A selenium blood test measures how much of this trace mineral is circulating in your serum or plasma, and it is usually ordered when a doctor suspects either a shortfall or an overload. Selenium is needed in tiny amounts, yet it sits at the center of your antioxidant defenses and of your thyroid’s ability to activate its own hormones. Because the distance between too little and too much is narrower for selenium than for almost any other nutrient, the number on your report deserves a careful reading rather than a quick glance. In this article you will learn what the test actually measures, how to interpret the reference ranges most American laboratories print, what low and high values tend to signal, which everyday factors distort the result, and when a follow-up conversation with your doctor is worth having.
What a selenium blood test measures
Selenium is a trace mineral, meaning your body needs it in microgram quantities rather than the milligram or gram amounts required for calcium or magnesium. Once absorbed, it is built into roughly twenty-five specialized proteins called selenoproteins. These include glutathione peroxidases, which neutralize damaging by-products of normal metabolism, and deiodinases, the enzymes that convert storage thyroid hormone into its active form. A blood test cannot see those enzymes directly, so it measures the mineral available to make them.
Serum, plasma, and whole blood
Most laboratories report serum or plasma selenium, which reflects your intake over the previous weeks. Whole blood selenium, which includes the mineral stored inside red blood cells, changes more slowly and reflects several months of intake. A third option, red cell glutathione peroxidase activity, gauges function rather than quantity but is rarely used outside research settings. If your report does not state which sample type was used, the reference interval printed beside your value is the surest clue, because whole blood values normally run higher than serum values.
Why a doctor orders it
Selenium is not part of a routine checkup. It is typically requested when someone has malabsorption, has been on long-term intravenous nutrition, follows a highly restricted diet, lives in an area with selenium-poor soil, takes high-dose supplements, or has thyroid symptoms that standard testing has not explained. Doctors also order it alongside other trace minerals, because deficiencies rarely travel alone. Track a closely related mineral by reading our zinc blood test levels guide.
Selenium reference ranges and how to read your report
Adult serum selenium is commonly reported with a reference interval of roughly 70 to 150 micrograms per liter, although each laboratory sets its own boundaries from the population it serves. The National Institutes of Health Office of Dietary Supplements considers serum or plasma concentrations of 8 micrograms per deciliter, which equals 80 micrograms per liter, to be sufficient in healthy people. Values a few points outside the printed range are not automatically a diagnosis; they are an invitation to look at your diet, your symptoms, and your other laboratory results together.
A practical reading table
| Result band | Typical serum value | What it usually suggests | Common next step |
|---|---|---|---|
| Clearly low | Below about 50 mcg/L | Genuine shortfall, often from malabsorption or a very restricted diet | Medical review of diet, gut health, and possible supplementation |
| Borderline low | About 50 to 70 mcg/L | Intake at the low end, or a value pulled down by inflammation | Dietary adjustment and a repeat test after two to three months |
| Within range | About 70 to 150 mcg/L | Adequate supply for selenoprotein production | No action beyond a balanced diet |
| Above range | Above about 150 mcg/L | Usually supplements, sometimes very high Brazil nut intake | Review every supplement label with your doctor |
| Markedly high | Above about 400 to 500 mcg/L | Excess intake with a real risk of selenosis | Prompt medical assessment and a review of exposure sources |
Units and why laboratories differ
American reports usually print micrograms per liter or micrograms per deciliter, while some laboratories use micromoles per liter. One microgram per deciliter equals ten micrograms per liter, and one micromole per liter equals about seventy-nine micrograms per liter. Converting before you compare two reports prevents a great deal of unnecessary worry. Soil selenium content also varies enormously across regions, so a population range built in one state may sit slightly higher or lower than a range built in another.
What low selenium levels can mean
Outright selenium deficiency is uncommon in the United States, because American soils and the food supply are comparatively rich in the mineral. When it does appear, it is usually the consequence of something else rather than a simple dietary gap.
Who is most likely to run low
People with Crohn disease, celiac disease, or a history of bowel surgery absorb less of what they eat. Those receiving long-term intravenous feeding depend entirely on what the formula provides. People on dialysis lose trace minerals during treatment, and people living with HIV often show lower values. Very restrictive elimination diets, severe alcohol use, and advanced liver disease also push levels down. Because low selenium frequently accompanies other micronutrient gaps, doctors often check several markers at once. Assess a second antioxidant nutrient by reading our vitamin E blood levels guide.
