What selenium is and why your body needs it

Selenium is a mineral your body uses to build proteins called selenoproteins, which do two main jobs: they protect your cells from damage and they help your thyroid work properly. You need only a small amount—about 55 micrograms a day for adults—but that small amount matters for how your immune system responds to infection and how your metabolism runs.

Your body cannot make selenium on its own, so it has to come from food or supplements. Unlike some minerals that pile up in your body if you take too much, selenium gets used up or excreted, which means you need a steady supply. Most people in North America get enough selenium from their diet without thinking about it, but some people—particularly those with certain digestive conditions or on very restricted diets—may not get enough.

Key Takeaways

  • Selenium helps your body build proteins that protect cells and support thyroid function, and you need about 55 micrograms daily.
  • Common food sources include Brazil nuts, seafood, eggs, chicken, and whole grains, and most people meet their needs through regular eating.
  • Too little selenium may affect thyroid health and immune response, while too much can cause nausea, hair loss, and nerve damage.
  • Your doctor can test your selenium level if you have symptoms of deficiency or take certain medications that affect mineral absorption.

How selenium protects your cells and supports your thyroid

Selenium works as part of selenoproteins, which act like bodyguards for your cells. One major selenoprotein is glutathione peroxidase, an enzyme that stops harmful molecules called free radicals from damaging your cell membranes and DNA. This protection matters especially in tissues that use a lot of oxygen and generate a lot of waste—your heart, brain, and immune cells.

Your thyroid gland is packed with selenoproteins. The thyroid makes hormones that control how fast your body burns calories and how your nervous system responds. Selenoproteins help the thyroid make those hormones and also protect the gland itself from oxidative stress. If your selenium level drops too low, your thyroid can struggle to work efficiently, which may slow your metabolism and affect your energy and mood.

Selenium also plays a role in how your immune system responds to viruses and bacteria. It helps immune cells communicate and mount an effective response without overreacting. This is why selenium deficiency has been linked to slower immune recovery in some studies, though getting enough selenium does not prevent illness on its own.

Foods that contain selenium

Brazil nuts are the most concentrated source of selenium—just two or three nuts can provide a full day's worth. However, the amount varies depending on where the nuts were grown, because selenium content in plants depends on how much selenium is in the soil. Seafood is another reliable source: oysters, clams, tuna, and halibut all contain significant amounts.

Eggs, chicken, turkey, and pork contain moderate amounts of selenium. Whole grains like brown rice, oats, and whole wheat bread also contribute selenium, as do legumes like lentils and chickpeas. Dairy products like yogurt and cheese have smaller amounts. If you eat a mix of these foods regularly, you are likely meeting your daily need without tracking it.

The amount of selenium in plant-based foods depends on the soil where they were grown. Regions with selenium-rich soil produce more selenium-dense vegetables and grains. This is why people in some parts of the world are more likely to have low selenium levels—not because they eat differently, but because their local soil contains less of the mineral.

What happens when you do not get enough selenium

Selenium deficiency is uncommon in developed countries but does occur in people with certain digestive disorders like Crohn's disease or celiac disease, which interfere with nutrient absorption. It can also happen to people on long-term dialysis or those who have had weight-loss surgery. Deficiency develops slowly because your body stores some selenium in muscle tissue and can draw on those reserves for a while.

Early signs of low selenium include fatigue, muscle weakness, and mood changes. Over time, deficiency can contribute to thyroid problems, slower immune response, and in severe cases, a condition called Keshan disease, which affects the heart. Keshan disease is rare in the United States but has been documented in regions where soil selenium is very low and people rely on locally grown food.

If you have a condition that affects nutrient absorption, take medications that interfere with mineral uptake, or follow a very restricted diet, talk with your doctor about whether your selenium level should be checked. A straightforward blood test can measure your selenium status.

Getting too much selenium and what that means

Selenium toxicity is rare from food alone because you would have to eat enormous amounts of Brazil nuts or seafood to overdose. However, it can happen if you take high-dose supplements without medical supervision. The upper safe limit for adults is 400 micrograms per day—more than seven times the recommended amount.

Symptoms of too much selenium include nausea, vomiting, hair loss, brittle nails, a garlic-like taste in your mouth, and nerve damage if exposure is prolonged. These symptoms usually go away once you stop taking excess selenium. If you are considering a selenium supplement, check the dose on the label and talk with your doctor first, especially if you take other supplements or have a medical condition.

Selenium supplements: when they might matter

Most people do not need a selenium supplement because food provides enough. However, your doctor might recommend one if you have a digestive condition that prevents absorption, if blood tests show your level is low, or if you follow a diet that excludes most selenium-containing foods. Supplements come as selenomethionine (the form found in plants) or sodium selenite (a synthetic form), and both are absorbed by your body.

If you take a supplement, a typical dose is 50 to 200 micrograms daily, depending on your needs. Taking more than this without medical supervision increases the risk of toxicity. Supplements work best when taken with food, and you should tell your doctor about any supplements you take, because selenium can interact with certain medications and other nutrients.

Selenium and specific health conditions

Research has looked at whether selenium might help with thyroid disease, heart health, and cancer prevention. For thyroid disease, studies show that people with autoimmune thyroid conditions may benefit from adequate selenium, though supplements have not been proven to reverse existing thyroid disease. If you have thyroid disease, your doctor can advise whether your selenium level should be checked.

For heart health, selenium is part of selenoproteins that protect heart tissue from oxidative damage. Some studies suggest that adequate selenium may support cardiovascular function, but this does not mean supplements prevent heart disease. For cancer prevention, the evidence is mixed—some older studies suggested selenium might lower cancer risk, but more recent research has not confirmed this, and high-dose supplements are not recommended for this purpose.

The takeaway is that selenium matters for normal body function, but getting enough through food is the goal. If you have a specific health condition and wonder whether your selenium level matters, your doctor is the right person to ask.

Frequently Asked Questions

How do I know if I am not getting enough selenium?

Deficiency symptoms include fatigue, muscle weakness, mood changes, and hair loss, but these can have many causes. The only way to know for sure is a blood test ordered by your doctor. Most people in North America get enough selenium from food, so testing is usually recommended only if you have a digestive disorder, have had weight-loss surgery, or follow a very restricted diet.

Are Brazil nuts a safe way to get selenium every day?

Two to three Brazil nuts provide a full day's selenium, but the amount varies by where they were grown. Eating a few nuts most days is safe, but eating a large handful daily could push you toward the upper limit. Rotating Brazil nuts with other selenium sources like seafood and eggs is a safer approach.

Can selenium supplements prevent thyroid disease?

Supplements cannot prevent thyroid disease, but adequate selenium supports normal thyroid function. If you have an autoimmune thyroid condition, your doctor may check your selenium level, but supplements are not a standard treatment. Getting enough selenium through food is the best approach for thyroid health.

What is the difference between selenomethionine and sodium selenite supplements?

Selenomethionine is the form found naturally in plants and is absorbed slightly better by your body. Sodium selenite is synthetic but also effective. Both forms work, and your body uses them similarly. The choice between them matters less than taking the right dose and talking with your doctor first.

Can I get too much selenium from eating seafood and eggs?

No. You would need to eat extreme amounts of these foods daily to reach toxic levels. Toxicity from food alone is essentially impossible. The risk comes from taking high-dose supplements without medical supervision.