Selenium is a trace mineral that your body uses to build a family of specialized proteins, called selenoproteins, involved in antioxidant defense, thyroid hormone activation, immune function, fertility, and brain health. Adults need just 55 micrograms per day, a tiny amount that powers at least 25 different selenoproteins across nearly every tissue in your body.
How Selenium Protects Your Cells
Selenium’s most fundamental job is antioxidant defense. It sits in the active site of enzymes called glutathione peroxidases, which neutralize hydrogen peroxide and other reactive molecules before they can damage your DNA, cell membranes, and proteins. The process works like a recycling system: the enzyme converts harmful peroxide into plain water, using a helper molecule called glutathione as fuel. Without enough selenium, this cleanup system slows down, and oxidative damage accumulates.
A second group of selenium-dependent enzymes, called thioredoxin reductases, recycles other small proteins that keep cells in a healthy, balanced state. These enzymes also influence how your genes are switched on and off by affecting certain transcription factors, the proteins that bind to DNA and control which genes are active. This is one reason selenium’s effects are so wide-ranging: it doesn’t just mop up damage, it helps regulate the underlying signals that keep cells functioning normally.
Selenium and Thyroid Hormone
Your thyroid gland produces a relatively inactive hormone (T4) that must be converted into its active form (T3) before your body can use it to regulate metabolism, energy, and temperature. The enzymes responsible for this conversion, called deiodinases, depend on selenium. Specifically, they rely on a selenium-containing amino acid at their core. When researchers replaced this selenium component with a sulfur-based alternative, enzyme activity dropped by a factor of 100. That single selenium atom makes the enzyme a far more effective catalyst, stripping an iodine atom from T4 to produce T3.
This is why people with low selenium levels sometimes experience symptoms that overlap with thyroid problems: fatigue, sluggishness, and difficulty regulating body temperature. The thyroid gland itself also contains high concentrations of selenium-based antioxidant enzymes, which protect it from the hydrogen peroxide generated during hormone production.
Immune Defense and Viral Infections
Selenium deficiency weakens the immune system on two fronts. First, lower antioxidant capacity means immune cells are less protected from the oxidative stress they generate while fighting infections. Second, low selenium status appears to increase the rate of mutations in certain viral genomes, potentially making viruses more harmful to the host.
Research on selenium supplementation and immune response shows real but nuanced results. In one study, participants supplemented with selenium for 10 weeks before receiving a flu vaccine showed changes in their immune cell activity compared to placebo. Higher doses (200 micrograms per day) boosted production of an anti-inflammatory signaling molecule and reduced a cytotoxic protein in certain immune cells, while a lower dose (50 micrograms per day) enhanced the killing capacity of those same cells. The takeaway: adequate selenium supports a balanced immune response, but more isn’t automatically better.
Male Fertility
Selenium plays a surprisingly specific role in sperm development. A selenium-dependent enzyme makes up over 50 percent of the structural capsule surrounding the mitochondria in the midpiece of mature sperm, the section that powers the tail’s movement. When selenium is deficient, this structure becomes unstable, directly impairing sperm motility.
Beyond structure, selenium protects sperm cells from oxidative DNA damage during their development. In a study of 69 infertile men given selenium supplements for three months, both supplemented groups showed significant improvements in sperm motility, though sperm count was unaffected. Selenium also participates in testosterone metabolism, adding another layer to its role in male reproductive health. Animal studies across multiple species have consistently linked selenium deficiency to impaired fertility, and supplementation has improved reproductive outcomes in sheep and mice.
Brain Health and Cognitive Function
The brain prioritizes selenium. Even during deficiency, your body redirects available selenium toward brain tissue, suggesting the mineral is critical for neurological function. Animal studies show that depleting certain brain selenoproteins causes irreversible damage, seizure susceptibility, impaired motor coordination, and cognitive decline.
In humans, the picture is still developing. Patients with Alzheimer’s disease show reduced activity of the antioxidant enzyme glutathione peroxidase in their blood. A systematic review found that selenium supplementation improved scores on cognitive tests in patients with both mild cognitive impairment and Alzheimer’s disease, alongside measurable increases in antioxidant enzyme activity. The connection likely runs through selenium’s antioxidant and anti-inflammatory properties, since elevated oxidative stress is thought to both cause and worsen Alzheimer’s.
What Happens When You Don’t Get Enough
Mild selenium deficiency may not cause obvious symptoms but quietly weakens antioxidant defenses, immune function, and thyroid hormone conversion. Severe deficiency, historically concentrated in regions of China and Siberia where soil selenium is extremely low, causes two recognized diseases. Keshan disease is a viral heart condition that damages the heart walls, primarily affecting children and young women. Kashin-Beck disease is a progressive joint and bone disorder that develops in growing children, gradually limiting mobility.
These diseases are rare outside selenium-poor regions, but subclinical deficiency is more common than many people realize, particularly in parts of Europe and among people with digestive conditions that impair nutrient absorption.
Food Sources and How Much You Need
The recommended daily intake for adults is 55 micrograms, regardless of age or sex. Most people in North America meet this through diet alone. The richest food sources include seafood, organ meats, poultry, eggs, and dairy. Grains and vegetables contain variable amounts depending on the selenium content of the soil where they were grown.
Brazil nuts are in a category of their own. A single Brazil nut contains 68 to 91 micrograms of selenium, meaning just one nut exceeds your entire daily requirement. Eating two or three a day consistently could push you toward the upper limit of safe intake.
Too Much Selenium: Selenosis
The tolerable upper intake level for adults is 400 micrograms per day. Consistently exceeding this can cause a condition called selenosis. The hallmark signs are hair loss, brittle or discolored nails, and neurological symptoms like numbness or tingling in the hands and feet. These symptoms typically develop from chronic overexposure rather than a single high dose.
The CDC has determined that 5 micrograms per kilogram of body weight per day would not cause adverse effects over a lifetime. For a 155-pound person, that works out to about 350 micrograms daily. Toxicity is almost always caused by supplement overuse or, in rare cases, eating large quantities of Brazil nuts regularly. If you eat a varied diet without high-dose selenium supplements, toxicity is unlikely.