Symptoms that may accompany a low result
Selenium deficiency has no signature symptom. People report fatigue, muscle weakness or aching, hair thinning, poor wound healing, and more frequent infections. Two named diseases have been linked to severe, long-standing deficiency in selenium-poor regions of the world: Keshan disease, which affects the heart muscle, and Kashin-Beck disease, which affects joints and growing bone. Neither is seen in ordinary American practice. Because fatigue has so many causes, doctors usually look at iron stores and inflammation in parallel. Rule out a far more common cause of tiredness by reading our ferritin blood level guide.
What high selenium levels can mean
High selenium is almost always a supplement story. The mineral is so concentrated in some foods that a single generous handful of Brazil nuts can exceed a full day’s requirement several times over, and stacking a multivitamin, an antioxidant formula, and a thyroid support product can quietly triple an intended dose.
Recognizing selenosis
Chronic excess produces a condition called selenosis. According to the National Institutes of Health Office of Dietary Supplements, its most recognizable signs are hair loss and brittle or shedding nails, often accompanied by a garlic odor on the breath, a metallic taste in the mouth, skin rash, nausea, diarrhea, fatigue, irritability, and nervous system changes. These symptoms build slowly, which is exactly why a blood measurement is more reliable than self-assessment.
How much is too much
| Group | Recommended daily intake | Tolerable upper intake level |
|---|---|---|
| Adults, 19 years and older | 55 micrograms | 400 micrograms |
| Pregnancy | 60 micrograms | 400 micrograms |
| Breastfeeding | 70 micrograms | 400 micrograms |
Those figures come from the National Institutes of Health and MedlinePlus. The practical lesson is that the useful dose and the risky dose are separated by a factor of roughly seven, which is a much smaller safety margin than most people assume when they buy a supplement. Excess of one trace mineral can also unbalance another, so doctors sometimes measure the group together. Examine a mineral whose balance is easily disturbed by supplements in our copper blood test results guide.
Selenium, your thyroid, and autoimmunity
The thyroid gland holds more selenium per gram of tissue than any other organ, which explains why so much research on this mineral is thyroid research.
Selenoproteins inside the thyroid
Making thyroid hormone generates hydrogen peroxide, a reactive molecule that would damage the gland if it were not cleared. Selenium-containing enzymes do that clearing. Other selenoproteins, the deiodinases, convert thyroxine into the more active triiodothyronine that your tissues use. A shortfall therefore has the potential to affect both the safety and the efficiency of hormone production. Understand the hormone that drives the whole system by reading our TSH blood test results guide.
Which thyroid tests belong alongside selenium
If selenium was ordered because of thyroid symptoms, it rarely stands alone. Thyroid-stimulating hormone shows how hard the pituitary is pushing the gland, free thyroxine shows what the gland is producing, and thyroid peroxidase antibodies show whether the immune system is attacking it. Interpret the antibody that defines autoimmune thyroiditis in our Anti-TPO blood test results guide. Explore the condition these markers most often point toward in our hypothyroidism symptoms and causes guide.
What can distort a selenium result
A single number is a snapshot of one morning, and several ordinary circumstances move that snapshot without anything being wrong with your nutrition.
Inflammation, protein status, and timing
Selenium travels in the blood attached to carrier proteins. During infection, injury, or any acute inflammatory episode, those carrier proteins redistribute and serum selenium falls temporarily, which can imitate a deficiency. The same happens when blood protein levels are low from liver disease or malnutrition. For that reason doctors often read selenium next to an inflammatory marker and a protein marker. Interpret the marker that flags an inflammatory episode in our CRP blood test results guide. Check the carrier protein that shapes many mineral results in our albumin test results guide.
Sample handling and laboratory differences
Trace mineral testing is unusually sensitive to contamination, because ordinary rubber stoppers and some collection materials contain traces of metals. Laboratories therefore use special trace-element tubes, and a result drawn into the wrong tube can read falsely high. Methods differ between laboratories as well, so a value should ideally be compared with a previous result from the same laboratory. If you supplement, note the timing: a dose taken shortly before the draw raises the reading without telling you much about your longer-term status. Compare how another trace mineral behaves in testing by reading our chromium blood test results guide.
When to see a doctor about your selenium result
Use these signals rather than the size of the deviation alone to decide how urgently to act.
- Hair loss, brittle or shedding nails, a persistent garlic breath odor, or a metallic taste alongside a high result deserve prompt medical review, because they are the classic signs of selenosis.
- Any result above the reference range while you are taking supplements is worth discussing before your next dose, and bringing every label with you shortens the conversation.
