A selenium blood test measures the amount of selenium in your blood, most often in serum or plasma. Selenium is a trace mineral, which means your body needs only a small amount, but that small amount supports important enzymes involved in thyroid hormone metabolism, antioxidant defense, immune function, and reproduction. The test is not part of a routine checkup for most people. It is usually ordered when a clinician suspects low selenium from poor absorption, long-term parenteral nutrition, dialysis, severe digestive disease, or a restricted diet, or when excess selenium is possible from supplements, Brazil nuts, occupational exposure, or contaminated products.
A normal result does not prove that every selenium-dependent process is perfect, and an abnormal result does not diagnose one disease by itself. The result is most useful when it is interpreted with your diet, supplement use, symptoms, medical conditions, and the reference range from the laboratory that performed the test.
- Adult serum selenium is often listed around 110–165 mcg/L, but ranges vary by laboratory and specimen type.
- Serum or plasma selenium reflects recent selenium intake, while whole blood can better reflect longer-term status.
- Low selenium can occur with malabsorption, dialysis, long-term parenteral nutrition, very low intake, or severe illness.
- High selenium is most often linked to supplements, frequent high-selenium Brazil nut intake, exposure, or dosing errors.
- Possible selenium excess symptoms include garlic-like breath, metallic taste, hair loss, brittle nails, nausea, diarrhea, fatigue, and nerve symptoms.
- Do not treat a borderline selenium result with high-dose supplements unless a clinician confirms the cause and dose.
Table of Contents
- What the Selenium Blood Test Measures
- Selenium Blood Test Normal Range
- What Low Selenium Results Can Mean
- What High Selenium Results Can Mean
- What Can Affect Selenium Blood Test Results
- How to Prepare and What to Tell Your Clinician
- What to Do With Abnormal Selenium Results
What the Selenium Blood Test Measures
A selenium blood test measures selenium concentration in a blood sample. Most clinical labs report serum selenium or plasma selenium, although some specialized testing uses whole blood, red blood cells, urine, hair, or nails. For routine medical interpretation, serum and plasma are the most common.
Selenium is needed to make selenoproteins. These are proteins that contain selenium in a special amino acid called selenocysteine. Important selenium-dependent proteins include glutathione peroxidases, thioredoxin reductases, iodothyronine deiodinases, and selenoprotein P. In plain language, selenium helps enzymes that protect cells from oxidative damage, helps regulate thyroid hormone activation and breakdown, and helps transport selenium through the body.
The blood test does not directly measure all selenium-related enzyme activity. It measures the amount of selenium present in the tested blood fraction. That distinction matters because two people can have similar selenium results but different health contexts. A person taking a high-dose supplement may have a high serum level without a medical benefit. A person with inflammation or severe illness may have a low serum level that partly reflects redistribution or illness rather than simple dietary deficiency.
Serum and plasma selenium are useful because they respond to recent intake. If someone starts or stops a selenium supplement, serum or plasma may change sooner than hair, nails, or whole blood. Whole blood includes selenium inside red blood cells and can give a broader picture of longer-term selenium exposure because red blood cells circulate for about 120 days.
Doctors may order selenium testing when there is a reason to suspect deficiency, excess, or a need to monitor replacement therapy. Common situations include:
- Long-term total parenteral nutrition, especially if trace minerals are missing or interrupted
- Short bowel syndrome, severe malabsorption, bariatric surgery complications, or major digestive surgery
- Long-term hemodialysis or other complex kidney-related nutrition issues
- Unexplained cardiomyopathy or muscle weakness in a setting where selenium deficiency is plausible
- Very restricted diets, severe food insecurity, or dependence on foods from low-selenium regions
- Symptoms or exposure history suggesting excess selenium
- Monitoring after prescribed selenium replacement
Selenium testing can also appear in a broader vitamin and mineral blood test panel, especially when a clinician is evaluating multiple nutrient concerns at once. In that setting, selenium should still be interpreted as its own marker because normal and abnormal patterns do not always match iron, zinc, magnesium, copper, iodine, or vitamin D.
