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Quick take
- What it measures: A selenium blood test measures the amount of selenium in a blood sample, usually as serum, plasma, or whole blood selenium.
- Why it is ordered: It may be used to evaluate suspected selenium deficiency, possible excess intake from supplements, occupational or environmental exposure, or nutritional status in people at higher risk for deficiency.
- Preparation: Many selenium tests do not require fasting, but trace-element testing is sensitive to collection handling. Follow the lab order exactly, and ask whether supplements or recent contrast imaging should affect timing.
- Results: Interpretation depends on the specimen type and the laboratory’s reference interval. Low results can suggest inadequate intake, malabsorption, or increased needs; high results can suggest excessive supplements or exposure.
- Next step: Do not start high-dose selenium based only on a symptom checklist. Review the result with a clinician, especially if you take supplements, have thyroid disease, have gastrointestinal disease, receive tube feeding or parenteral nutrition, or have possible workplace exposure.
What is a selenium blood test?
A selenium blood test is a laboratory test that measures selenium in a sample of blood. Selenium is an essential trace mineral, meaning the body needs small amounts but too much can be harmful. It is incorporated into selenoproteins, which support antioxidant activity, thyroid hormone metabolism, immune function, and other cellular processes. The National Institutes of Health Office of Dietary Supplements explains that adult recommended intakes are measured in micrograms, not milligrams, which is one reason supplementation should be approached carefully rather than treated like a general wellness boost. NIH Office of Dietary Supplements
In everyday medical use, selenium testing is not a routine screening test for most healthy adults. Most people in the United States get enough selenium from foods such as seafood, meat, poultry, eggs, dairy products, grains, and Brazil nuts. The amount in plant foods varies with soil selenium, so dietary patterns and geography can matter, but true deficiency is still uncommon in the general U.S. population. Testing becomes more relevant when there is a clinical reason to suspect deficiency, excess intake, malabsorption, specialized nutrition support, or unusual exposure.
The test is performed from a blood draw, usually from a vein in the arm. Depending on the ordering code, the lab may measure selenium in serum, plasma, or whole blood. These are related but not interchangeable. Your result should be interpreted using the reference interval printed on the same report, because values can differ by specimen type, method, age group, and laboratory.
When is a selenium blood test ordered?
A clinician may order selenium testing when the result could change the next step in care. Common reasons include suspected deficiency, monitoring nutritional replacement, possible supplement overuse, and evaluation after a high-level environmental or occupational exposure.
Possible deficiency or inadequate selenium status
Selenium deficiency is rare in many well-nourished adults, but it can occur in certain settings. Risk is higher when intake is very low, absorption is impaired, or nutrition is delivered in a way that omits trace elements. Examples include severe malnutrition, some gastrointestinal disorders or surgeries that reduce absorption, long-term parenteral nutrition without adequate trace-element replacement, and some medically restricted diets. The Merck Manual notes that selenium deficiency is uncommon but can be associated with muscle weakness and, in severe cases, cardiomyopathy in particular deficiency patterns. Merck Manual Consumer Version
Symptoms alone are not enough to diagnose selenium deficiency. Fatigue, hair changes, muscle symptoms, mood changes, and frequent infections can have many causes, including thyroid disease, iron deficiency, B12 deficiency, medication effects, autoimmune disease, poor sleep, and stress. A selenium blood test may be one part of a broader evaluation rather than a stand-alone answer.
Possible excess selenium or toxicity
Selenium can cause harm when intake is chronically too high. The NIH Office of Dietary Supplements describes selenosis as a toxicity syndrome that can include hair loss, nail changes, gastrointestinal symptoms, skin rash, fatigue, irritability, and nervous-system symptoms. The adult upper limit has been revised in newer NIH materials to 255 micrograms per day, lower than the older 400 microgram per day value still seen in some references. NIH Office of Dietary Supplements
Excess selenium most often becomes a concern when someone takes multiple supplements, uses a high-dose selenium product, or eats very selenium-rich foods in large amounts. Brazil nuts are a common example because selenium content can vary widely by source. A multivitamin, thyroid-support formula, immune supplement, hair-and-nail supplement, and separate selenium capsule can unintentionally stack the dose.