- A low result combined with digestive symptoms, unintended weight loss, or a known bowel condition points toward an absorption problem that needs investigating in its own right.
- A low result with thyroid symptoms should be interpreted together with your thyroid panel rather than in isolation.
- Numbness, tingling, or unsteadiness with either a very high or a very low result warrants medical attention without waiting for a repeat test.
Between those situations, a borderline value usually calls for a dietary review and a repeat measurement after two to three months, since blood selenium responds to intake gradually. Balance the picture with another mineral commonly checked at the same time by reading our magnesium blood test results guide.
Latest scientific advances
Research published over the past three years has sharpened, rather than overturned, what doctors already believed about selenium. Three findings stand out for anyone holding a test result.
Selenium supplements and Hashimoto thyroiditis
A systematic review and meta-analysis in the journal Thyroid pooled thirty-five randomized trials and found that selenium supplementation modestly lowered thyroid-stimulating hormone in people with Hashimoto thyroiditis who were not yet taking thyroid hormone replacement, and lowered thyroid peroxidase antibodies whether or not people were on replacement. Side effects were no more frequent than with placebo. What this means for you: if you have autoimmune thyroiditis, selenium may nudge two laboratory numbers in a favorable direction, but the review found no change in the active thyroid hormones themselves, so it is a possible adjunct rather than a treatment. The certainty of the evidence was rated moderate, which means future trials could still shift the picture.
Selenium status and long-term health outcomes
A 2025 systematic review and meta-analysis in Redox Biology combined twenty population studies, following large groups of people over many years. Higher selenium biomarkers, meaning measurements of the mineral in blood, went along with lower death rates overall and from heart disease and cancer specifically. What this means for you: an adequate selenium status appears to sit alongside better long-term health, but this is an observational pattern, not proof that taking a supplement produces the benefit. The authors themselves concluded that trials in genuinely deficient populations are still needed. Reading it as an argument for high-dose supplementation would go well beyond what the data support.
Where the evidence is still unsettled
A 2025 narrative review in the Journal of Trace Elements in Medicine and Biology surveyed the whole field and reached a deliberately cautious conclusion: selenium looks most promising in autoimmune thyroid disease, especially with the selenomethionine form, but the evidence remains inconsistent for hypothyroidism, thyroid problems in pregnancy, goiter, and thyroid cancer. An earlier overview of systematic reviews in Nutrients reached the same verdict, noting that antibody reductions were real but rested on low-certainty evidence and faded by the twelve-month mark. What this means for you: results depend heavily on where you live and on how much selenium you started with, and every one of these reviews repeats the same warning about the toxicity risk of excess intake. Testing before supplementing is the practical conclusion the literature keeps pointing to.
Glossary of key terms
| Term | Definition |
|---|---|
| Trace mineral | A nutrient the body needs in microgram amounts. Selenium, zinc, copper, and chromium all belong to this group. |
| Selenoprotein | Any protein that incorporates selenium into its structure. Humans make about twenty-five of them, and they carry out most of selenium’s work. |
| Glutathione peroxidase | A family of selenium-dependent enzymes that neutralize hydrogen peroxide and other reactive by-products of normal metabolism. |
| Deiodinase | A selenium-dependent enzyme that converts thyroxine, the storage form of thyroid hormone, into the more active triiodothyronine. |
| Oxidative stress | An imbalance between reactive molecules produced during metabolism and the antioxidant systems that clear them. |
| Selenosis | The clinical picture of chronic selenium excess, marked by hair loss, nail changes, garlic breath odor, and digestive or nervous system symptoms. |
| Tolerable upper intake level | The highest daily intake considered unlikely to cause harm in the general population. For adult selenium it is 400 micrograms per day. |
| Selenomethionine | The main natural food form of selenium, in which the mineral replaces sulfur inside the amino acid methionine. It is also the form used in most supplements studied in trials. |
| Thyroid peroxidase antibody | An immune protein directed against a thyroid enzyme. Its presence indicates autoimmune thyroid disease such as Hashimoto thyroiditis. |
| Reference interval | The range of values found in most healthy people tested by a given laboratory. It is a comparison tool, not a personal health target. |
Frequently asked questions
What is the selenium blood test called on a lab order?
It is most often listed as selenium, serum or plasma, and sometimes abbreviated to Se. Some laboratories offer selenium, whole blood, and a few include it inside a broader trace element or heavy metals panel alongside zinc and copper. If your order simply says selenium, the sample type is usually serum. The distinction matters when you compare results, since whole blood values normally read higher than serum values from the same person on the same day.
