A low selenium blood test means your measured selenium level is below the reference range used by the lab, or below the level expected to support normal selenium-dependent enzymes. Selenium is a trace mineral, so the body needs only tiny amounts, but those small amounts help make selenoproteins involved in thyroid hormone metabolism, antioxidant defense, immune function, and reproduction. A low result is not usually diagnosed from the number alone. It needs context: your diet, supplement use, kidney or digestive conditions, inflammation, and whether the sample was serum, plasma, or whole blood. True selenium deficiency is uncommon in many high-income countries, but it can happen with very low intake, long-term parenteral nutrition, malabsorption, kidney dialysis, severe illness, or diets based mostly on foods grown in low-selenium soil. The safest response is to confirm the result, look for the cause, and correct intake without drifting into excess.
- A low selenium result usually means low recent selenium status, especially when serum or plasma selenium is below the lab’s adult reference range.
- Common adult serum reference intervals vary by lab, but one commonly used adult range is about 110–165 mcg/L; values below about 40 mcg/L suggest more severe deficiency.
- Selenium supports thyroid hormone metabolism, but a low selenium result does not automatically mean thyroid disease.
- Food-first correction is usually safer than high-dose supplements, because selenium has a narrow gap between enough and too much.
- Follow-up often includes thyroid tests, nutrition review, and repeat selenium testing, especially if symptoms, malabsorption, dialysis, or supplement use are present.
- Urgent care is needed for serious symptoms, such as chest pain, fainting, severe shortness of breath, new weakness, or signs of severe thyroid dysfunction.
Table of Contents
- What a Low Selenium Blood Test Means
- Selenium Testing, Units, and Reference Ranges
- Common Causes of Low Selenium
- Symptoms and Health Effects of Selenium Deficiency
- Low Selenium and Thyroid Health
- What to Check Next After a Low Result
- How to Raise Selenium Safely
- When to Seek Medical Care
What a Low Selenium Blood Test Means
A low selenium blood test means the selenium measured in your blood sample is lower than expected for the test method and specimen type. In most everyday clinical use, selenium is measured in serum or plasma. These results mainly reflect recent selenium intake and short-term status. Whole blood selenium can reflect a longer time window because it includes selenium inside red blood cells.
Selenium is needed to build selenoproteins. These proteins help the body control oxidative stress, support immune signaling, and process thyroid hormones. One important selenium-containing protein, glutathione peroxidase, helps neutralize peroxides that can damage cell membranes. Another group, the iodothyronine deiodinases, helps activate and deactivate thyroid hormones.
A low result can mean one of several things:
- Your diet has not supplied enough selenium.
- You are not absorbing selenium well.
- You are losing or using more during illness, dialysis, or inflammation.
- The sample was affected by collection, timing, or lab-method issues.
- Your result is low compared with that lab’s range, but not low enough to cause symptoms.
This is why selenium results should not be read like a simple yes-or-no diagnosis. A mildly low value in a healthy person with no symptoms may need a diet review and repeat testing. A clearly low value in someone with malabsorption, bowel surgery, long-term tube feeding, or parenteral nutrition deserves a more careful medical workup.
It also helps to separate “low selenium level” from “selenium deficiency disease.” A low lab value is a measurement. Selenium deficiency is a clinical state where low selenium is likely affecting body function. Many people with mildly low results do not have obvious symptoms, while people with severe or long-lasting deficiency can develop more serious problems.
Selenium Testing, Units, and Reference Ranges
Selenium testing is usually ordered when a clinician suspects deficiency, excess intake, unusual exposure, malabsorption, nutritional support problems, or the need to monitor selenium replacement. It is not a routine blood test for most people.
The most common specimen types are serum, plasma, and whole blood. Serum and plasma are more common for checking current selenium status. Whole blood may be useful when longer-term status is being assessed, but it is less commonly seen on routine lab reports.
Selenium may be reported in different units. The most common are:
- mcg/L, also written as micrograms per liter
- mcg/dL, or micrograms per deciliter
- ng/mL, which is numerically the same as mcg/L
For conversion, 1 mcg/dL equals 10 mcg/L. So a selenium value of 8 mcg/dL equals 80 mcg/L.