Selenium Blood Test Normal Range
The normal range for a selenium blood test depends on the specimen type, lab method, age, and reporting unit. For adult serum selenium, a commonly used reference interval is about 110–165 mcg/L. Some laboratories use slightly different intervals, such as a lower cutoff near 70, 80, or 100 mcg/L, especially when they are using different populations, methods, or clinical decision points.
Always compare your result with the reference range printed on your own lab report. Selenium is measured at very low concentrations, and trace-element testing is sensitive to collection tubes, contamination, and method differences.
| Result or range | Common meaning | Important note |
|---|---|---|
| About 110–165 mcg/L in adult serum | Often reported as an adult serum reference interval | Use your lab’s exact range first |
| About 80 mcg/L or higher | Often considered sufficient for selenoprotein synthesis in healthy people | This is not the same as a full diagnostic cutoff for every patient |
| Below about 40 mcg/L | May be seen in more severe deficiency states | Needs clinical evaluation, especially with symptoms or nutrition risk |
| Above the lab’s upper limit | May suggest high intake, supplementation, exposure, or contamination | Symptoms and supplement history are important |
Units can be confusing. Selenium may be reported as mcg/L, ug/L, mcg/dL, or µmol/L depending on the lab and country. The symbols mcg and µg mean the same thing: micrograms.
A useful conversion is:
- 1 mcg/dL = 10 mcg/L
- 110 mcg/L = 11 mcg/dL
- 165 mcg/L = 16.5 mcg/dL
So if one source says serum or plasma selenium of 8 mcg/dL is generally sufficient, that equals 80 mcg/L. This helps explain why a person may see different-looking numbers across medical sources even when the underlying interpretation is similar.
Children have age-specific selenium reference ranges. Infants and younger children often have lower expected serum concentrations than adults, and pediatric ranges change with age. A child’s result should be interpreted with a pediatric reference interval, not an adult one.
A “normal” selenium result usually means the blood concentration falls within the expected range for the tested population. It does not mean more selenium would be better. Selenium has a narrow safety margin compared with many nutrients. Both deficiency and excess can be harmful, and high-dose selenium supplements can push levels upward without improving health.
What Low Selenium Results Can Mean
A low selenium result means the selenium concentration in the tested blood sample is below the lab’s reference interval or below the clinical target your clinician is using. Mildly low results are more common than severe deficiency, and the cause is often related to nutrition, absorption, dialysis, severe illness, or medical treatment rather than one single disease.
Low selenium is uncommon in many parts of North America because food is shipped across regions and average intake is usually adequate. It can be more likely in areas where soil selenium is low and local diets rely heavily on locally grown foods. Plant foods vary widely in selenium content because the soil where they grow matters. Animal foods tend to be more consistent because livestock feed is often controlled.
Medical causes can matter more than ordinary diet. People who receive long-term total parenteral nutrition need carefully balanced trace minerals, including selenium. People with severe intestinal disease, major bowel surgery, short bowel syndrome, chronic diarrhea, or malabsorption may not absorb enough. People receiving long-term hemodialysis may have lower selenium status because of dietary limits, inflammation, reduced intake, and losses during treatment.
Low selenium may be checked when symptoms fit the context. Possible signs associated with significant deficiency include muscle weakness, fatigue, changes in immune function, hair or nail changes, and in severe cases heart muscle problems. These symptoms are not specific. Many other conditions, including thyroid disease, anemia, vitamin B12 deficiency, iron deficiency, chronic inflammation, and kidney disease, can cause similar symptoms.
Selenium has an important relationship with thyroid biology because several deiodinase enzymes use selenium. These enzymes help activate and deactivate thyroid hormones. Low selenium can also worsen the effects of iodine deficiency in some settings. That does not mean selenium supplements treat most thyroid problems. Thyroid symptoms should be evaluated with thyroid-specific tests rather than selenium alone.
