody levels. These are lab markers. The review does not say symptoms improved. Results for antibodies varied a lot between studies. | 3 | |
| Thyroid Support | Mixed findings | Graves' eye disease (orbitopathy): One 2011 trial in 159 people with mild Graves' orbitopathy (eye disease linked to an overactive thyroid) found selenium improved quality of life and slowed progression compared with placebo. A 2026 review of five trials found a small improvement in one score but no clear change in quality of life or vision. The evidence is mixed. | 8 4 | |
| Cellular Protection | Evidence against | Preventing cancer: Large trials give a clear answer: selenium supplements did not lower the chance of getting cancer in healthy adults. A Cochrane review rated this high certainty. People with higher selenium in the blood had less cancer in observational studies, but that kind of study cannot show cause. In the biggest trial, men who already had higher selenium levels had more high-grade prostate cancer on selenium. | 12 7 5 | |
| Heart Health | Evidence against | Preventing heart disease: A Cochrane review of 12 trials found selenium-only supplements did not lower deaths, heart disease deaths or heart disease events. Non-HDL cholesterol was lower in one trial. The authors say the evidence does not support selenium supplements to prevent heart disease. | 9 | |
| Blood Sugar Balance | Adverse effect or risk | Evidence against | Type 2 diabetes and blood sugar: Selenium has not been shown to prevent diabetes, and trials point the other way. In one long trial, more people on selenium developed type 2 diabetes. A 2018 review of trials found an 11% higher risk, borderline. A trial in older Danes found no lasting change in HbA1c. | 10 11 13 |
| Healthy Aging | Very low certainty | Living longer (selenium status and deaths): In 20 cohort studies, people with higher blood selenium were less likely to die during follow-up. This is an observational link: it cannot show that taking selenium helps, and it does not test supplements. | 1 | |
| Immune Support | Low certainty | Sepsis in hospital (intravenous selenium): In people very ill with sepsis, intravenous selenium did not clearly lower deaths in a review of 13 trials. Some secondary outcomes such as ICU stay looked better. The authors did not suggest it for routine use. This is a hospital treatment, not a pill for healthy people. | 6 | |
| Immune Support | Not enough evidence | Immune function, fertility and other claims: The NHS and MedlinePlus say selenium helps the immune system and reproduction. We read no trial showing that extra selenium improves these in people with enough already. Evidence for these uses is insufficient here. | 15 14 | |
| Fertility Support | Not enough evidence | Immune function, fertility and other claims: The NHS and MedlinePlus say selenium helps the immune system and reproduction. We read no trial showing that extra selenium improves these in people with enough already. Evidence for these uses is insufficient here. | 15 14 |
B. Body systems
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Immune System | Biological association | Moderate certainty | Studied for: Meeting the body's need (preventing deficiency) Studied for: Hashimoto thyroiditis (lab markers) | 15 14 3 12 7 5 6 |
| Endocrine System | Biological association | Moderate certainty | Studied for: Meeting the body's need (preventing deficiency) Studied for: Hashimoto thyroiditis (lab markers) | 15 14 3 8 4 10 11 13 |
| Cardiovascular System | Biological association | Evidence against | Studied for: Preventing heart disease Studied for: Living longer (selenium status and deaths) | 9 1 |
E. Health conditions
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Selenosis (selenium toxicity) | Adverse effect or risk | Documented risk | Hair and nail loss and other signs when intake is too high. Preparation or compound: Excess selenium from supplements or Brazil nuts |
15 14 2 |
| Type 2 diabetes | Adverse effect or risk | Trial signal | Small increase in risk in trials. Preparation or compound: Selenium supplements |
13 10 |
| Hashimoto disease | Lab markers only | Lower TSH and antibodies in a 2024 review. Preparation or compound: Selenium supplements |
3 |
I. Biomarkers
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Thyroid peroxidase antibodies (TPOAb) | Biological association | Moderate certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 3 |
| TSH (thyroid-stimulating hormone) | Biological association | Moderate certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 3 |
| Clinical Activity Score (Graves' orbitopathy) | Biological association | Low certainty | Measured in trials: lowered. A lab change is not the same as a health benefit. | 4 8 |
| HbA1c (average blood sugar over about 3 months) | Biological association | Low certainty | Measured in trials: unchanged. A lab change is not the same as a health benefit. | 11 |
| Blood selenium (status) | Biological association | Very low certainty | Measured in trials: associated. A lab change is not the same as a health benefit. | 1 13 |
K. Biological pathways
L. Active compounds
| Connected to | Kind of connection | Evidence status | Context | Studies |
|---|---|---|---|---|
| Selenomethionine | Biological association | Human | The usual dietary form; EFSA says almost 90% of selenium in plants is selenomethionine. Preparation or compound: Main form in plant foods and the form used in the SELECT trial |
2 7 |
| Sodium selenite and sodium selenate | Biological association | Human | Inorganic supplement forms named in the existing nutrient summary; not tested here against other forms. | 2 |
| Selenium-enriched yeast | Biological association | Human | Used in the Danish PRECISE trial; content varies by product. | 11 |
Each connection is stored with the ingredient, the connected record, the kind of connection, the evidence status, the supporting studies, the preparation or compound involved, and a review date (last reviewed October 26, 2026).