Occupational or environmental exposure
Blood and urine selenium tests can also be used after recent high-level exposure. The Agency for Toxic Substances and Disease Registry states that blood and urine tests are most useful for people recently exposed to high levels, while toenail testing can reflect longer-term exposure; however, these tests do not predict with certainty whether health effects will occur. ATSDR
Potential exposure settings include certain industrial processes, electronics and semiconductor work, copper refining byproducts, glass or ceramic manufacturing, rubber vulcanization, contaminated water in specific regions, or accidental ingestion of high-dose compounds. If symptoms are acute or exposure is significant, testing should be coordinated through occupational medicine, poison control, or a clinician familiar with toxic exposure evaluation.
Serum selenium vs whole blood selenium
“Selenium blood test” can refer to more than one specimen type. This matters because serum, plasma, and whole blood can answer slightly different questions.
| Test type | What it reflects | Common use | Important limitation |
|---|---|---|---|
| Serum or plasma selenium | Selenium circulating in the liquid portion of blood | Often used to assess recent intake or current circulating status | Can be affected by recent intake, inflammation, specimen handling, and lab method |
| Whole blood selenium | Selenium in plasma plus blood cells | May better reflect longer-term tissue stores than serum alone | Reference intervals differ from serum; results should not be compared directly |
| Urine selenium | Recent excretion | Sometimes used in exposure evaluation | Not the usual nutritional-status test |
| Hair or toenail selenium | Longer-term exposure patterns | Selected environmental or research contexts | External contamination and interpretation issues limit routine medical use |
Mayo Clinic Laboratories describes its serum selenium test as useful for assessing recent intake, while its whole blood selenium test is described as useful for assessing tissue stores. Mayo Clinic Laboratories serum selenium Mayo Clinic Laboratories whole blood selenium This is why two people can both say they had a “selenium blood test” but receive different reference ranges and units.
How to prepare for a selenium blood test
Preparation depends on the exact lab order. Many consumer-facing selenium tests state that no special preparation is required, and Labcorp OnDemand lists no special preparation for its selenium test. Labcorp OnDemand Still, trace-element testing is more sensitive to contamination and specimen handling than many routine chemistry tests, so preparation is not only about what you eat before the draw.
Ask about supplements before the draw
Tell the ordering clinician about every supplement you take, including multivitamins, prenatal vitamins, thyroid-support products, hair/nail products, “immune” blends, protein powders, and individual mineral capsules. Do not stop a prescribed supplement or medical nutrition product unless the ordering clinician tells you to. If the goal is to see your usual selenium exposure, your clinician may want you to continue your normal routine. If the goal is to evaluate a possible excess from supplements, they may want a different timing plan.
Ask about recent contrast imaging
Some metal and trace-element tests can be affected by recent imaging contrast. Mayo Clinic Laboratories advises that specimens for certain ICP-MS-based metal tests should not be collected for at least 96 hours after gadolinium- or iodine-containing contrast media because these substances can interfere with testing. Mayo Clinic Laboratories If you recently had a CT scan, MRI, angiography, or other contrast study, mention it before the blood draw.
Use the correct tube and collection process
Selenium is a trace element, so the collection tube matters. Laboratories commonly require royal-blue-top trace-element tubes or other metal-free supplies. Mayo’s serum selenium instructions specify trace-element blood collection materials and prompt separation of serum, while Labcorp’s whole blood selenium listing specifies a royal-blue-top EDTA tube. Mayo Clinic Laboratories Labcorp If the wrong tube is used, the specimen can be rejected or, worse, yield a misleading result.
Fasting is usually not the main issue
Unless your order includes other tests that require fasting, selenium testing alone often does not require fasting. If your blood draw includes glucose, insulin, lipid testing, or another panel, follow the most restrictive instruction provided by the ordering clinician or lab. Drinking water is usually encouraged before a blood draw unless you were told otherwise.
