Selenium: Roles, Food Sources, Evidence and Toxicity
Selenium roles, food sources including Brazil nuts, official intake levels by country, what trials found for cancer, diabetes and thyroid disease, and the upper limits for selenosis.
Content last updated . This page has not been medically reviewed.
What is Selenium good for?
What the body uses it for, and how strong the evidence is. A normal job in the body is not proof that extra supplements add benefit; each card says which.
Antioxidant enzymes
Established roleSelenium helps the body make antioxidant enzymes that help prevent cell damage.
Supplements: Trials of supplements in people with adequate intake did not show lower cancer or heart disease risk.
Immune system function
Established roleSelenium helps the immune system work properly.
Supplements: Supplements correct low intake; no good evidence of extra benefit when intake is adequate.
Thyroid hormone production
Established roleSelenium is needed to make thyroid hormones.
Supplements: In Hashimoto thyroiditis, trials show lower TSH and antibody markers; these are lab markers, not symptom relief.
Reproduction
Established roleThe NHS lists helping reproduction as one of selenium's roles.
Supplements: Not shown that supplements improve fertility when intake is adequate.
Protecting cells and tissues
Established roleSelenium helps prevent damage to cells and tissues.
Supplements: Supplements correct low intake; no good evidence of extra benefit when intake is adequate.
Preventing Keshan disease (heart muscle)
Deficiency linkLack of selenium causes Keshan disease, a heart muscle condition that killed many children in parts of China before supplementation.
Supplements: Relevant only where intake is very low.
Bone and joint health in low-selenium regions
Deficiency linkSelenium deficiency has been linked to Kashin-Beck disease, a joint and bone disease.
Supplements: Relevant only where intake is very low.
Brain development in severe deficiency
Deficiency linkMedlinePlus links selenium deficiency to myxedematous endemic cretinism (intellectual disability).
Supplements: Relevant only where intake is very low.
Connected topics
Open a group to see how each topic connects and how strong the evidence is.
Health goals (5)
- Cellular ProtectionEstablished role
The body uses selenium to make antioxidant enzymes that help prevent cell damage. This is a normal role, not proof that extra selenium protects cells further.
- Immune SupportEstablished role
Selenium helps the immune system work properly. Supplements only clearly matter when intake is low; we found no good evidence of extra immune benefit when intake is adequate.
- Thyroid SupportEstablished role
Selenium is needed to make thyroid hormones. In Hashimoto thyroiditis, trials found lower TSH and thyroid antibody levels, but these are lab markers, not symptoms.
- Blood Sugar BalanceDocumented risk
A 2018 review found selenium supplements slightly raised the risk of type 2 diabetes in trials (about 11 percent relative, borderline). This is a caution, not a benefit.
- Fertility SupportEstablished role
The NHS lists reproduction among the things selenium helps with. We did not read evidence that supplements improve fertility in people with adequate intake.
Body systems (13)
- Cardiovascular SystemDeficiency link
Severe selenium deficiency causes Keshan disease, a heart muscle condition. Supplements have shown no effect on heart disease risk in people who are not deficient.
- Endocrine SystemEstablished role
Selenium is needed for thyroid hormone production, so it matters mainly for the thyroid part of the hormone system.
- Heart & CirculationDeficiency link
Severe selenium deficiency causes Keshan disease, a heart muscle condition. Supplements have shown no effect on heart disease risk in people who are not deficient.
- Hormone HealthEstablished role
Selenium is needed for thyroid hormone production. Evidence for other hormones was not read.
- Immune SupportEstablished role
Selenium helps the immune system work properly; extra supplements are not shown to add benefit when intake is adequate.
- Immune SystemEstablished role
Selenium helps the immune system work properly; extra supplements are not shown to add benefit when intake is adequate.
- Integumentary SystemDocumented risk
Too much selenium (selenosis) can cause hair and nail loss, and in the largest trial supplements increased hair loss and dermatitis.
- Skin, Hair & BeautyDocumented risk
Too much selenium (selenosis) can cause hair and nail loss. Selenium is not shown to improve hair, skin or nails in people with adequate intake.
- Bones, Joints & MusclesDeficiency link
Selenium deficiency has been linked to Kashin-Beck disease, a joint and bone disease seen in low-selenium regions.
