The two jobs of selenium

Why does the body need selenium, and what can a blood test tell you about your supply? This trace element is part of enzymes that protect cells from oxidative damage and activate thyroid hormones. A blood test offers clues about your status. How much the result actually tells you also depends on your diet, any inflammation and your overall health background.

What selenium is and why the body needs it

The body needs selenium to build selenoproteins. These proteins include enzymes that protect cells from oxidative damage or activate thyroid hormones. Because the body cannot make this trace element itself, it has to come from food. Selenium is absorbed through the gut. The liver then handles its transport, partly through selenoprotein P.

When a lab report says “selenium,” it usually means the concentration in serum or plasma. The value is a snapshot and an indirect sign of how much selenium is available to build these enzymes and keep them working. It does not directly show how active the selenoproteins are inside cells.

How selenium protects cells and activates thyroid hormones

Normal metabolism and immune responses produce reactive oxygen species. Selenium-containing enzymes, above all the glutathione peroxidases and thioredoxin reductases, help break down these byproducts before they damage DNA, fats or proteins.

Thyroid hormones come mainly in two forms: T4, the storage form, and T3, the active form. To convert T4 into T3, the body needs deiodinases. These enzymes contain selenium as well. Low selenium status can alter this conversion and so throw thyroid hormone activity off balance.

Why doesn’t a higher selenium level automatically mean a stronger antioxidant effect? For glutathione peroxidase, a saturation level of roughly 90 µg/L is given. Serum selenium can keep rising with higher intake without the enzyme becoming any more active. The exact threshold, however, has not yet been firmly tied to a source: the reference cited for optimal selenium status is Hurst et al. (2010), PubMed, DOI.

The relationship between selenium status and risk is described as U-shaped: both very low and very high status may carry risks. Observational data from regions with marked deficiency show an association between very low selenium levels, a higher risk of heart muscle disease and impaired immune defense. Some cohorts also report an association between very high intake and a higher risk of type 2 diabetes. That cohort statement, however, is not directly supported by the cited source: according to its title, Stranges et al. (2007), PubMed, DOI, describes a randomized trial.

The SELECT study, short for “Selenium and Vitamin E Cancer Prevention Trial,” found that selenium supplements did not lower prostate cancer risk in well-supplied populations. Safety concerns have also been raised in connection with combining selenium and high-dose vitamin E. Whether they can be attributed to exactly this combination remains open. The source cited is Lippman et al. (2009), PubMed, DOI. Beyond an adequate supply, extra selenium offers no established additional benefit and may carry risks.

Inflammation can make a blood value especially hard to interpret. During infections or inflammation, the liver shifts its protein production and selenoprotein P levels fall. As a result, less selenium may show up in serum even when the body’s overall stores are sufficient. Selenium is therefore considered a negative acute-phase marker: its blood level can drop during an acute inflammatory response. A low value during or shortly after an illness does not reliably indicate a true deficiency.

How to interpret a selenium result

What the reference range tells you

A reference range describes the values measured in most people. It does not set a value that is optimal for everyone. Its limits depend on the measurement method and the population studied. Many labs give roughly 70–150 µg/L for serum selenium.

Enzyme activity data suggest that certain selenoproteins already reach a functional plateau in the lower to middle part of this range. In other words, their activity does not rise further even with a higher supply. For glutathione peroxidase, about 90 µg/L is usually given as the saturation level; whether this threshold can be attributed precisely to the cited study remains open. Selenoprotein P rises at first as intake increases. Once the tissues are supplied, the increase levels off. The literature cited is Hurst et al. (2010), PubMed, DOI.

Correcting an inadequate supply can help. Once the supply is adequate, however, pushing it higher does not keep adding benefit. Age, diet, pregnancy and inflammation also affect how a result should be read.

When levels are high

High levels usually reflect high intake from supplements or selenium-rich foods. The selenium content of Brazil nuts varies widely depending on the soil. People who eat them regularly may end up exceeding the tolerable upper intake level for the day. Multivitamins and hair, skin and nail products can also contain selenium without this being obvious at first glance.

