What is selenium and what does it do?

Selenium is an essential trace mineral used to build selenoproteins that support antioxidant defense, thyroid hormone metabolism, and immune function.

What foods are high in selenium?

Brazil nuts are a concentrated source, and selenium is also found in seafood, meats, eggs, dairy, and grains (amounts vary by soil content).

How much selenium should I take per day?

Many adults aim around 55 mcg/day from diet and supplements combined, and supplemental intakes are often kept within about 55–200 mcg/day unless a clinician advises otherwise.

What are signs of selenium deficiency?

Deficiency can contribute to impaired thyroid function and weakened immune response, and may be more likely with very low dietary intake or certain medical conditions.

Can you take too much selenium?

Yes. Excess intake can cause selenium toxicity (selenosis), with symptoms such as gastrointestinal upset, hair or nail changes, and a garlic-like breath odor; avoid high-dose use without medical guidance.

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Selenium

Moderate study situation·7/10

A higher score indicates better study conditions, but not necessarily effectiveness.

Selenium is an essential trace mineral crucial for antioxidant defense, thyroid function, and immune health.

Cancer-inhibitor Essential Nutrient Antioxidant Immune Booster

dose range

55–200 µg

Once daily or split with meals.

Consumption

Take with or without food; taken with food may improve absorption slightly and reduce GI discomfort.

Not fat-soluble.

Best time

Morning or with main meals is common.

Description

Selenium is an essential micronutrient that acts as a cofactor for selenoproteins with antioxidant, thyroid, and immune regulatory functions. It is naturally found in foods such as Brazil nuts, seafood, and grains. Both deficiency and excess can have health consequences, making appropriate intake important.

benefits

Main effects with level of evidence

Moderate

Note on evidence

Evidence for selenium's role in thyroid health and immune support is moderate; evidence for cancer prevention is mixed.

Supports Thyroid Health

Strong

Selenium is vital for the synthesis and metabolism of thyroid hormones and helps protect the thyroid gland from oxidative damage.

Particularly relevant for: People with thyroid disorders · Adults with low dietary selenium

S1 · American Journal of Clinical Nutrition S2 · NIH Office of Dietary Supplements (ODS) Selenium Fact Sheet S5 · EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on Dietary Reference Values for selenium

Antioxidant Protection

Moderate

As a component of selenoproteins such as glutathione peroxidase, selenium helps neutralize harmful free radicals, supporting cellular and DNA integrity.

Particularly relevant for: Adults · Older adults

S2 · NIH Office of Dietary Supplements (ODS) Selenium Fact Sheet S6 · Examine.com – Selenium

Immune System Support

Moderate

Adequate selenium is associated with optimal immune response, as it modulates inflammation and enhances the function of white blood cells.

Particularly relevant for: People at risk of infection · Older adults

S2 · NIH Office of Dietary Supplements (ODS) Selenium Fact Sheet

May Lower Risk of Some Cancers (Mixed Evidence)

Mixed

Observational studies suggest that higher selenium status may lower the risk of certain cancers (e.g., prostate, lung, colorectal), but data from supplementation trials are inconsistent.

Particularly relevant for: General population

S3 · British Journal of Nutrition S2 · NIH Office of Dietary Supplements (ODS) Selenium Fact Sheet

Safety & Information

Important safety information & known risks

For educational purposes only • not medical advice

Selenium is safe at recommended dosages, but excess intake can cause toxicity; use caution with chronic supplementation or if in a high-risk group.

warning notices

High selenium intake (>400 mcg/day) can cause selenosis with symptoms such as gastrointestinal distress, hair loss, nail brittleness, and neurological abnormalities.
Individuals with kidney dysfunction, certain skin conditions, or those taking anticoagulants should consult a healthcare provider before supplementing.
Selenium supplements should be avoided or used with caution in individuals with known hypersensitivity to selenium compounds.
Do not exceed 400 mcg of selenium per day to avoid risk of selenosis.
Avoid selenium supplementation if you have known hypersensitivity to selenium compounds.
Use selenium supplements only under medical supervision if you are taking anticoagulants, undergoing chemotherapy, or have severe skin conditions.
Monitor selenium intake in individuals with chronic kidney disease or iodine deficiency.

Known interactions with medications

Anticoagulants (increased bleeding risk)
Chemotherapy drugs (potential altered efficacy/toxicity)

Contraindications / special precautions

Known hypersensitivity to selenium or selenium compounds
Acute selenosis or chronic excessive selenium intake

interactions

Important interactions between active ingredient ↔ supplement / medication / food

Selenium interacts with certain drugs and nutrients, potentially altering absorption or effects.

Selenium may increase bleeding risk when taken with anticoagulant medications.

drug · Anticoagulants

CautionModerate

Why

Some forms of selenium may influence blood clotting or platelet function.

recommendation

Use only with clinician supervision if taking anticoagulants.

S2 · NIH Office of Dietary Supplements (ODS) Selenium Fact Sheet S4 · WebMD Selenium Monograph

Selenium may interact with chemotherapy drugs and affect treatment efficacy or toxicity.

drug · Chemotherapy drugs

CautionLimited

Why

Possible interactions between selenium and some chemotherapeutic agents have been reported; clinical significance is unclear.

recommendation

Consult an oncologist before selenium supplementation during chemotherapy.

S2 · NIH Office of Dietary Supplements (ODS) Selenium Fact Sheet

Selenium status influences iodine metabolism and thyroid hormone synthesis.

nutrient · Iodine

MonitorModerateIodine

Why

Adequate selenium is necessary for proper thyroid hormone metabolism, especially in areas of marginal iodine intake.

recommendation

Ensure proper intake of both nutrients, especially in regions with iodine deficiency.

S1 · American Journal of Clinical Nutrition S2 · NIH Office of Dietary Supplements (ODS) Selenium Fact Sheet

mechanisms of action

Brief explanations of how it works

Cofactor in selenoproteins for antioxidant defense

Selenium acts as a cofactor for glutathione peroxidase and other enzymes that reduce cellular oxidative stress, protecting lipids, proteins, and DNA from damage.

S2 · NIH Office of Dietary Supplements (ODS) Selenium Fact Sheet S6 · Examine.com – Selenium