The evidence
The research centre for Selenium. Every record here was checked against PubMed or the publisher's page; we never list a study we have not verified.
Evidence by outcome
Bars show how many studies we found for each outcome, not how well it works. People are added up across trials only (reviews are not added, to avoid double counting).
How we rate evidence
High certainty
Several well-run trials or reviews agree; further research is unlikely to change the picture much.
Moderate certainty
Good trials point the same way, but some doubt remains.
Low certainty
Small, short or inconsistent trials. The finding could change with better research.
Very low certainty
Very little or very weak human data. Treat as a possibility only.
Mixed findings
Trials or reviews disagree with each other.
Not enough evidence
Not enough human research to say either way.
Evidence against
Good research found no benefit, or found harm.
We take the reviewers' own risk-of-bias and GRADE statements where they exist, and otherwise judge from study size, design, consistency, directness and whether a product matches what is sold. Many publications do not make evidence high. Method note: We started from our existing selenium summary, read the NHS, MedlinePlus pages and the EFSA opinion, and re-read PubMed abstracts for reviews and trials. Only abstracts were read for studies. The NIH Office of Dietary Supplements sheet was blocked and is flagged as not read.
Study library: every study we checked 16
| Study | Design | People | Product, dose, length | Result |
|---|---|---|---|---|
| Cui 2025 Associations of selenium status with all-cause and cause-specific mortality: a systematic review and meta-analysis of cohort studies Redox Biol · PMID 40690813 · DOI |
Observational |
20 population cohort studies | Blood selenium markers (status), not supplements Mineral supplement (not a plant) Varied · vs Lower versus higher selenium markers |
Each one standard-deviation higher selenium marker went with 13% lower all-cause death (RR 0.87, 95% CI 0.83 to 0.90), 11% lower heart disease death and 15% lower cancer death. The authors say trials in selenium-deficient groups are still needed. All-cause mortality RR 0.87 per SD (95% CI 0.83 to 0.90) Methods, limits and fundingOutcomes: All-cause, heart disease and cancer deaths Limitations: Observational: cannot show that selenium caused the difference; healthier people may have higher levels. Does not test supplements. Funding and conflicts: Funding not stated in the abstract. Read: abstract |
| EFSA Panel on Nutrition 2023 Scientific opinion on the tolerable upper intake level for selenium EFSA J · PMID 36698500 · DOI |
Regulatory opinion n/a |
Systematic reviews of excess selenium and health in people | Selenium from food and supplements Mineral supplement (not a plant) |
EFSA chose hair loss (alopecia) as the key early sign of too much selenium. It found a lowest-observed-adverse-effect level of 330 micrograms a day in SELECT and set an upper level of 255 micrograms a day for adults, including pregnant and breastfeeding women; upper levels for infants are 45 (4 to 6 months) and 55 (7 to 11 months) micrograms a day. Adults are unlikely to exceed it except regular users of high-dose supplements or regular eaters of Brazil nuts. Supplements for toddlers and children should be used with caution. The full text lists Brazil nuts as the richest food source, with a very large range between batches. Adult upper level 255 micrograms a day (EFSA, Jan 2023) (Not applicable) Methods, limits and fundingOutcomes: Upper intake level Limitations: An upper limit is a safety ceiling for the population, not a recommended amount. Funding and conflicts: Public regulator opinion. Read: full-text |
| Huwiler 2024 Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials Thyroid · PMID 38243784 · DOI |
Systematic review meta analysis |
2358 Adults with Hashimoto thyroiditis in 35 studies (TPO antibody result: 29 cohorts, 2,358 people) |
Selenium supplements (form varied) versus control Mineral supplement (not a plant) Varied Varied · vs Placebo or no selenium |
In people not on thyroid hormone replacement, selenium lowered TSH a little (SMD -0.21, 95% CI -0.43 to -0.02; 869 people). TPO antibodies fell (SMD -0.96, -1.36 to -0.56; 90% heterogeneity). Thyroid hormone levels, TGAb and thyroid volume did not change significantly. Adverse effects were similar to control (OR 0.89, 0.46 to 1.75). The authors rate overall certainty as moderate. TPOAb SMD -0.96; TSH SMD -0.21 (TPOAb 95% CI -1.36 to -0.56) Methods, limits and fundingOutcomes: TSH, thyroid antibodies (TPOAb), thyroid hormones, adverse events Limitations: These are lab markers; the abstract does not report that symptoms improved. TPOAb results varied a lot between studies. Funding and conflicts: Authors declare no conflicts. Read: abstract |
