Selenium supports thyroid function and may prevent conditions like hypothyroidism.

Selenium may boost the immune system by increasing T-cells and natural killer cells.

Selenium can help manage asthma by reducing oxidative stress.

Selenium provides various benefits, such as protecting thyroid and reproductive health. It's found naturally in the soil and many foods, such as Brazil nuts, and is also available as a dietary supplement. Selenium is an essential mineral, meaning it performs many important functions, like boosting the immune system, guarding against infertility and cognitive decline, and improving thyroid function. 

Certain groups may be at risk for selenium deficiency, which can cause health complications. Taking selenium supplements may have some side effects, so taking a dose right for you is important.

Selenium plays a key role in the proper function of the thyroid, an endocrine gland that affects metabolism, growth, and energy levels.

This may be because selenium boosts the activity of thyroxine 5-deiodinase, an enzyme that causes the thyroid gland to release iodine. A deficiency in selenium or iodine may lead to hypothyroidism (underactive thyroid), a condition that causes fatigue, depression, unwanted weight gain, and other symptoms.

Selenium has been found to have antioxidant and anti-inflammatory properties. This means it may help to boost the immune system, lower the risk of infection, and reduce the effects of oxidative stress due to the circulation of free radicals in the body. 

Studies also suggest sufficient selenium levels are linked to higher amounts of T-cells, or lymphocytes—a certain kind of white blood cells that fight off infection. For example, people who have contracted human immunodeficiency virus (HIV) experience a decline in T-cells, which impairs the immune system. Some research has shown people with HIV are more likely to develop acquired immunodeficiency syndrome (AIDS) if they have a selenium deficiency.

Selenium also may increase the activity of the body’s natural killer (NK) cells, which fight off pathogens and destroy virally infected cells, such as cancer cells. 

Research indicates selenium deficiency is associated with a significantly higher risk of prostate cancer, as well as colon and breast cancer. However, no evidence proves that increasing selenium intake prevents cancer in humans.

For some people with asthma, selenium supplementation could help to reduce the severity of symptoms.

Studies have found lower selenium concentrations in the blood are associated with more asthma attacks and greater inhaler use. This has led some researchers to argue that selenium’s antioxidant effects and ability to reduce oxidative stress could help manage asthma symptoms and prevent respiratory distress.

There is a growing body of evidence to suggest selenium plays a role in promoting fertility and reproductive health, as well as healthy pregnancy outcomes.

One systematic review found higher serum (blood) selenium concentrations were linked to various positive markers of female fertility, such as higher numbers of oocytes (immature egg cells) and ovarian follicles. 

Other studies have found selenium deficiency may increase the risk of some pregnancy complications, including miscarriage, fetal growth restriction, premature birth, preeclampsia (high blood pressure during pregnancy), low birth weight, and gestational diabetes. Some healthcare providers prescribe selenium supplements during pregnancy to lower the chance of these complications, especially in people who may have a higher risk.

Selenium may have a neuroprotective effect due to its ability to reduce inflammation and oxidative stress. Inflammation often contributes to the development of Alzheimer’s disease and other forms of dementia. 

A review of studies found selenium supplementation improved cognitive test results among both people with mild cognitive impairment (MCI) and Alzheimer’s disease.

The recommended dietary allowance (RDA) of selenium is typically 55 micrograms (mcg) per day for adults. People who are pregnant or breastfeeding should get 60-70 mcg of selenium in their diet each day. 

Many foods are naturally high in selenium, especially Brazil nuts, meat, and seafood. Common food sources of selenium include:

Brazil nuts: 544 mcg per ounce (6-8 nuts), or 989% of your daily value (DV)

Yellowfin tuna: 92 mcg per 3-ounce serving, or 167% DV

Halibut: 47 mcg per 3-ounce serving, or 85% DV

Sardines: 45 mcg per 3-ounce serving, or 82% DV

Ham: 42 mcg per 3-ounce serving, or 76% DV

Shrimp: 40 mcg per 3-ounce serving, or 73% DV

Cooked macaroni: 37 mcg per cup, or 67% DV

Beef steak: 33 mcg per 3-ounce serving, or 60% DV

Turkey: 31 mcg per 3-ounce serving, or 56% DV

Chicken: 28 mcg per 3-ounce serving, or 40% DV

If your healthcare provider recommends a selenium supplement, you can find it at many pharmacies, drugstores, and health food stores. As a supplement, selenium is available both as a multivitamin and by itself.

You may need to take a selenium supplement if you are deficient in the mineral. Selenium deficiency can lead to a number of health complications, such as:

However, selenium deficiency is relatively rare in North America. Most people in the United States get an adequate amount of selenium from their diet and don’t need selenium supplements. 

However, you may be more at risk of selenium deficiency if you:

To avoid selenium deficiency, it's important to consume around the recommended daily allowance (RDA) of selenium for your age group. The RDA for both men and women is as follows:

Infants ages 0-6 months: 15 mcg

Infants ages 7-12 months: 20 mcg

Children ages 1-3: 20 mcg

Children ages 4-8: 30 mcg

Children ages 9-13: 40 mcg

Teens ages 14-18: 55 mcg

Adults: 55 mcg

Whether you’re getting selenium from food or supplements, ingesting excessive amounts can be toxic. For example, eating too many Brazil nuts (which are very rich in selenium) can lead to selenium toxicity. 

Before taking a selenium supplement, talk to your healthcare provider. They can let you know if an increased selenium intake is safe for you based on your individual health.

Certain medications may impact selenium levels, and supplemental selenium may affect some drugs. For example, the chemotherapy drug cisplatin (Platinol)—often used to treat ovarian, bladder, lung, and other cancers—can lower selenium levels. The reason for this interaction is unclear.

Talk to your healthcare provider about any other medications or supplements you’re currently taking, and if they might interact with selenium supplements.

Selenium is often listed on supplement ingredient labels as sodium selenate or selenomethionine.

Like other dietary supplements, selenium is not regulated by the FDA. Look for supplements that are tested by reputable third parties. Testing organizations like U.S. Pharmacopeia and ConsumerLab verify that supplement products contain the ingredients listed and don't contain a harmful level of contaminants. Check for the third-party organization's seal on the bottle.

When consumed in very high amounts, selenium can cause a number of health problems. Based on age, the daily upper limit of selenium that you should consume from food and supplements is as follows:

Infants ages 0-6 months: 45 mcg

Infants ages 6-12 months: 60 mcg

Children ages 1-3: 90 mcg

Children ages 4-8: 150 mcg

Children ages 9-13: 280 mcg

Teens ages 14-18: 400 mcg

Adults: 400 mcg

Consuming an excessive amount of selenium regularly can cause the following side effects:

Damaged or brittle hair and nails

Diarrhea

Discolored teeth

Fatigue

Facial flushing

Foul-smelling breath, resembling garlic breath

Metallic taste in the mouth

Nausea and vomiting

Irritability and mood swings

Rashes

In severe cases, selenium toxicity can cause serious and even fatal health complications, such as:

Selenium is a natural mineral that plays an important role in many of the body’s most essential functions. For example, research suggests selenium helps to protect respiratory function, fertility, reproductive health, thyroid function, and the body’s ability to fight off infection.

Most people in the U.S. don’t need to take selenium supplements. However, you should talk to your healthcare provider if you think you may have a selenium deficiency. They may recommend consuming more selenium as part of your diet or as a nutritional supplement.

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10 Sources

Health.com uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

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