Many labs use their own reference intervals because results can vary by method, specimen type, age, and population. One commonly used adult serum reference interval is 110–165 mcg/L. Some nutrition references consider serum or plasma selenium of about 80 mcg/L or higher enough to support selenoprotein synthesis in healthy people. Severe deficiency is more concerning when serum selenium is very low, such as below about 40 mcg/L, especially when symptoms or risk factors are present.
| Result pattern | Possible meaning | Usual next step |
|---|---|---|
| Below the lab range | Low selenium status, especially if serum or plasma was tested | Review diet, supplements, risk factors, and repeat or confirm if needed |
| Very low, such as below about 40 mcg/L | More concerning for clinically meaningful deficiency | Medical evaluation for malabsorption, nutrition support issues, dialysis, or severe illness |
| Near the lower end of normal | May be adequate, borderline, or lab-dependent | Interpret with diet, symptoms, and thyroid or nutrition markers |
| High-normal or high | Possible high intake, supplement use, or excess exposure | Avoid unnecessary selenium supplements and review total intake |
A selenium result should be interpreted using the exact lab range printed on your report. If you are comparing results over time, try to use the same lab and specimen type. Switching from serum to whole blood, or from one lab method to another, can make the trend harder to interpret. For a broader look at test interpretation, a dedicated selenium blood test normal range guide can help clarify why reference intervals differ.
Sample handling also matters. Trace element tests require careful collection. Many labs use metal-free tubes and specific handling instructions. Some contrast agents used in imaging tests may interfere with certain metal tests, so the lab may recommend waiting before collection after contrast exposure. Do not assume a surprising result is final if the sample was drawn under unusual conditions.
Common Causes of Low Selenium
Low selenium usually comes from low intake, poor absorption, increased losses, or a medical situation that changes selenium metabolism. In many countries, severe deficiency is uncommon because grains, meats, seafood, eggs, and mixed diets often provide enough. Risk rises when diet is restricted, absorption is impaired, or nutrition is medically dependent.
Low intake is one cause. Selenium in plant foods depends heavily on soil selenium. Two people eating similar plant-based diets can have different selenium intakes if their grains, beans, and vegetables come from different regions. Animal foods are usually more consistent because animal feed is often controlled, but intake still varies by food pattern. People who eat very limited diets, very low-protein diets, or strict vegan diets without reliable selenium sources may be more vulnerable.
Digestive and absorption problems can also lower selenium. Risk is higher with:
- Short bowel syndrome
- Extensive bowel surgery
- Crohn disease or severe inflammatory bowel disease
- Untreated celiac disease
- Chronic diarrhea
- Bariatric surgery with poor intake or malabsorption
- Long-term inability to eat a balanced diet
People receiving long-term parenteral nutrition are a classic risk group if trace minerals are not properly included and monitored. Tube feeding can also contribute if the formula or intake is inadequate, although modern formulas often include selenium.
Kidney dialysis can lower selenium in some people. Dialysis may remove selenium, and appetite changes or dietary restrictions can reduce intake. The degree of risk varies, so supplementation should be individualized rather than automatic.
Severe illness, inflammation, infection, and trauma can complicate interpretation. Selenium in blood may fall during inflammatory states even when total body selenium status is harder to define. In these cases, a low result may reflect both nutrition and the body’s acute response to illness. A clinician may interpret selenium alongside albumin, C-reactive protein, kidney function, liver markers, and the overall clinical picture.
Smoking and chronic oxidative stress have also been linked with lower selenium status in some population studies. This does not mean selenium supplements offset smoking risk. It means the result should be viewed as part of a larger health pattern.
A low result can also follow supplement changes. For example, someone who stopped a multivitamin containing selenium may see levels drift lower if diet is not supplying much selenium. The opposite can happen with high-dose selenium products, Brazil nut overuse, or “thyroid support” supplements. Selenium deficiency and excess can both occur from supplement decisions, so total intake matters.
Symptoms and Health Effects of Selenium Deficiency
Mild low selenium may cause no obvious symptoms. Many symptoms blamed on selenium deficiency are nonspecific, meaning they can also come from low iron, low B12, low vitamin D, thyroid disease, poor sleep, chronic illness, depression, medication effects, or low calorie intake.
Possible symptoms and findings with more meaningful deficiency can include fatigue, muscle weakness, poor immune resilience, hair or nail changes, fertility concerns, and abnormal thyroid patterns. Severe deficiency can contribute to cardiomyopathy, a disease of the heart muscle, especially in high-risk settings. Historically, severe selenium deficiency has been associated with Keshan disease, a cardiomyopathy described in low-selenium regions. Selenium deficiency has also been associated with Kashin-Beck disease, a bone and joint disorder in certain endemic regions, although its cause is complex and not explained by selenium alone.
Selenium deficiency can affect antioxidant enzyme activity. When glutathione peroxidase activity falls, cells may have less protection from oxidative damage. This is one reason selenium has long been studied in heart, immune, thyroid, and reproductive health.
Still, symptoms should not be used to self-diagnose selenium deficiency. Fatigue plus a low-normal selenium result does not prove selenium is the cause. A person with fatigue might need a more complete check for anemia, thyroid disease, kidney or liver problems, inflammation, sleep apnea, depression, medication effects, or other nutrient deficiencies. If iron status is also part of the question, patterns such as low ferritin with normal hemoglobin can cause symptoms even when a CBC looks acceptable.