- Musculoskeletal SystemDeficiency link
Selenium deficiency has been linked to Kashin-Beck disease, a joint and bone disease seen in low-selenium regions.
- Nervous SystemDocumented risk
Selenosis can include irritability and mild nerve damage.
- Reproductive HealthEstablished role
The NHS says selenium helps in reproduction. Effects of supplements were not reviewed in the sources we read.
- Reproductive SystemEstablished role
The NHS says selenium helps in reproduction. Effects of supplements were not reviewed in the sources we read.
Body parts (9)
- CellsEstablished role
Selenium helps the body make antioxidant enzymes that prevent cell damage.
- HairDocumented risk
Hair loss is an early sign of too much selenium (selenosis).
- HeartDeficiency link
Keshan disease, a heart muscle condition, is caused by severe selenium deficiency.
- Immune SystemEstablished role
Selenium helps the immune system work properly.
- ThyroidEstablished role
Selenium is needed for thyroid hormone production; in Hashimoto thyroiditis, trials show lower TSH and antibody markers.
- BonesDeficiency link
Kashin-Beck disease (joint and bone disease) has been linked to selenium deficiency.
- JointsDeficiency link
Kashin-Beck disease (joint and bone disease) has been linked to selenium deficiency.
- Nervous SystemDocumented risk
Selenosis can include mild nerve damage and irritability.
- Reproductive OrgansEstablished role
The NHS says selenium helps in reproduction.
Functional actions (2)
- AntioxidantEstablished role
The body needs selenium to make antioxidant enzymes. Taking extra selenium has not been shown to lower cancer or heart disease risk.
- Endocrine SupportEstablished role
Selenium is required for thyroid hormone synthesis.
Root causes (3)
- Nutrient DeficiencyDeficiency link
Low selenium causes Keshan disease and is linked to Kashin-Beck disease. Deficiency is rare in the US but can occur with long-term intravenous feeding.
- Digestive DysfunctionDeficiency link
Severe gut disorders such as Crohn disease may impair selenium absorption.
- Oxidative StressEstablished role
Selenium is part of antioxidant enzymes, so low status can reduce this protection. Supplements have not been shown to reduce disease risk in people with adequate intake.
Habits (1)
- Balanced NutritionEstablished role
Selenium comes from food such as Brazil nuts, fish, meat and eggs. The NHS says people who eat meat, fish or nuts should get enough from diet; plant content depends on the soil.
Biomarkers (1)
- TSHPreliminary or mixed
In Hashimoto thyroiditis without thyroid hormone treatment, selenium supplements lowered TSH modestly in a meta-analysis (moderate certainty). This is a lab marker, not a symptom outcome.
Conditions & symptoms (7)
- Keshan DiseaseDeficiency link
A heart muscle condition caused by lack of selenium.
- Selenium DeficiencyDeficiency link
Rare in the US; can occur with long-term intravenous feeding, and severe gut disorders such as Crohn disease may impair absorption.
- AlopeciaDocumented risk
Hair loss is a sign of selenosis, and selenium supplements increased alopecia in the SELECT trial.
- Hashimoto DiseasePreliminary or mixed
Supplements lowered TSH and thyroid antibodies in a 2024 meta-analysis (moderate certainty); symptom and long-term outcomes were not shown.
- Kashin-Beck DiseaseDeficiency link
A joint and bone disease linked to selenium deficiency.
- Diabetes Mellitus, Non-Insulin-DependentDocumented risk
Trials of 200 mcg a day showed a small increased diabetes risk (RR 1.11, 95% CI 1.01 to 1.22); a possible safety signal.
- Endemic CretinismDeficiency link
MedlinePlus links selenium deficiency to myxedematous endemic cretinism (intellectual disability).
Safety first. Selenium has a fairly narrow gap between a useful intake and too much. Too much causes selenosis, with hair and nail loss, nausea, irritability, fatigue and mild nerve damage. [1][2] Count food, multivitamins and any stand-alone selenium together, and ask a clinician or pharmacist before adding a supplement, especially in pregnancy, with thyroid disease or with long-term medicines.