| Kostadinov 2026 Efficacy of Selenium Supplementation in Graves' Orbitopathy: A Systematic Review and Meta-Analysis of Randomized Controlled Trials with Trial Sequential Analysis J Clin Med · PMID 42355878 · DOI |
Systematic review meta analysis |
303 5 randomised trials (303 people) of Graves' orbitopathy |
Selenium versus placebo or no selenium Mineral supplement (not a plant) Varied Varied · vs Placebo or none |
Selenium lowered the Clinical Activity Score a little (mean difference -1.05, 95% CI -1.61 to -0.48). No significant differences in eyelid opening, quality of life (RR 1.72, 0.43 to 6.92) or visual function. The authors call the finding cautious because there were few trials and they differed. Clinical Activity Score MD -1.05 (95% CI -1.61 to -0.48) Methods, limits and fundingOutcomes: Clinical Activity Score, eyelid opening, quality of life, visual function Limitations: Few small trials, large differences between them. Funding and conflicts: Authors declare no conflicts. Read: abstract |
| Kristal 2014 Baseline selenium status and effects of selenium and vitamin E supplementation on prostate cancer risk J Natl Cancer Inst · PMID 24563519 · DOI |
Observational |
3117 Men from the SELECT trial: case-cohort (1,739 prostate cancer cases, 489 high-grade, 3,117 comparison men) |
Selenium supplementation (selenium-only and selenium plus vitamin E arms combined), by baseline toenail selenium Mineral supplement (not a plant) See SELECT See SELECT · vs Placebo |
Selenium supplements had no effect in men with lower selenium status but raised the risk of high-grade prostate cancer by 91% (P = .007) in men with higher selenium status. The authors advise men to avoid selenium or vitamin E supplements above recommended dietary intakes. High-grade prostate cancer +91% in men with higher baseline status (P = .007) Methods, limits and fundingOutcomes: Prostate cancer risk by baseline selenium status Limitations: A follow-up analysis of a randomised trial (not a new trial); subgroup finding. Funding and conflicts: Funded by the US National Institutes of Health and others. Read: abstract |
| Li 2019 A meta-analysis of randomized controlled trials: Efficacy of selenium treatment for sepsis Medicine (Baltimore) · PMID 30817625 · DOI |
Systematic review meta analysis |
13 randomised trials in people with sepsis (a life-threatening infection response) | Selenium given in hospital (intravenous), versus placebo Mineral supplement (not a plant) Not given in the abstract Varied · vs Placebo |
No clear effect on death at day 28 (RR 0.94, 95% CI 0.82 to 1.06), 90 days or 6 months. Some secondary outcomes were better (shorter ICU stay, fewer ventilator pneumonias, RR 0.61, 0.42 to 0.89). The authors say intravenous selenium could not be suggested for routine use. Death at day 28 RR 0.94 (no clear effect) (95% CI 0.82 to 1.06) Methods, limits and fundingOutcomes: Death, organ failure, hospital outcomes Limitations: Hospital-only intravenous use in very ill people; not about pills for healthy people. Funding and conflicts: Authors declare no competing interests. Read: abstract |
| Lippman 2009 Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT) JAMA · PMID 19066370 · DOI |
Randomized trial |
35533 Relatively healthy men aged 50 or 55 and older (US, Canada, Puerto Rico) |
L-selenomethionine, alone or with vitamin E (all rac-alpha-tocopheryl acetate) Mineral supplement (not a plant) 200 micrograms a day of selenium from L-selenomethionine Median 5.46 years follow-up (planned 7 to 12 years) · vs Placebo |
Prostate cancer hazard ratio for selenium alone was 1.04 (99% CI 0.87 to 1.24) versus placebo. No difference for any other cancer outcome. Type 2 diabetes was not significantly higher with selenium (RR 1.07, 99% CI 0.94 to 1.22). The trial was stopped early for lack of benefit. Prostate cancer HR 1.04 (selenium alone) (99% CI 0.87 to 1.24) Methods, limits and fundingOutcomes: Prostate cancer; other cancers; type 2 diabetes Limitations: Only the form and dose tested; men with fairly good selenium status. Funding and conflicts: Funded by the US National Cancer Institute and others (research support N.I.H.). Read: abstract |
| Marcocci 2011 Selenium and the course of mild Graves' orbitopathy N Engl J Med · PMID 21591944 · DOI |