Selenium status can also overlap with other nutrient issues. Iodine, iron, zinc, vitamin D, vitamin B12, folate, and protein intake can all affect energy, thyroid function, blood cell production, and immune health. Selenium is one piece of the nutrition picture, not a stand-alone explanation for every symptom.
Low Selenium and Thyroid Health
Selenium is closely tied to thyroid biology because the thyroid gland uses several selenium-dependent enzymes. The thyroid makes thyroid hormone, but the body also has to activate and deactivate thyroid hormone in tissues. Selenium-containing deiodinase enzymes help convert thyroxine, or T4, into triiodothyronine, or T3, and help regulate thyroid hormone activity.
The thyroid also deals with oxidative stress during hormone production. Selenium-containing antioxidant enzymes help control peroxide byproducts created during thyroid hormone synthesis. This is one reason selenium is often discussed in Hashimoto thyroiditis, thyroid antibodies, and hypothyroidism.
A low selenium result does not automatically mean your thyroid is underactive. Thyroid function is usually assessed with TSH and free T4, sometimes with free T3, thyroid peroxidase antibodies, thyroglobulin antibodies, and thyroid ultrasound when clinically appropriate. Selenium may support thyroid physiology, but it does not replace thyroid hormone testing or thyroid medication when medication is needed.
In Hashimoto thyroiditis, selenium supplementation has been studied because many patients have thyroid peroxidase antibodies, called TPO antibodies. Recent reviews suggest selenium may reduce TPO antibody levels in some study groups, especially when baseline selenium status is low, but the effects on symptoms, thyroid ultrasound, long-term disease progression, and medication needs are less certain. Lowering an antibody number is not the same as curing autoimmune thyroid disease.
This distinction matters. Someone with low selenium and Hashimoto thyroiditis may benefit from correcting true deficiency. But taking high-dose selenium when selenium status is already adequate may add risk without clear benefit. The safest approach is to measure, correct deficiency, and avoid long-term high-dose use unless a clinician is monitoring it.
Selenium and iodine also interact. Iodine is needed to make thyroid hormone, while selenium helps process thyroid hormones and protect thyroid tissue from oxidative stress. Severe deficiency of either nutrient can disrupt thyroid function. Correcting one nutrient while ignoring the other may not solve the problem. For example, a person with low selenium and possible low iodine may need a broader thyroid-nutrition review rather than a single supplement decision. A related low iodine blood test result should be interpreted carefully because iodine testing has its own limitations.
High selenium is not better for the thyroid. Excess selenium can cause toxicity, and some supplements marketed for thyroid support contain selenium along with iodine, kelp, tyrosine, ashwagandha, or other ingredients that may not be appropriate for everyone. People with thyroid disease should be especially careful with multi-ingredient thyroid products.
What to Check Next After a Low Result
A low selenium result should lead to a practical review, not panic. The first step is to confirm what was measured. Look at the specimen type, units, reference range, and whether the result was serum, plasma, or whole blood. Then review the reason the test was ordered.
Useful follow-up questions include:
- Was the sample collected in the correct trace-element tube?
- Was the result mildly low or clearly low?
- Are there symptoms that fit a broader nutrient or thyroid pattern?
- Is the person taking selenium, multivitamins, Brazil nuts, or thyroid supplements?
- Is there malabsorption, dialysis, bowel surgery, chronic diarrhea, or parenteral nutrition?
- Are inflammatory markers or albumin abnormal?
- Has the result changed over time?
If thyroid symptoms are present, follow-up commonly includes TSH and free T4. Thyroid antibodies may be useful if autoimmune thyroid disease is suspected. Free T3 is sometimes ordered, but it is not usually the first test for routine hypothyroidism evaluation. Thyroid tests should be interpreted using the lab’s reference range and the person’s age, pregnancy status, medication use, and symptoms.
If the concern is broader nutrition, a clinician may also check CBC, ferritin, B12, folate, vitamin D, zinc, copper, magnesium, albumin, kidney function, liver markers, and inflammatory markers. This is especially helpful when fatigue, hair shedding, weakness, frequent infections, anemia, or restrictive dieting is part of the picture. A broader vitamin and mineral blood test panel may be useful when several deficiencies are possible, but testing should still be targeted to the person’s risks.
If selenium is low because of malabsorption, simply taking a standard supplement may not fix the underlying issue. The same is true for people on dialysis, parenteral nutrition, or complex medical diets. These situations need clinician-guided dosing and monitoring.