Overview
Selenium is an essential trace mineral. The NHS (UK) says it helps the immune system work properly and supports reproduction and protection of cells and tissues. [1] MedlinePlus explains that it helps the body make antioxidant enzymes that play a role in preventing cell damage. [2] Evidence labels used on this page: established role / deficiency, no benefit shown, safety signal, promising but limited, Proper evidence not found.Food sources
The NHS lists Brazil nuts, fish, meat and eggs. [1] MedlinePlus adds shellfish, grains, chicken, liver and garlic, and says how much selenium is in vegetables depends on the soil where they grew. [2] Brazil nuts are a very good source, but a 2025 trial paper describes their selenium content as highly variable, and a 2025 literature review found Brazil nuts also stand out for radium content, which it said appears neither clearly harmful nor clearly harmless at one or two nuts a day. [3][4] The sources we read do not give selenium per nut. See Selenium toxicity, Brazil nuts and upper limits.Official intake levels
Amounts are micrograms (mcg) of selenium a day. Read body, year and group before comparing.| Body and year | Group | Level |
|---|---|---|
| US Dietary Reference Intakes, via MedlinePlus (reviewed 2025) | Children 1 to 3; 4 to 8; 9 to 13 | RDA 20; 30; 40 mcg [2] |
| US Dietary Reference Intakes, via MedlinePlus | Age 14 and older; pregnancy; breastfeeding | RDA 55; 60; 70 mcg [2] |
| NHS (UK), page reviewed 2020 | Men 19 to 64; women 19 to 64 | 75 mcg; 60 mcg [1] |
| NHS (UK) | Supplements | 350 mcg or less a day unlikely to cause harm [1] |
| EFSA (EU), 2023 opinion in the 2025 overview | Adults, including pregnancy and breastfeeding | Tolerable upper intake level 255 mcg from all dietary sources [5] |
| EFSA (EU) | Children 1 to 3; 4 to 6; 7 to 10; 11 to 14; 15 to 17 | Tolerable upper intake level 70; 95; 130; 180; 230 mcg [5] |
| Health Canada dietary reference intake tables | Adults | Upper level 400 mcg [6] |
Upper levels differ between bodies; none is a target. The NIH Office of Dietary Supplements pages were blocked to us, so US figures come from MedlinePlus.
Deficiency
MedlinePlus says deficiency is rare in the United States but may occur when a person is fed through a vein for long periods, and that severe gut disorders such as Crohn disease may affect absorption. It links deficiency to Keshan disease (a heart muscle condition) and Kashin-Beck disease (joint and bone disease). [2] Deficiency is for a clinician to assess, not to guess from symptoms. We did not read a source on selenium blood tests for individuals: Proper evidence not found.Evidence by use
| Question | What the source found | Strength |
|---|---|---|
| Cancer prevention | Cochrane (2018): 10 trials, 27,232 people. In low-risk-of-bias trials the risk ratio for any cancer was 1.01 (95% CI 0.93 to 1.10; high certainty). [7] | No benefit shown |
| Prostate cancer | SELECT (35,533 men; 200 mcg a day as selenomethionine) found selenium did not prevent prostate cancer. [8] After longer follow-up the hazard ratio for selenium was 1.09 (99% CI 0.93 to 1.27, not significant). [9] | No benefit shown |
| Type 2 diabetes risk | A 2018 meta-analysis found 200 mcg a day increased diabetes risk by 11% (RR 1.11, 95% CI 1.01 to 1.22). [10] A 2014 meta-analysis of four trials found 1.09 (95% CI 0.99 to 1.20). [11] | Safety signal; mixed |
| Heart disease | A 2022 review of 884 trials found selenium showed no effect on cardiovascular disease or type 2 diabetes risk. [12] | No benefit shown |
| Hashimoto thyroiditis | A 2024 meta-analysis (35 studies) found selenium lowered TSH and thyroid peroxidase antibodies; moderate certainty; adverse effects were similar to control. [13] These are lab markers, not symptoms or outcomes. See Selenium and the thyroid. | Promising but limited |
| Fertility, immunity or general "antioxidant" support in people with adequate intake | Proper evidence not found. We read NHS and MedlinePlus pages and searched PubMed; the reviews found did not cover these uses. | Marketing claim |
MedlinePlus says taking a selenium supplement in addition to food is not currently recommended for cancer prevention or protection from heavy metals. [2] In the largest trial, SELECT, selenium also increased alopecia and dermatitis. [7] Trials in one population (SELECT enrolled men aged 50 or 55 and older) do not transfer to others. [8]
Too much: selenosis and limits
The NHS says too much selenium causes selenosis, which in its mildest form can lead to loss of hair and nails. [1] MedlinePlus lists hair loss, nail problems, nausea, irritability, fatigue and mild nerve damage, and says toxicity is rare in the US. [2] Combination products should be added up; see amounts and duplicate ingredients.Forms, labels and quality
Selenium appears in multivitamins and stand-alone products in several chemical forms. We did not find a source in what we read showing one form is better for health outcomes: Proper evidence not found. Check micrograms of selenium per serving; see reading supplement labels and quality testing.Medicines, life stages and conditions
- Medicines. No specific medicine interaction was found in the sources we read, which is not proof that none exists. Review all supplements with a pharmacist; see medication interactions.
