Selenium: What Your Levels Mean

What Is Selenium?

If your clinician has mentioned selenium, you may be wondering what it is and why it matters. Selenium is a trace mineral your body needs in small amounts to work normally. You get it from food. Dietary sources include Brazil nuts, seafood, organ meats, eggs, poultry, and grains. The amount in plant foods depends heavily on the selenium in the soil where they grow, so your intake can vary by region and food source.

Your body only needs modest amounts, yet selenium plays an outsized role in several core biological processes. It is built into proteins called selenoproteins, which work as antioxidant and metabolic enzymes.

Why Selenium Matters in the Body

Selenium is best known as a building block of glutathione peroxidase enzymes, which help protect your cells from oxidative damage. It also supports other key functions:

• Thyroid hormone metabolism: Selenium-dependent enzymes (deiodinases) help convert the thyroid hormone T4 into its more active form, T3.
• Antioxidant defense: Selenoproteins help neutralize reactive oxygen species that can damage tissues over time.
• Immune function: Enough selenium supports normal immune responses.
• Reproductive health: Selenium contributes to normal sperm development and other reproductive processes.

Because of its ties to thyroid function, oxidative stress, and overall cellular health, selenium is sometimes reviewed as part of a broader longevity and preventive-care workup.

Why Selenium Levels Are Tested

A selenium test is not part of routine bloodwork for most people. Your clinician may consider checking it in specific situations, such as:

• Suspected deficiency in people with malabsorption conditions, long-term parenteral (IV) nutrition, or very restricted diets
• Evaluation of certain thyroid or muscle symptoms when deficiency is a consideration
• Monitoring people taking high-dose selenium supplements, where excess is a concern
• Suspected toxicity from over-supplementation or occupational exposure

Selenium status can be measured in several ways, including serum or plasma selenium and, in some labs, red blood cell selenium or glutathione peroxidase activity. Each one reflects a slightly different aspect of selenium status, which is one reason a clinician should interpret your results.

Understanding Your Results

Selenium is most often reported from serum or plasma in micrograms per liter (mcg/L), and sometimes in micromoles per liter (mcmol/L). A frequently cited adult reference range for serum or plasma selenium is roughly 70 to 150 mcg/L, but this varies meaningfully between laboratories and testing methods.

Reference ranges are assay-dependent. Different labs use different equipment and population data, so always compare your result to the specific range printed on your own report rather than to a general figure. A value that is “normal” at one lab may fall outside the range at another.

Low Selenium

Low selenium is uncommon in regions with selenium-rich soil and varied diets, but it can happen with malabsorption, severe dietary restriction, or long-term IV nutrition without enough supplementation. Severe, prolonged deficiency has historically been linked to specific conditions affecting the heart muscle and joints in selenium-poor regions. More general signs that may prompt evaluation can include fatigue, muscle weakness, or thyroid-related symptoms, though these are nonspecific and have many possible causes.

High Selenium

High selenium most often reflects too much intake, usually from high-dose supplements rather than food. Chronic excess, sometimes called selenosis, can be linked to symptoms such as a garlic-like odor on the breath, brittle or changed nails and hair, hair loss, gastrointestinal upset, irritability, and skin changes. Because selenium has a relatively narrow window between adequate and excessive intake, taking more than recommended amounts is generally discouraged without clinical guidance.

How to Support Healthy Selenium Levels

For most people, a balanced diet provides enough selenium without supplements. Whole-food sources such as seafood, eggs, poultry, and a small amount of Brazil nuts can add meaningfully to your intake. Brazil nuts in particular are very concentrated, so a little goes a long way.

If you are thinking about a selenium supplement, talk with a clinician first, especially if you already eat selenium-rich foods or take a multivitamin, because intake can add up quickly. Decisions about testing, supplementing, or adjusting your diet should fit your full health picture.

Next Steps

Selenium is one of many markers that can offer insight into your nutritional status, thyroid health, and overall wellness. Reviewing it alongside your symptoms and other labs gives the most useful picture. At ENNU Life in Louisville, Kentucky, our team provides personalized, preventive-focused care for patients across the Louisville metro and Kentucky.

To understand which markers may be relevant for you, start with our health assessment.

Educational only, not medical advice; consult a licensed clinician.