Repeat testing may be reasonable after dietary changes or supplementation. The timing depends on the degree of deficiency and the treatment plan, but many clinicians reassess over weeks to a few months rather than days. Selenium status does not need to be checked repeatedly without a clear reason.
How to Raise Selenium Safely
The usual adult recommended intake for selenium is 55 mcg per day. Pregnancy increases the recommended intake to 60 mcg per day, and lactation increases it to 70 mcg per day. These are daily intake targets, not blood test targets.
Food is often the best first step for mild low selenium. Good sources include seafood, meat, poultry, eggs, dairy, grains, and Brazil nuts. Brazil nuts are famous for selenium because they can contain very high amounts, but their selenium content is highly variable. One ounce may contain several hundred micrograms. Because of that, Brazil nuts can correct intake but can also push intake too high if eaten in large amounts daily.
A practical food approach might include:
- Fish, shellfish, poultry, eggs, or lean meats several times per week if these fit your diet
- Whole grains, oats, rice, beans, and dairy as part of a balanced pattern
- Brazil nuts only in small amounts, such as one nut occasionally or a few per week, unless a clinician gives different advice
- A review of vegan or highly restricted diets to make sure selenium has a reliable source
Supplements can help when food is not enough or when deficiency is clear. Common forms include selenomethionine, selenium yeast, sodium selenite, and sodium selenate. Many multivitamins contain about 55 mcg. Selenium-only supplements often contain 100–200 mcg, and some contain more.
More is not safer. The U.S. adult tolerable upper intake level is 400 mcg per day from all sources. That includes food, multivitamins, selenium pills, Brazil nuts, and combination supplements. Some people should stay well below that level, especially if their blood selenium is already adequate.
Possible signs of too much selenium include:
- Garlic-like breath odor
- Metallic taste
- Nausea, diarrhea, or abdominal discomfort
- Hair loss
- Brittle nails or nail changes
- Skin rash
- Irritability or fatigue
- Tingling, numbness, or nerve symptoms in more serious cases
High selenium can be harmful, so do not treat a low-normal thyroid pattern with high-dose selenium unless deficiency is documented and a clinician is guiding the plan. If a past result showed high selenium, a high selenium blood test pattern should be addressed by reducing unnecessary sources and reviewing supplements.
People on thyroid medication should not stop or change prescribed medication because of selenium. Correcting a deficiency may support normal physiology, but it is not a substitute for levothyroxine or other medically indicated thyroid treatment.
When to Seek Medical Care
Seek medical care promptly if a low selenium result comes with serious symptoms such as chest pain, fainting, severe shortness of breath, new swelling, rapid heartbeat, severe weakness, confusion, or worsening neurological symptoms. These symptoms are not specific to selenium, but they can signal heart, thyroid, electrolyte, or neurological problems that need urgent evaluation.
Make a non-urgent appointment if you have a low selenium result plus ongoing fatigue, muscle weakness, hair shedding, brittle nails, digestive symptoms, infertility concerns, thyroid symptoms, restrictive eating, or unexplained weight change. Also schedule follow-up if you have Crohn disease, celiac disease, short bowel syndrome, bariatric surgery, kidney dialysis, long-term tube feeding, or parenteral nutrition.
Bring the full lab report, not just the number. The clinician needs the specimen type, units, reference interval, collection date, and any notes about sample quality. Also bring a list of supplements, including multivitamins, thyroid support products, hair and nail formulas, fertility supplements, and Brazil nut intake. Selenium can appear in products where people do not expect it.
Children, pregnant people, and people with chronic kidney disease or complex digestive disease should not self-treat low selenium with high-dose supplements. Their needs and risks differ, and monitoring matters.
A low selenium blood test is usually manageable. The best response is measured: confirm the result, identify the cause, correct intake safely, and check related thyroid or nutrition markers when symptoms or risk factors point in that direction.
References
- Selenium – Health Professional Fact Sheet 2025 (Official Fact Sheet)
- SES – Overview: Selenium, Serum 2026 (Laboratory Test Catalog)
- Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials 2024 (Systematic Review)
- Effects of different supplements on Hashimoto’s thyroiditis: a systematic review and network meta-analysis 2024 (Systematic Review)
- Hypothyroidism 2022 (Review)
- The Influence of Nutritional Intervention in the Treatment of Hashimoto’s Thyroiditis—A Systematic Review 2023 (Systematic Review)
Disclaimer
A low selenium blood test should be interpreted with your full health history, diet, medications, supplement use, and the reference range from the lab that performed the test. Do not start high-dose selenium, stop thyroid medication, or treat suspected deficiency in pregnancy, kidney disease, dialysis, malabsorption, or long-term nutrition support without medical guidance. Seek urgent care for severe shortness of breath, chest pain, fainting, confusion, or new severe weakness.