Randomized trial |
159 159 people with mild Graves' orbitopathy (eye disease linked to an overactive thyroid) |
Selenium (and a comparison drug, pentoxifylline) versus placebo Mineral supplement (not a plant) Selenium 100 micrograms twice daily 6 months of treatment, then 6 months follow-up · vs Placebo |
At 6 months selenium, but not pentoxifylline, was linked to better quality of life (P<0.001), less eye involvement (P=0.01) and slower progression (P=0.01) than placebo. Results held at 12 months. No adverse events were evident with selenium. Better quality of life, less eye involvement vs placebo (P-values only in abstract) Methods, limits and fundingOutcomes: Overall eye assessment; quality of life; Clinical Activity Score Limitations: One trial, mild disease, from Europe; the later review below found mixed results. Funding and conflicts: Funded by the University of Pisa and the Italian Ministry for Education, University and Research. Read: abstract |
| Rees 2013 Selenium supplementation for the primary prevention of cardiovascular disease Cochrane Database Syst Rev · PMID 23440843 · DOI |
Systematic review meta analysis |
19715 12 randomised trials (19,715 people) in the general population and people at high heart risk |
Selenium-only supplements versus placebo or no treatment Mineral supplement (not a plant) Varied At least 3 months for pooled trials · vs Placebo or no treatment |
No significant effect on all-cause mortality (RR 0.97, 95% CI 0.88 to 1.08), heart disease death (RR 0.97, 0.79 to 1.2) or all heart disease events (RR 1.03, 0.95 to 1.11). A small, non-significant rise in type 2 diabetes (RR 1.06, 0.97 to 1.15). In SELECT, hair loss (RR 1.28) and mild dermatitis (RR 1.17) were higher with selenium. The authors say the limited evidence does not support selenium supplements to prevent heart disease. Heart disease events RR 1.03 (no effect) (95% CI 0.95 to 1.11) Methods, limits and fundingOutcomes: All-cause mortality, heart disease deaths and events, type 2 diabetes, cholesterol, adverse effects Limitations: Two big US trials dominate; selenium-only supplements. Funding and conflicts: Conflict statement: none known. Read: abstract |
| Stranges 2007 Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial Ann Intern Med · PMID 17620655 · DOI |
Randomized trial |
1202 1,202 people from dermatology clinics in low-selenium areas of the eastern US, without diabetes at the start |
Selenium supplement versus placebo Mineral supplement (not a plant) 200 micrograms a day Average 7.7 years · vs Placebo |
Type 2 diabetes developed in 58 selenium and 39 placebo recipients (hazard ratio 1.55, 95% CI 1.03 to 2.33). Risk was highest in people with the highest blood selenium at the start (HR 2.70, 1.30 to 5.61). The authors conclude selenium does not seem to prevent type 2 diabetes and may raise the risk. Type 2 diabetes HR 1.55 (95% CI 1.03 to 2.33) Methods, limits and fundingOutcomes: New type 2 diabetes (a secondary outcome) Limitations: Diabetes was a secondary outcome; self-reported diagnoses; mostly older white participants. Funding and conflicts: Funding not stated in the abstract. Read: abstract |
| Stranges 2019 Effect of selenium supplementation on changes in HbA1c: Results from a multiple-dose, randomized controlled trial Diabetes Obes Metab · PMID 30280459 · DOI |
Randomized trial |
491 491 Danish volunteers aged 60 to 74 with low selenium status |
Selenium-enriched yeast versus placebo yeast Mineral supplement (not a plant) 100, 200 or 300 micrograms a day 6 months and 2 years · vs Placebo yeast |
At 6 months HbA1c fell by 1.5 mmol/mol (95% CI -2.8 to -0.2) in the lowest-amount group and by 0.7 (-2.0 to 0.6) in the others compared with placebo, with no clear difference across groups (P = 0.16). After 2 years HbA1c fell in all groups, with no difference between selenium and placebo. The authors say this fits a possible U-shaped response. No difference in HbA1c from placebo at 2 years (95% CIs in abstract; none excludes no effect at 2 years) Methods, limits and fundingOutcomes: HbA1c (long-term blood sugar marker) Limitations: Older Europeans with low selenium; one centre. Funding and conflicts: Funding not stated in the abstract. Read: abstract |
| Vinceti 2018 Selenium for preventing cancer Cochrane Database Syst Rev · PMID 29376219 · DOI |
Systematic review meta analysis |
27232 Adults in 10 randomised trials (27,232 people); plus 70 observational cohort studies (over 2,360,000 people) |
Selenium supplements (various forms) versus placebo; plus selenium status in cohort studies Mineral supplement (not a plant) Varied by trial; not pooled in the abstract Varied · vs Placebo |