A low selenium blood test result becomes more meaningful when it fits a clear risk pattern: limited intake, poor absorption, dialysis, long-term parenteral nutrition, or known low-selenium exposure. A borderline result in an otherwise healthy person may need repeat testing, diet review, or a broader nutrition assessment before treatment.
Common causes of low selenium
Low selenium can come from one or more of these factors:
- Low dietary intake, especially in low-selenium regions
- Very restricted diets with little seafood, meat, eggs, dairy, grains, or other selenium-containing foods
- Vegan or vegetarian diets that rely heavily on foods grown in low-selenium soil
- Malabsorption from inflammatory bowel disease, short bowel syndrome, celiac disease, pancreatic insufficiency, or major bowel surgery
- Long-term total parenteral nutrition without adequate trace mineral replacement
- Long-term hemodialysis
- Severe chronic illness, inflammation, or critical illness
- HIV infection or other conditions linked with poor nutrition or absorption
- Certain medications or chemotherapy contexts that affect trace mineral status
Low selenium should not be treated automatically with high doses. For many adults, the recommended dietary allowance is 55 mcg/day, with higher needs during pregnancy and lactation. Therapeutic dosing should be individualized because supplements can accumulate and cause toxicity.
What High Selenium Results Can Mean
A high selenium result means the selenium concentration is above the reference range for the specimen type. In practice, high selenium is most often related to intake or exposure. The most common sources are selenium supplements, multivitamins, “thyroid support” products, Brazil nuts, occupational exposure, or accidental overdose from mislabeled or misformulated products.
Selenium excess is called selenosis when it causes clinical toxicity. Early symptoms can include a garlic-like odor on the breath and a metallic taste in the mouth. More developed symptoms can include nausea, diarrhea, skin rash, fatigue, irritability, hair loss, brittle nails, nail loss, and nerve symptoms such as tingling or numbness. Severe acute toxicity is rare but can cause serious gastrointestinal, neurological, breathing, kidney, or heart complications.
The adult tolerable upper intake level in the United States is 400 mcg/day from food and supplements combined. Some authorities use lower adult upper limits. This does not mean 399 mcg/day is a good target. It means chronic intake above that level increases concern for harm. Many people need far less than that, and routine high-dose supplementation is usually unnecessary unless prescribed.
Brazil nuts deserve special attention because their selenium content varies widely. A small number of nuts can contain a large selenium dose, depending on where they were grown. Eating one Brazil nut occasionally is different from eating several every day while also taking a selenium supplement.
A high selenium blood test result should prompt a careful review of all sources, including:
- Stand-alone selenium supplements
- Multivitamins
- Hair, nail, thyroid, fertility, immune, or antioxidant formulas
- High-dose “detox” products
- Brazil nuts or Brazil nut powders
- Protein powders or fortified nutrition products
- Occupational or environmental exposure
- Recent intravenous nutrition or trace mineral therapy
Do not ignore a high result just because selenium is “natural” or “essential.” Essential nutrients can still be toxic at high intake. Selenium is a clear example because the healthy intake range is relatively narrow.
When high selenium needs faster attention
Contact a clinician promptly if a high selenium result appears with hair loss, nail changes, persistent gastrointestinal symptoms, tremor, confusion, weakness, shortness of breath, chest pain, fainting, or known overdose. Bring the supplement bottles or product labels. If a product was mislabeled or taken in a much larger dose than intended, the dose history may matter as much as the blood number.
What Can Affect Selenium Blood Test Results
Selenium blood results can shift for reasons that have little to do with long-term nutrient status. That is why a single result should be interpreted with context.
Specimen type is one major factor. Serum and plasma reflect recent intake more than whole blood. Whole blood includes red blood cell selenium and may better reflect longer-term status. Hair and nail selenium can reflect exposure over months, but they are more vulnerable to external contamination from shampoos, treatments, dust, occupational materials, and environmental contact.