How to interpret selenium blood test results
The most important rule is simple: compare your value with the reference interval on your own lab report. Selenium reference intervals vary by specimen type and age. For example, Mayo Clinic Laboratories lists an adult serum selenium reference interval of 110–165 mcg/L and an adult whole blood reference interval of 150–241 ng/mL for its specific tests. Mayo Clinic Laboratories serum selenium Mayo Clinic Laboratories whole blood selenium Those numbers should not be treated as universal cutoffs for every lab.
| Result pattern | Possible meaning | Practical next step |
|---|---|---|
| Within the lab’s reference interval | Selenium status is not obviously low or high by that lab’s method | Review whether symptoms need evaluation for other causes |
| Low | May reflect low intake, malabsorption, severe illness, specialized nutrition issues, or specimen/interpretation factors | Discuss diet, GI history, medications, nutrition support, and whether repeat testing or related labs are needed |
| High | May reflect supplement excess, occupational/environmental exposure, high-selenium foods, or recent exposure | Inventory all supplements and exposures; seek medical guidance before continuing selenium-containing products |
| Very high with symptoms | Possible selenium toxicity or acute exposure, depending on context | Contact a clinician promptly; for acute ingestion or severe symptoms, contact poison control or emergency care |
Low selenium result
A low selenium result does not automatically mean you should take a high-dose supplement. The right response depends on the degree of abnormality, diet, health conditions, and why the test was ordered. In a person with poor intake or malabsorption, a clinician may consider dietary changes, a standard-dose supplement, or a supervised replacement plan. In a person receiving parenteral nutrition or tube feeding, the nutrition prescription may need review to ensure trace elements are included appropriately.
Because selenium is involved in thyroid hormone metabolism, people sometimes test selenium when they have thyroid symptoms. A low value may be relevant, but it does not diagnose hypothyroidism, Hashimoto’s thyroiditis, Graves’ disease, or any specific thyroid condition. Thyroid evaluation usually requires tests such as TSH, free T4, and sometimes thyroid antibodies, interpreted with symptoms and exam findings.
High selenium result
A high selenium result should prompt a careful look at supplements and exposures. Symptoms that can occur with excess selenium include gastrointestinal upset, a garlic-like breath odor, metallic taste, hair loss, brittle or deformed nails, skin changes, numbness, tingling, and other neurologic symptoms. ATSDR notes that high selenium exposure can cause neurologic effects and brittle hair and deformed nails, and Labcorp’s test information also lists garlic breath odor, brittle nails, brittle hair, skin changes, and nervous-system abnormalities among toxic features. ATSDR Labcorp
If you have a high result, do not assume that “more antioxidant support” is beneficial. Selenium has a narrow useful range compared with many nutrients. Bring the bottles or ingredient lists of all supplements to your clinician or pharmacist. If workplace exposure is possible, ask whether occupational medicine evaluation, urine testing, repeat blood testing, or workplace safety review is appropriate.
Limitations of selenium testing
Selenium testing is useful in the right context, but it has limits. First, a blood value is a biomarker, not a complete diagnosis. It must be interpreted with symptoms, diet, medical history, supplement use, kidney and liver status, inflammatory illness, and exposure history. Second, reference intervals differ by laboratory and specimen type. Third, trace-element specimens are vulnerable to collection problems, including the wrong tube or contamination. Fourth, a normal result does not rule out every condition that can cause fatigue, hair loss, immune symptoms, or thyroid complaints.
Finally, selenium testing does not prove that a supplement will improve symptoms. Some people feel better after correcting a true deficiency, but many nonspecific symptoms have other explanations. Taking selenium when you already have adequate levels can increase the risk of excess without addressing the actual cause of symptoms.
How much does a selenium blood test cost?
Cost depends on whether the test is ordered through insurance, a clinician’s office, a hospital outpatient lab, or a direct-to-consumer/self-pay service. Insurance coverage varies by plan and medical necessity. If you are using insurance, ask about the ordering diagnosis code, network status of the lab, deductible, coinsurance, and whether the test is considered covered for your situation.
For self-pay comparison, look at the total cost to the patient rather than the headline test price. The total may include the lab test price, a required physician oversight or order fee, draw fee, processing fee, taxes where applicable, and any cancellation or rescheduling terms. Also compare specimen type, collection network, state availability, age restrictions, result turnaround time, and whether results are reviewed by an ordering clinician.