In trials at low risk of bias, any cancer RR was 1.01 (95% CI 0.93 to 1.10; 3 studies, 19,475 people; high-certainty). Prostate cancer RR 1.01 (0.90 to 1.14). The authors conclude that well-run trials show no benefit of selenium supplements for cancer risk (high certainty). Trials raised concern about high-grade prostate cancer and type 2 diabetes. Low-risk-of-bias trials suggested increased melanoma risk. Observational cohorts linked the highest selenium exposure to lower cancer (OR 0.72, 0.55 to 0.93) but certainty was very low. Any cancer RR 1.01 (trials, high certainty) (95% CI 0.93 to 1.10) Methods, limits and fundingOutcomes: Any cancer incidence, cancer mortality, site-specific cancers, adverse effects Limitations: Trials were 88% men. Observational findings are weak (confounding, mismeasured exposure). Funding and conflicts: Conflict statement: none known for 9 of 10 authors; one author co-authored included studies. Read: abstract |
| Vinceti 2018 Selenium exposure and the risk of type 2 diabetes: a systematic review and meta-analysis Eur J Epidemiol · PMID 29974401 · DOI |
Systematic review meta analysis |
50 observational studies and 5 randomised trials | Selenium exposure (blood or intake) and selenium supplements versus placebo Mineral supplement (not a plant) Supplement trials pooled for 200 micrograms a day Varied · vs Placebo |
In trials, selenium supplements raised diabetes risk by 11% (RR 1.11, 95% CI 1.01 to 1.22), more in women than men. In observational studies, higher blood selenium went with higher diabetes risk (RR 3.6 at 140 micrograms per litre versus under 45; 95% CI 1.4 to 9.4). The authors say selenium may raise the risk of type 2 diabetes; the increase is small but could matter at population level. Trials: type 2 diabetes RR 1.11 (95% CI 1.01 to 1.22) Methods, limits and fundingOutcomes: New type 2 diabetes Limitations: Observational data can be confounded; trial effect borderline. Funding and conflicts: Funding not stated in the abstract. Read: abstract |
| s-medlineplus-selenium MedlinePlus: Selenium in diet · Source |
Health agency n/a |
Methods, limits and fundingRead: full-text | ||
| s-nhs-selenium NHS: Vitamins and minerals - selenium · Source |
Health agency n/a |
Methods, limits and fundingRead: full-text | ||
| s-ods-selenium NIH Office of Dietary Supplements: Selenium fact sheet for health professionals · Source |
Health agency n/a |
Methods, limits and fundingRead: not-read |
What we do not know yet
- No trial read tested selenium for correcting deficiency; the deficiency facts come from health-agency pages.
- Hashimoto: no symptom or long-term outcome data in the review read.
- Per-portion food amounts and US upper limits were not available from a source we could open.
- Interaction data with medicines is not established.
- Fertility, immune and pregnancy outcomes from trials were not reviewed.
How new research is added
New studies are found by a saved PubMed search and placed in a review queue before the page changes.
Lab markers
What changed in blood or saliva tests during trials. A changed number is not the same as feeling or being healthier, and we give no "normal ranges".
Hormonal 1
TSH (thyroid-stimulating hormone)
Lowered in trials
A pituitary hormone that rises when the thyroid is underactive.
Moderate certaintyResults and what it means
Why it was measured
Standard thyroid function test
Statistical vs. clinical meaning
A small fall in TSH.
Safety implication
None found in the abstract.
| Study | Result | People | Product |
|---|---|---|---|
| 3 | SMD -0.21 (95% CI -0.43 to -0.02) in people not on thyroid hormone replacement | Adults with Hashimoto thyroiditis without hormone replacement (869 people) | Selenium supplements versus control |
Metabolic 1
HbA1c (average blood sugar over about 3 months)
Unchanged in trials
A blood test that reflects average blood sugar.
Low certaintyResults and what it means
Why it was measured
Used to diagnose and follow diabetes
Statistical vs. clinical meaning
No lasting effect on HbA1c in this trial.
Safety implication
See type 2 diabetes safety signal.
| Study | Result | People | Product |
|---|---|---|---|
| 11 | After 2 years HbA1c fell in all groups with no difference between selenium and placebo | 491 older Danes with low selenium status | Selenium-enriched yeast versus placebo yeast |
Other 3
Thyroid peroxidase antibodies (TPOAb)
Lowered in trials
Antibodies against a thyroid enzyme. They are higher in autoimmune thyroid disease.