Recent supplement use can raise serum or plasma selenium. A person who takes 200 mcg/day for a few weeks may see a higher result than their usual baseline. A person who stops a supplement before testing may see a different value later. This is why clinicians often ask about supplement timing before interpreting the result.
Diet can also affect results. Seafood, meat, poultry, eggs, grains, dairy, and Brazil nuts can all contribute selenium. Soil content strongly affects many plant foods, so the same food can contain different amounts depending on geography. A person who recently changed diet, started a nutrition shake, or began eating Brazil nuts daily may have a blood result that reflects that change.
Inflammation and severe illness can complicate interpretation. Some selenium-related biomarkers, including selenoprotein P and glutathione peroxidase activity, may be affected by inflammation and oxidative stress. Serum selenium can also fall during acute illness, trauma, or critical illness. In those cases, the result may reflect both selenium status and the body’s response to illness.
Kidney function and dialysis can affect selenium balance. Urinary excretion is part of selenium homeostasis, and dialysis can influence blood levels. People on dialysis should not self-treat abnormal selenium because kidney disease often changes mineral handling, diet, and supplement safety.
Contamination is another issue. Trace-element tests require careful collection. The wrong tube, environmental contamination, or handling errors can distort results. If a selenium result is unexpected and does not fit the person’s diet, symptoms, or supplement history, repeating the test with proper trace-element collection may be reasonable.
Selenium does not act alone. It overlaps with iodine, thyroid hormones, vitamin E, zinc, copper, iron status, and overall protein intake. For example, someone with fatigue and hair shedding may need evaluation for thyroid disease, iron deficiency, zinc issues, or other causes rather than selenium alone. Mineral interpretation is often clearer when related markers are reviewed together, such as zinc and copper balance when supplement use is heavy or prolonged.
How to Prepare and What to Tell Your Clinician
Most selenium blood tests do not require fasting unless they are ordered with other labs that do. Follow the instructions on your lab order. If selenium is part of a larger panel, the fasting requirement may come from glucose, insulin, lipid, or other tests rather than selenium itself.
Before the test, tell your clinician about every supplement you take. This includes multivitamins, thyroid support products, fertility formulas, antioxidant blends, hair and nail supplements, protein powders, and trace mineral drops. Selenium is often hidden in formulas where people do not expect it. The label may list selenium as selenomethionine, selenium yeast, sodium selenite, or sodium selenate.
Do not stop prescribed supplements unless your clinician tells you to. If selenium is being monitored because you receive parenteral nutrition or prescribed replacement, the timing of the test should match the monitoring plan. Stopping on your own can make the result less useful.
For non-prescribed supplements, ask whether to pause them before testing. Some clinicians prefer to test while you are taking your usual routine to see real-world exposure. Others may ask you to stop nonessential supplements for a period before testing if the goal is to estimate baseline status. The best approach depends on why the test was ordered.
Bring or photograph labels when possible. Product labels help clinicians calculate total selenium intake. A person might take a multivitamin with 55 mcg, a thyroid formula with 200 mcg, and a separate antioxidant product with another 100 mcg without realizing the combined daily intake is high.
Also mention diet patterns and exposures, including:
- Frequent Brazil nut intake
- High seafood intake
- Vegan, vegetarian, or very restricted diets
- Recent major diet changes
- Tube feeding, nutrition shakes, or fortified meal replacements
- Digestive disorders or surgeries
- Dialysis
- Long-term IV nutrition
- Occupational exposure to selenium compounds, metal refining, glass manufacturing, pigments, electronics, or industrial materials
- Recent poisoning concern or supplement dosing error
The blood draw itself is like most routine blood tests. A clinician or phlebotomist collects blood from a vein, often using a tube appropriate for trace mineral testing. Because selenium is a trace element, proper collection materials matter more than they do for many routine chemistry tests.