What is a normal range for a selenium blood test?
Most American laboratories report an adult serum reference interval of roughly 70 to 150 micrograms per liter, and the National Institutes of Health Office of Dietary Supplements treats 80 micrograms per liter or higher as sufficient in healthy people. Ranges vary between laboratories because they are built from local populations, and regional soil selenium differs considerably across the country. Always read your value against the interval printed on your own report rather than one you found elsewhere.
What causes high selenium levels in blood?
Supplements are the usual explanation, particularly when several products are taken together and each contributes selenium without the total being obvious. Very high Brazil nut consumption is the other common cause, since a small number of nuts can deliver more than a day’s requirement. Occupational exposure and, rarely, mislabeled or contaminated products account for the remainder. A recent supplement dose taken close to the blood draw can also lift a single reading.
What are the symptoms of too much selenium?
The earliest and most recognizable signs are hair loss and nails that become brittle, ridged, or shed. Many people also notice a garlic-like odor on the breath and a metallic taste in the mouth. Nausea, diarrhea, skin rash, fatigue, and irritability can follow, and prolonged excess may cause numbness or tingling. Because these symptoms appear gradually and resemble many other conditions, they are easy to miss without a blood measurement to anchor them.
How can I lower my selenium levels naturally?
In practice, lowering selenium means removing the source. That usually involves stopping or reducing supplements under medical guidance, checking every product label including multivitamins and thyroid or antioxidant formulas, and limiting Brazil nuts to an occasional few rather than a daily handful. Blood levels fall gradually over weeks to months once intake drops. Do not stop a prescribed supplement on your own, and ask for a repeat measurement rather than assuming the problem has resolved.
Should I take a selenium supplement if my thyroid is underactive?
Not automatically. Research suggests selenium may help lower thyroid antibodies in autoimmune thyroiditis, but it does not replace thyroid hormone treatment and the evidence is weaker for hypothyroidism generally. Benefit also depends on whether you were low to begin with, which is precisely what a blood test answers. Because the gap between a helpful and a harmful dose is narrow, this is a decision to make with the doctor who manages your thyroid, ideally after measuring your level.
Sources
- Office of Dietary Supplements, National Institutes of Health — Selenium: Fact Sheet for Health Professionals — NIH ODS, updated 2025 — nih.gov
- MedlinePlus, National Library of Medicine — Selenium in diet — MedlinePlus Medical Encyclopedia, reviewed 2025 — medlineplus.gov
- Cleveland Clinic — Mineral Deficiency: Symptoms, Causes and Treatment — Cleveland Clinic Health Library, 2025 — my.clevelandclinic.org
- Huwiler VV, Maissen-Abgottspon S, Stanga Z, Mühlebach S, Trepp R, Bally L, Bano A — Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials — Thyroid, 2024 — doi.org/10.1089/thy.2023.0556
- Cui Z, Xie R, Lu X, Schomburg L, Brenner H, Schöttker B — Associations of selenium status with all-cause and cause-specific mortality: a systematic review and meta-analysis of cohort studies — Redox Biology, 2025 — doi.org/10.1016/j.redox.2025.103755
- Linn YH, Aziz S, Mok SL — Selenium status and supplementation in thyroid disorders: A narrative review of current evidence — Journal of Trace Elements in Medicine and Biology, 2025 — doi.org/10.1016/j.jtemb.2025.127799
- Wang YS, Liang SS, Ren JJ, Wang ZY, Deng XX, Liu WD, Yan YL, Song GH, Li XX — The Effects of Selenium Supplementation in the Treatment of Autoimmune Thyroiditis: An Overview of Systematic Reviews — Nutrients, 2023 — doi.org/10.3390/nu15143194
Further reading
- Measure the hormone your thyroid actually releases by reading our free T4 lab results guide.
- Follow the active hormone that selenium-dependent enzymes produce in our free T3 lab results guide.
- Decode the antibody test that screens for autoimmune thyroid disease in our TPO lab test explainer.
- Recognize the opposite thyroid problem by reading our hyperthyroidism symptoms and treatments guide.
- Put your trace mineral results in context with the panel most often drawn beside them in our complete blood count results guide.
Understand your lab results with BloodSense
A selenium value only becomes meaningful next to your thyroid markers, your inflammatory markers, and your own symptoms. BloodSense reads your report in context, explains in plain language what a low or high selenium level suggests, and shows how it relates to results such as TSH, free T4, zinc, and copper. It helps you understand your numbers and prepare better questions for your doctor; it does not diagnose, and it does not replace medical care.
