Selenium
Also known as: Se
Medically reviewed by Nano Health Insights Editorial Team · Last reviewed 2026-06-29
Selenium is an essential trace mineral; the adult RDA is 55 mcg/day and too little or too much can both cause harm.
What it is
Selenium is an essential trace mineral; the adult RDA is 55 mcg/day and too little or too much can both cause harm. It is needed in very small amounts, but it is built into at least 25 human selenoproteins that support thyroid hormone metabolism, antioxidant defense, reproduction, DNA synthesis, and immune function. Selenium is naturally present in foods, added to some products, and sold as supplements. The tolerable upper intake level for adults is 400 mcg/day, which matters because chronic excess can cause toxicity.
Selenium intake varies widely because the selenium content of plant foods depends heavily on the soil where they are grown. In India, this means selenium intake can differ by region and by dietary pattern. People who eat a varied diet usually get enough, but deficiency can occur in settings of poor intake, malabsorption, or long-term specialized nutrition without adequate selenium.
Common food sources include seafood, meat, eggs, dairy, cereals, and nuts. Brazil nuts are often highlighted because they can contain very large amounts, but the selenium content is highly variable, so they are not a precise or consistently safe way to supplement.
How it works
Selenium works mainly through selenoproteins, which contain the amino acid selenocysteine. Important examples include glutathione peroxidases, thioredoxin reductases, and selenoprotein P.
These proteins help with several core functions:
- Antioxidant defense: Selenium-dependent enzymes help limit oxidative damage by reducing peroxides and supporting cellular redox balance.
- Thyroid function: Selenium is involved in enzymes that activate and deactivate thyroid hormones. The thyroid contains relatively high selenium concentrations.
- Immune function: Selenium affects immune cell signaling and responses to infection, although the clinical meaning of supplementation in people who are not deficient is less clear.
- Reproduction and DNA synthesis: Selenium has roles in sperm function and in basic cellular processes.
Selenium appears in foods and supplements in different chemical forms. These forms are absorbed well, but they are handled somewhat differently by the body.
| Form | Where found | Notes |
|---|---|---|
| Selenomethionine | Many foods, many supplements | Organic form; commonly used in supplements |
| Selenocysteine | Foods of animal origin, selenoproteins | Functional form incorporated into proteins |
| Selenite / selenate | Some supplements, environment | Inorganic forms |
After absorption, selenium is used to make selenoproteins or is excreted, mainly in urine, to help maintain balance.
Evidence and uses
The clearest use of selenium is prevention and treatment of deficiency. In people with low selenium status, restoring adequate intake supports normal enzyme function and overall health. Severe deficiency has been linked to specific disorders in low-selenium regions, including Keshan disease, a cardiomyopathy described in parts of China.
For the general population, the evidence is more mixed. Selenium is essential, but more is not necessarily better.
Where evidence is strongest
- Meeting nutritional requirements: Strong evidence supports selenium as an essential nutrient.
- Correcting deficiency: Strong evidence supports supplementation when deficiency is present or likely.
- Parenteral or medical nutrition: Selenium is included in clinical nutrition formulas because deficiency can develop without it.
Where evidence is mixed or limited
- Cancer prevention: Large trials have not shown a consistent benefit of selenium supplements for preventing cancer in generally well-nourished adults. Some studies have raised concern about possible harms in certain groups.
- Cardiovascular disease: Evidence does not support routine selenium supplementation for heart disease prevention in the general population.
- Thyroid disease: Selenium has been studied in autoimmune thyroid conditions such as Hashimoto thyroiditis and Graves orbitopathy. Some trials show changes in antibody levels or symptoms, but results are inconsistent, and routine use is not universally recommended.
- Infections and immunity: Selenium status matters for immune function, but supplementation beyond adequate intake has not shown clear broad benefit in healthy people.
Recommended dietary allowances from the NIH Office of Dietary Supplements are:
| Group | RDA |
|---|---|
| Adults 19+ years | 55 mcg/day |
| Pregnancy | 60 mcg/day |
| Lactation | 70 mcg/day |
In practice, food-first intake is preferred unless a clinician identifies deficiency, poor intake, malabsorption, or another reason to supplement.