- Pregnancy and breastfeeding. The US RDA is higher (60 and 70 mcg). [2] Ask your midwife or doctor about multivitamins; see Pregnancy and breastfeeding.
- Children and older adults. EFSA sets lower upper levels for children (table above). [5] Ask a pediatrician before giving any supplement.
- Kidney or liver conditions, and before surgery. Not addressed in the sources we read; that is not proof of safety. Tell the team about every supplement; see supplements before surgery.
Frequently asked questions
Is a Brazil nut a day too much selenium?
The sources we read do not state selenium per nut, and one describes Brazil nut selenium as highly variable. [3] Check your total against the limits in the table (NHS 350 mcg for supplements, EFSA 255 mcg from all sources). [1][5]
Can selenium prevent cancer?
A Cochrane review found no reduction in overall cancer incidence in the best-quality trials. [7]
What are signs of too much selenium?
Hair and nail loss in mild cases, and in other cases nausea, irritability, fatigue and mild nerve damage. [1][2] Contact a clinician if you notice these while taking a supplement.
Does selenium help an autoimmune thyroid condition?
A 2024 meta-analysis found lower TSH and antibody markers in Hashimoto thyroiditis, but these are lab markers; ask the clinician managing your thyroid. [13]
Do I need a selenium supplement?
The NHS says if you eat meat, fish or nuts you should be able to get all the selenium you need from your daily diet. [1]
References
- NHS (UK). Vitamins and minerals: Others (chromium, molybdenum, selenium and more; page last reviewed 3 August 2020). Link. Read 2026-10-23.
- MedlinePlus (US National Library of Medicine). Selenium in diet (review date 21 January 2025). Link. Read 2026-10-23.
- Improving the selenium supply of vegans and omnivores with Brazil nut butter compared to a dietary supplement in a randomized controlled trial. Eur J Nutr. 2025. Link. Read 2026-10-23.
- Radium levels in Brazil nuts: A review of the literature. Nutr Bull. 2025. Link. Read 2026-10-23.
- European Food Safety Authority. Overview on Tolerable Upper Intake Levels as derived by the SCF and the EFSA NDA Panel, version 11 (August 2025). Link. Read 2026-10-23.
- Health Canada. Dietary reference intakes: reference values for elements (tables). Link. Read 2026-10-23.
- Vinceti M et al. Selenium for preventing cancer. Cochrane Database Syst Rev. 2018. Link. Read 2026-10-23.
- Lippman SM 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. Link. Read 2026-10-23.
- Klein EA et al. Vitamin E and the risk of prostate cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2011. Link. Read 2026-10-23.
- Selenium exposure and the risk of type 2 diabetes: a systematic review and meta-analysis. Eur J Epidemiol. 2018. Link. Read 2026-10-23.
- Selenium supplementation and the risk of type 2 diabetes mellitus: a meta-analysis of randomized controlled trials. Endocrine. 2014. Link. Read 2026-10-23.
- An P et al. Micronutrient Supplementation to Reduce Cardiovascular Risk. J Am Coll Cardiol. 2022. Link. Read 2026-10-23.
- Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials. Thyroid. 2024. Link. Read 2026-10-23.