Moderate certaintyResults and what it means
Why it was measured
Marker of autoimmune activity in Hashimoto thyroiditis
Statistical vs. clinical meaning
Lower antibody levels in pooled trials. It is not shown to change symptoms.
Safety implication
None found in the abstract.
| Study | Result | People | Product |
|---|---|---|---|
| 3 | SMD -0.96 (95% CI -1.36 to -0.56) with selenium; results differed a lot between studies | Adults with Hashimoto thyroiditis (29 cohorts, 2,358 people) | Selenium supplements (forms varied) versus control |
Clinical Activity Score (Graves' orbitopathy)
Lowered in trials
A score doctors use to rate how active the eye disease is.
Low certaintyResults and what it means
Why it was measured
Tracks inflammation of the eye socket
Statistical vs. clinical meaning
A small lower score in a few trials.
Safety implication
No adverse events with selenium in the 2011 trial.
| Study | Result | People | Product |
|---|---|---|---|
| 4 | Mean difference -1.05 (95% CI -1.61 to -0.48); heterogeneity 52% | Adults with Graves' orbitopathy (5 trials, 303 people) | Selenium versus placebo or none |
| 8 | The score fell in all groups, with a bigger fall in the selenium group | 159 people with mild disease | Selenium versus placebo for 6 months |
Blood selenium (status)
Associated in trials
Selenium measured in blood or plasma, showing how much the body has.
Very low certaintyResults and what it means
Why it was measured
Used in studies to compare people with lower and higher status
Statistical vs. clinical meaning
An association, not a treatment result.
Safety implication
Higher blood selenium also went with higher diabetes risk in observational studies (RR 3.6 at the highest level compared; 95% CI 1.4 to 9.4).
| Study | Result | People | Product |
|---|---|---|---|
| 1 | Higher selenium markers went with lower all-cause death in cohort studies (RR 0.87 per SD) | 20 cohort studies | Observation only |
Safety
What studies and official sources say about side effects, medicines and who should check first. Each point shows what kind of evidence it comes from.
Main points to know 4
- Too much selenium is harmful, and the safe range is narrow. Selenium is essential but toxic in excess. The NHS says too much causes selenosis, in its mildest form loss of hair and nails. MedlinePlus lists hair loss, nail problems, nausea, irritability, fatigue and mild nerve damage. EFSA set an upper intake level of 255 micrograms a day for adults from all sources (food plus supplements), based on hair loss, and says adults are unlikely to exceed it except regular users of high-dose supplements or regular eaters of Brazil nuts. This is a safety ceiling reported by the regulator, not advice to take that amount. Authoritative guidance 2 15 14
- Brazil nuts can carry very large and uneven amounts. EFSA says Brazil nuts are the richest food source and that 5 or 6 nuts could supply roughly 100 to 2,000 micrograms of selenium, depending on where they grew. Regular eating of Brazil nuts can push intake above the upper level. Authoritative guidance 2
- Supplements did not prevent disease and raised some risks in trials. High-certainty trial evidence shows no cancer prevention. In SELECT, men with higher selenium status had more high-grade prostate cancer on selenium. Trials suggested a small rise in type 2 diabetes (RR 1.11), more hair loss and mild dermatitis, and possibly more melanoma. Men and anyone with diabetes risk should talk to a doctor before taking selenium supplements. Controlled trial 12 7 5 10 13 9
- Thyroid condition: ask first. Selenium affects thyroid hormones. Trials in Hashimoto thyroiditis found lower TSH and antibodies and similar adverse events to control, but a doctor should guide any use if you have a thyroid condition or take thyroid medicine. Systematic review 3
Side effects 5
| Effect | How often | Severity and recovery | Evidence |
|---|---|---|---|
| Selenosis: hair loss, nail changes, nausea, irritability, tiredness, mild nerve damage EFSA chose hair loss (alopecia) as the key early sign. | Not given. MedlinePlus says toxicity is rare in the US. | mild to moderate in the sources read EFSA calls hair loss mild and likely reversible. | Authoritative guidance 15 14 2 |
| Hair loss and mild dermatitis in a large trial | SELECT, via Cochrane: hair loss RR 1.28 (95% CI 1.01 to 1.62); grade 1 to 2 dermatitis RR 1.17 (1.0 to 1.35). | mild Not described in the abstract. | Controlled trial 9 12 |
| Type 2 diabetes Small, borderline risk increase; the Danish trial showed no HbA1c change. | Trials pooled: RR 1.11 (95% CI 1.01 to 1.22); one trial HR 1.55 (1.03 to 2.33). | long-term condition Not described. | Controlled trial 13 10 |
| High-grade prostate cancer in men with higher selenium status Subgroup of a randomised trial; also hinted at in the Cochrane review. | +91% in a SELECT sub-analysis (P = .007). | serious Not applicable. | Controlled trial 5 12 |
| Possible melanoma risk increase Low certainty signal. | Cochrane: low-risk-of-bias trials suggested increased risk; no figure in the abstract. | serious Not applicable. | Systematic review 12 |
Liver
Liver injury from selenium was not searched in LiverTox and none was found in the abstracts or pages read.