Safety and interactions
Selenium is safe within recommended amounts, but chronic high intake can cause selenosis, or selenium toxicity. The adult upper limit is 400 mcg/day from all sources.
Possible signs and symptoms of excess include:
- Garlic-like breath odor
- Nausea or diarrhea
- Brittle hair or hair loss
- Brittle or discolored nails
- Skin rash
- Irritability or fatigue
- Nervous system abnormalities in more severe cases
Supplement labels may list selenium in micrograms, and some multivitamins already contain it. Taking multiple products at once can push total intake too high. Brazil nuts can also contribute large, unpredictable amounts.
Potential interactions are not as prominent as with some minerals, but selenium can still matter in clinical care. Chemotherapy regimens, dialysis, gastrointestinal disease, and long-term parenteral nutrition can affect selenium status or how supplementation is used. People with thyroid disease should not self-treat with selenium instead of standard care. If you take supplements regularly, especially combination products, it is sensible to review them with a clinician or pharmacist.
When to see a clinician
Consider medical advice if you have symptoms or risk factors that could suggest low or high selenium status, especially if you use supplements.
Examples include:
- Unexplained hair loss or brittle nails while taking selenium products
- Long-term digestive disorders that impair absorption
- Kidney dialysis or long-term medical nutrition
- Thyroid disease and questions about whether selenium is appropriate
- Use of several supplements that may overlap in selenium content
- Pregnancy or breastfeeding if you are considering supplementation beyond a standard prenatal product
A clinician can help decide whether diet changes are enough or whether testing and targeted supplementation make sense.
Limitations and open questions
Selenium research is complicated because the gap between adequate intake and excessive intake is narrower than for many nutrients, and baseline selenium status differs across populations. A supplement that helps a deficient group may do nothing, or even cause harm, in a selenium-replete group.
Another challenge is measurement. Serum or plasma selenium is commonly used, and concentrations of about 8 mcg/dL or higher are often considered sufficient in healthy people, but no single test perfectly captures long-term status or functional adequacy. Hair and nail selenium can reflect longer-term intake, while functional markers such as glutathione peroxidase 3 and selenoprotein P can be influenced by inflammation and other factors.
Evidence in humans is limited or mixed for many proposed uses outside deficiency correction, especially cancer prevention, routine immune support, and broad anti-aging claims. The best-supported message is simple: selenium is essential, deficiency should be corrected, and routine high-dose supplementation is not a proven shortcut to better health.
FAQs
What does selenium do in the body?
Selenium is part of at least 25 selenoproteins that help with antioxidant defense, thyroid hormone metabolism, reproduction, and DNA synthesis. It is especially important for enzymes such as glutathione peroxidases and thioredoxin reductases. Because it is a trace mineral, the body needs only small amounts, but deficiency or excess can both cause problems.
How much selenium do adults need each day?
The recommended dietary allowance for most adults is 55 mcg per day. Needs rise to 60 mcg per day in pregnancy and 70 mcg per day during lactation. The tolerable upper intake level for adults is 400 mcg per day from all sources combined.
Which foods are good sources of selenium?
Seafood, meat, eggs, dairy products, cereals, and some nuts provide selenium. Brazil nuts can be very high in selenium, but the amount varies a lot depending on where they were grown, so they are not a precise dosing method. In plant foods, selenium content depends strongly on soil levels, which can vary by region, including within India.
Can selenium supplements help thyroid problems?
Selenium is involved in thyroid hormone metabolism, so researchers have studied it in conditions such as Hashimoto thyroiditis and Graves orbitopathy. Some trials show modest changes in thyroid antibodies or symptoms, but results are inconsistent and routine supplementation is not recommended for everyone with thyroid disease. People with thyroid conditions should discuss selenium with their clinician rather than self-prescribing.
Can you take too much selenium?
Yes. Chronic excess can cause selenosis, and the adult upper limit is 400 mcg per day. Warning signs include garlic-like breath, nausea, diarrhea, hair loss, and brittle nails. Risk rises when people combine a multivitamin, a separate selenium supplement, and high-selenium foods such as Brazil nuts without checking the total amount.