| Example self-pay option | Listed selenium test price | Required fee shown | Effective listed total | Collection details noted publicly |
|---|---|---|---|---|
| Labcorp OnDemand | $109 | No separate mandatory fee shown on the selenium test page | $109 | In-person blood draw at a Labcorp location; listed result timing 2–4 days after sample arrives at the lab |
| Walk-In Lab | $99 | $6 per-order physician fee shown | $105 before any temporary discounts | Blood draw through a participating lab network; page lists Labcorp and Quest test codes |
| Ulta Lab Tests | $58.95 for a serum/plasma selenium listing; $67.95 for a micronutrient whole blood listing | Mandatory extra fees were not shown in the searched listing excerpt | At least the listed test price; confirm checkout total before purchase | Blood draw; listed processing time varies by specific selenium product |
Public prices can change and promotions may not apply to every order. If you are comparing options, confirm the checkout total, available lab locations near you, whether the test can be ordered in your state, and whether the test is serum/plasma or whole blood before paying. If your clinician needs a specific specimen type, do not substitute a cheaper test without asking.
What to do after you get your result
If your result is normal, ask whether selenium is still relevant to your symptoms or whether other causes should be evaluated. If your result is low, review dietary intake, gastrointestinal health, medications, and nutrition support with your clinician. A practical plan may include food changes, a modest supplement dose, or repeat testing after a defined interval. If your result is high, stop and review nonessential selenium-containing supplements with a clinician, especially if you have symptoms of toxicity.
Seek prompt medical guidance if you have a very high result, neurologic symptoms, severe vomiting or diarrhea after a suspected ingestion, chest pain, shortness of breath, fainting, confusion, or a significant workplace or environmental exposure. For acute poisoning concerns in the United States, Poison Control can provide immediate guidance at 1-800-222-1222.
FAQs about the selenium blood test
Is a selenium blood test the same as a heavy metals test?
Not exactly. Selenium is a trace element that can be essential at low levels and toxic at high levels. Some labs group selenium with metals or trace elements because the testing method and collection requirements are similar, but selenium is not interpreted the same way as lead or mercury.
Do I need to fast for a selenium blood test?
Usually not for selenium alone. However, follow the instructions on your lab order because other tests drawn at the same visit may require fasting.
Should I stop selenium supplements before the test?
Ask the ordering clinician. Stopping supplements may be appropriate in some situations, but if the goal is to measure your usual exposure, your clinician may want you to continue your normal routine until the blood draw.
Can a selenium blood test diagnose thyroid disease?
No. Selenium status can be relevant to thyroid biology, but a selenium result does not diagnose thyroid disease. Thyroid evaluation usually relies on thyroid-specific blood tests and clinical assessment.
What is a normal selenium blood level?
Normal depends on the specimen type and laboratory. For example, Mayo Clinic Laboratories lists different adult reference intervals for serum selenium and whole blood selenium. Use the reference interval printed on your own report.
What causes low selenium?
Possible causes include very low dietary intake, malabsorption, severe malnutrition, certain medically restricted diets, and nutrition support that does not include adequate trace elements. The result should be interpreted with the full clinical picture.
What causes high selenium?
High selenium may come from high-dose supplements, multiple products that all contain selenium, unusually high intake of selenium-rich foods, contaminated products, or occupational/environmental exposure.
How long do results take?
Turnaround time varies. Labcorp lists 2–4 days for its whole blood selenium test, while Mayo lists 2–8 days for its whole blood selenium test. Timing can be longer if the specimen is sent to a reference laboratory or if collection occurs before a weekend or holiday. Labcorp Mayo Clinic Laboratories
Is hair testing better than blood testing for selenium?
Hair and toenail testing may reflect longer-term exposure in some settings, but they are not the usual first-line medical test for selenium nutrition. External contamination and interpretation issues can limit usefulness. For recent high-level exposure, ATSDR notes that blood and urine tests are most useful. ATSDR
Can I order a selenium blood test myself?
In many U.S. locations, adults can purchase selenium testing through direct-access lab services that include clinician order review and in-person blood collection. Availability, age rules, state restrictions, and fees vary, so confirm the details before checkout.
Sources
- NIH Office of Dietary Supplements: Selenium Health Professional Fact Sheet
- Agency for Toxic Substances and Disease Registry: Selenium ToxFAQs
- Mayo Clinic Laboratories: Selenium, Serum
- Mayo Clinic Laboratories: Selenium, Blood
- Labcorp: Selenium, Whole Blood
- Labcorp OnDemand: Selenium Test
- Walk-In Lab: Selenium Blood Test
- Merck Manual Consumer Version: Selenium Deficiency