What to Do With Abnormal Selenium Results
The right next step depends on how abnormal the result is, whether symptoms are present, and why the test was ordered. A mildly low or mildly high result in a healthy person is handled differently from a very abnormal result in someone on parenteral nutrition, dialysis, or high-dose supplements.
For a low result, the first step is usually to confirm the context. A clinician may review diet, digestive health, medications, kidney treatment, and other nutrient markers. If the result is unexpected, repeat testing may be ordered. If there is a clear deficiency risk, treatment may include dietary changes or a controlled selenium supplement dose.
Food-based correction may be enough for mild low intake. Selenium-rich foods include seafood, meat, poultry, eggs, dairy, grains, and Brazil nuts. Brazil nuts can be useful but are easy to overdo. Because their selenium content varies widely, they are not a precise treatment tool.
For a high result, the first step is usually to identify and reduce excess intake or exposure. This may mean stopping non-prescribed selenium supplements, avoiding stacked formulas, reducing frequent Brazil nut intake, or addressing occupational exposure. If toxicity symptoms are present, medical evaluation is more urgent.
| Result pattern | Common next step | Why it matters |
|---|---|---|
| Borderline low, no symptoms | Review diet, repeat if needed, check related risks | A single mild result may not prove true deficiency |
| Clearly low with malabsorption or parenteral nutrition | Clinician-guided replacement and monitoring | Deficiency can become clinically important in high-risk settings |
| High with supplement use | Calculate total dose and stop or reduce nonessential sources | Chronic excess can cause hair, nail, digestive, and nerve symptoms |
| High with toxicity symptoms | Prompt medical assessment | Symptoms and exposure history guide urgency |
| Unexpected result | Repeat with proper trace-element collection | Contamination or specimen issues can distort trace mineral testing |
It is also reasonable to check related tests when symptoms are broad. For fatigue, weakness, hair changes, palpitations, or thyroid-like symptoms, selenium may be only one part of the evaluation. Depending on the situation, clinicians may order a CBC, ferritin, iron studies, thyroid-stimulating hormone, free T4, vitamin B12, folate, zinc, copper, kidney markers, liver markers, inflammatory markers, or other nutrient tests.
Selenium supplementation should be precise. Many over-the-counter selenium products contain 100–200 mcg per dose, and some contain 400 mcg. That can be too much for routine use, especially if the person also gets selenium from food and a multivitamin. More is not better with selenium.
Long-term high-dose selenium has not been proven to prevent cancer, heart disease, or general aging in people who already have adequate intake. It may increase risk in some settings. For most people, the safest plan is to meet normal dietary needs, avoid unnecessary high-dose supplements, and treat abnormal blood results based on a clinician’s interpretation.
When to call your clinician
Call your clinician if your selenium result is outside the reference range and you have symptoms, take selenium-containing supplements, receive dialysis, have significant digestive disease, receive parenteral nutrition, are pregnant, or have known exposure. Seek urgent care for severe vomiting, confusion, tremor, breathing trouble, chest pain, fainting, or suspected overdose.
References
- SES – Overview: Selenium, Serum 2026 (Laboratory Reference)
- Selenium – Health Professional Fact Sheet 2026 (Official Fact Sheet)
- Scientific opinion on the tolerable upper intake level for selenium 2023 (Scientific Opinion)
- Prevention of Keshan Disease by Selenium Supplementation: a Systematic Review and Meta-analysis 2018 (Systematic Review)
- Acute selenium toxicity associated with a dietary supplement 2010 (Clinical Report)
- 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum 2017 (Guideline)
Disclaimer
A selenium blood test should be interpreted by a qualified clinician who can review your lab’s reference range, symptoms, diet, supplements, and medical history. Do not start high-dose selenium or stop prescribed trace mineral therapy based only on a single result. Seek urgent medical help if you suspect selenium overdose or have severe neurological, breathing, heart, or gastrointestinal symptoms.
