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Selenium is an essential trace element, meaning your body requires it in very small amounts to function properly but cannot produce it on its own—you have to get it from your diet. Despite its “ultratrace” status, selenium plays an enormous role in health because it’s a key part of special proteins called selenoproteins. These proteins help your body handle oxidative stress, produce thyroid hormones, and regulate immune responses.
One of selenium’s most important jobs is to help make glutathione peroxidase, an antioxidant enzyme. Antioxidants are substances that neutralize free radicals—highly reactive molecules that can damage DNA, proteins, and cell membranes, contributing to aging and diseases like cancer or heart disease. By helping make antioxidant enzymes, selenium reduces this cellular damage and promotes longevity.
Selenium is also essential for thyroid function. It helps make iodothyronine deiodinases, which are enzymes that activate your main thyroid hormone (called T3, or triiodothyronine) by converting it from its storage form (called T4, or thyroxine). Without enough selenium, this conversion slows down, which can lead to symptoms of low thyroid function—such as fatigue, weight gain, sensitivity to cold, and poor mental clarity.
Your immune system also relies on selenium. It supports the production and activity of natural killer cells (which destroy infected or cancerous cells) and helps regulate inflammation, the body’s healing response to infection or injury. Some studies have shown that selenium deficiency can reduce the number and activity of CD4 cells, a type of white blood cell important for fighting infections—especially in people with HIV.
Although selenium deficiency is rare in developed countries, it can happen with poor nutrition, digestive disorders that impair nutrient absorption, or long-term IV feeding (total parenteral nutrition, or TPN). Deficiency can lead to Keshan disease, a form of heart failure first identified in regions of China where soil is selenium-poor. Other signs of deficiency include muscle weakness, a weakened immune system, mood changes, and pale or brittle nails.
Too much selenium—usually from supplements or from foods grown in selenium-rich soil—can be harmful. Known as selenosis, this condition causes symptoms like nausea, hair loss, fatigue, nerve pain, and brittle nails or teeth. There have been outbreaks of selenium poisoning in people who took poorly manufactured supplements with hundreds of times the recommended dose.
For adults, the recommended daily intake is 55 micrograms, and the tolerable upper intake level (the highest daily amount likely to be safe) is 400 micrograms. The therapeutic range is narrow—meaning the line between enough and too much is smaller than for most other minerals—so targeted supplementation is best used when a deficiency is confirmed or there is increased demand (e.g., during pregnancy or in thyroid conditions).
Selenium’s role in health goes beyond the thyroid and immune system. Some research suggests that it may help:
You can get selenium from whole foods, especially:
Selenium also exists in different forms. The ones naturally found in food—selenomethionine and selenocysteine—are efficiently absorbed. Supplement forms include selenite and selenate, which are inorganic but also well absorbed.
In topical form, selenium is used as selenium sulfide, commonly found in shampoos for treating dandruff, seborrheic dermatitis (a skin condition causing scaly patches), and tinea versicolor (a fungal skin infection). Topical selenium does not get absorbed into the bloodstream.