How often. No frequency was found.
Monitoring. No agency monitoring advice was found.
This is a safety connection. It is not evidence that Selenium supports liver health.
Medicines and supplements 3
| With | Why it matters | Importance | Evidence | What to do |
|---|---|---|---|---|
| Vitamin E supplements supplement | Not explained in the abstract. | In SELECT, vitamin E alone raised prostate cancer risk in men with lower selenium status (63%, P = .02). This is trial subgroup data, not a drug interaction. | Controlled trial Low certainty 5 | If you are considering both, ask a doctor or pharmacist. |
| Medicines in general (blood thinners, chemotherapy, thyroid medicine) prescription | No mechanism found in the sources read. | No source read states a specific interaction. The NIH Office of Dietary Supplements sheet, which usually covers this, could not be opened (403), so this is not confirmed. Absence of evidence from a limited search is not proof of no interaction. | Not established Not enough evidence 16 | Ask your pharmacist if you take prescription medicines. |
| Other selenium-containing products (multivitamins, Brazil nuts) supplement | Intake adds up across sources. | EFSA says the upper level counts food and supplements together. | Authoritative guidance Moderate certainty 2 | Count all sources if you take supplements. |
Who should check first 3
| Situation | Advice | Evidence |
|---|---|---|
| Men and anyone at risk of prostate cancer or type 2 diabetes | Selenium supplements are not shown to help and raised risks in trials. Ask a doctor before using them. | Controlled trial 5 13 |
| Thyroid disease | Selenium affects thyroid markers. Let your doctor guide it. | Systematic review 3 |
| Infants and young children | EFSA says selenium supplements in toddlers and children should be used with caution, on individual needs. | Authoritative guidance 2 |
Overdose, product quality and regulators
Summary. Selenium is toxic in excess (selenosis).
Animal. Not reviewed.
Human doses studied. EFSA found the lowest level with an adverse effect (hair loss) at 330 micrograms a day in SELECT and set 255 micrograms a day as the adult upper level.
High dose or long term signals. Long-term trials: hair loss, dermatitis, a diabetes signal and a high-grade prostate cancer signal in men with higher selenium.
Overdose response. Not described in the sources read beyond selenosis signs.
Genotoxicity. Not researched.
Product quality was not reviewed from a source we could open.
Brazil nut variability. EFSA reports a very large range of selenium between Brazil nut batches (up to 400 micrograms in one nut). 2
| Body | Action | Date |
|---|---|---|
| EFSA (EU) (European Union) | Tolerable upper intake level for selenium: 255 micrograms a day for adults including pregnant and breastfeeding women; infants 45 (4 to 6 months) and 55 (7 to 11 months). Caution with supplements for toddlers and children. source | 2023-01-20 |
| NHS (UK) (United Kingdom) | Says taking 350 micrograms or less a day of selenium supplements is unlikely to cause harm; too much causes selenosis. source | 2020-08-03 |
| MedlinePlus (US NLM) (United States) | Selenium supplements in addition to food are not currently recommended for cancer prevention; too much causes selenosis; lists Dietary Reference Intakes. source | 2025-01-21 |
When to get medical help
- Repeated vomiting, severe stomach pain, garlic-smelling breath or new nerve symptoms such as numbness after taking a lot of selenium or eating many Brazil nuts: get medical help.
- Sudden hair or nail loss with tiredness while taking selenium supplements: stop and talk to a doctor.
Stop using the product and contact a clinician or emergency service. In the US, Poison Control is 1-800-222-1222.
General information only, not medical advice. Ask a clinician or pharmacist before use, especially with medicines, in pregnancy or with a health condition. See the Natural Remedy Safety guide.
Frequently asked questions
What is selenium?
Selenium is an essential trace mineral. The body uses it in antioxidant enzymes and for thyroid hormones.
What foods have selenium?
Do most people need a selenium supplement?
Does selenium prevent cancer?
Can selenium cause prostate cancer?
Does selenium help the thyroid?
In Hashimoto thyroiditis, a 2024 review found lower TSH and thyroid antibodies, with moderate certainty. Symptoms were not shown to improve. Talk to your doctor before using it.
Is selenium good for the heart?
A Cochrane review of 12 trials found no effect on deaths or heart disease events.
Does selenium affect diabetes?
Can you get too much selenium?
Are Brazil nuts risky?
They can be. EFSA says a few Brazil nuts can carry a very large and uneven amount of selenium, and regular eaters can exceed the upper level.
Is selenium safe in pregnancy and for children?
Is there anything we could not check?
The NIH Office of Dietary Supplements fact sheet could not be opened, so US upper limits and detailed interaction data are not stated here.
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Editorial transparency
- Written by
- AlternativeMed editorial team. Research compiled with AI assistance, then checked against the sources listed.
- Medical review
- None. This page has not been reviewed by a physician or pharmacist.
- First published
- 2026-10-26
- Last substantive update
- 2026-10-26
- How we research
- We started from our existing selenium summary, read the NHS, MedlinePlus pages and the EFSA opinion, and re-read PubMed abstracts for reviews and trials. Only abstracts were read for studies. The NIH Office of Dietary Supplements sheet was blocked and is flagged as not read.
- How we rate evidence
- See "How we rate evidence" in the Evidence tab.
- Independence
- No product is promoted on this page. No affiliate links are shown. Funding is stated where the abstracts gave it; many are public research funds. No industry funding was seen in the abstracts read.
Sources 16
Each source was opened and checked. Where we read only the abstract, it says so. The small numbers in the text point here.
- Cui Z, Xie R, Lu X, Schomburg L, Brenner H, Schottker B. Associations of selenium status with all-cause and cause-specific mortality: a systematic review and meta-analysis of cohort studies. Redox Biol. 2025. PMID 40690813. DOI 10.1016/j.redox.2025.103755. (abstract)
- EFSA Panel on Nutrition, Novel Foods and Food Allergens (Turck D, Bohn T, et al.). Scientific opinion on the tolerable upper intake level for selenium. EFSA J. 2023. PMID 36698500. DOI 10.2903/j.efsa.2023.7704. (full-text)
- Huwiler VV, Maissen-Abgottspon S, Stanga Z, et al.. Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials. Thyroid. 2024. PMID 38243784. DOI 10.1089/thy.2023.0556. (abstract)
- Kostadinov N, Kirovakov Z, Penchev P. Efficacy of Selenium Supplementation in Graves' Orbitopathy: A Systematic Review and Meta-Analysis of Randomized Controlled Trials with Trial Sequential Analysis. J Clin Med. 2026. PMID 42355878. DOI 10.3390/jcm15124710. (abstract)
- Kristal AR, Darke AK, Morris JS, et al.. Baseline selenium status and effects of selenium and vitamin E supplementation on prostate cancer risk. J Natl Cancer Inst. 2014. PMID 24563519. DOI 10.1093/jnci/djt456. (abstract)
- Li S, Tang T, Guo P, et al.. A meta-analysis of randomized controlled trials: Efficacy of selenium treatment for sepsis. Medicine (Baltimore). 2019. PMID 30817625. DOI 10.1097/MD.0000000000014733. (abstract)
- 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. PMID 19066370. DOI 10.1001/jama.2008.864. (abstract)
- Marcocci C, Kahaly GJ, Krassas GE, et al.. Selenium and the course of mild Graves' orbitopathy. N Engl J Med. 2011. PMID 21591944. DOI 10.1056/NEJMoa1012985. (abstract)
- Rees K, Hartley L, Day C, Flowers N, Clarke A, Stranges S. Selenium supplementation for the primary prevention of cardiovascular disease. Cochrane Database Syst Rev. 2013. PMID 23440843. DOI 10.1002/14651858.CD009671.pub2. (abstract)
- Stranges S, Marshall JR, Natarajan R, et al.. Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Ann Intern Med. 2007. PMID 17620655. DOI 10.7326/0003-4819-147-4-200708210-00175. (abstract)
- 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. PMID 30280459. DOI 10.1111/dom.13549. (abstract)
- Vinceti M, Filippini T, Del Giovane C, et al.. Selenium for preventing cancer. Cochrane Database Syst Rev. 2018. PMID 29376219. DOI 10.1002/14651858.CD005195.pub4. (abstract)
- 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. PMID 29974401. DOI 10.1007/s10654-018-0422-8. (abstract)
- . MedlinePlus: Selenium in diet. . . (full-text)
- . NHS: Vitamins and minerals - selenium. . . (full-text)
- . NIH Office of Dietary Supplements: Selenium fact sheet for health professionals. . . (not-read)
Where to buy
Check the label first. Whatever you buy, look for these:
- The amount of the active ingredient per serving, not just the front-label claim
- A third-party testing mark (USP, NSF or ConsumerLab)
- Other ingredients and allergens
- Ask a pharmacist about interactions with your medicines
Talk to a pharmacist or doctor before you start any supplement, especially if you take medicines. See the Safety tab for interactions.
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