Less often, very high levels result from manufacturing errors, mislabeled supplements or mistakes in parenteral nutrition (intravenous feeding). Living in a region with selenium-rich soils can also contribute to high levels.

A tolerable upper intake level of 400 µg per day is given for adults. A limit like this is meant to reduce the risk of toxic effects. It is unclear, however, which reference body this figure comes from and whether it applies to Germany or Europe. The 400 µg per day is therefore not a general intake recommendation.

Persistently high levels can go along with selenosis, meaning selenium poisoning. Possible symptoms include brittle hair and nails and a metallic or garlic-like breath odor. With higher exposure, digestive complaints and neurological changes may follow.

When levels are low

Low levels can result from too little intake, impaired absorption in the gut or increased needs. In regions with selenium-poor soils, including parts of Europe and Asia, diet-related undersupply is more common.

Diseases and procedures that reduce absorption in the gut can also lower levels. These include celiac disease, inflammatory bowel disease and bariatric surgery. The same applies to long-term total parenteral nutrition if it does not contain enough selenium.

Needs rise during pregnancy. At the same time, serum levels may dip slightly because blood volume expands and the placenta and fetus need selenium. During infections or inflammatory flare-ups, levels may fall temporarily even though the body as a whole is adequately supplied. With a low result, it is therefore worth looking at when the blood was drawn: did it coincide with an illness?

What can change your selenium level between two blood draws

Selenium from food and supplements

Selenium enters the body through food and supplements. A mixed diet with protein sources such as fish and seafood, eggs and meat, plus grains or legumes, provides enough in most regions. In plant foods, the content depends on the soil they grow in and so varies by origin.

Brazil nuts contain a lot of selenium, but the amount is hard to predict. A single nut may contain relatively little or may already supply more than the recommended intake for a whole day. The “Recommended Dietary Allowance” (RDA) for adults is given as 55 µg per day. This is not a verified German reference value. Needs are higher during pregnancy and breastfeeding.

If you take several supplements and multivitamins, what counts is the total. Together they can push daily intake above the tolerable upper level without anyone noticing. The label of any single product does not show this total.

Absorption problems and medical conditions

Celiac disease, inflammatory bowel disease and bariatric surgery can impair selenium absorption. With long-term parenteral nutrition, status depends on how much selenium is provided. Dialysis and advanced kidney disease change how the body handles trace elements.

In autoimmune thyroiditis, there are links to selenium status through the deiodinases and the antioxidant systems. In pregnancy, needs rise while the levels measured in blood may dip slightly.

Acute illness and inflammation

Active infections, inflammatory flare-ups or heavy physical strain can temporarily lower serum selenium. As a negative acute-phase marker, selenium responds in these situations regardless of diet or actual body stores. A blood draw during an acute illness can therefore suggest poorer status than actually exists. Results taken under stable conditions without acute inflammation are the easiest to interpret.

Which other lab values can help with interpretation

• Thyroid-stimulating hormone (TSH): Selenium-containing deiodinases convert T4 into T3. A low selenium level together with rising TSH could point to inadequate activation of thyroid hormones. Under this interpretation, which has not yet been clinically tested, the pattern would not necessarily stem only from a disease of the thyroid itself. On this basis, TSH is not a standalone diagnostic criterion for selenium deficiency.
• Free triiodothyronine (free T3): Free T3 is produced by selenium-dependent conversion via the deiodinases. Low selenium status could reduce free T3 even when TSH and T4 are normal. How to interpret this clinically is open. It does not follow that free T3 is suitable as a functional indicator of selenium’s effect on the thyroid; it is not a standalone diagnostic criterion for selenium deficiency.
• High-sensitivity C-reactive protein (hs-CRP): Elevated hs-CRP together with low serum selenium may indicate that inflammation is temporarily lowering the selenium level. In that case there need not be a true diet-related deficiency, because selenium is a negative acute-phase marker.
• Thyroid peroxidase antibodies (TPO antibodies): Selenium helps protect the thyroid from the oxidative stress produced during hormone synthesis. Cohorts with autoimmune thyroiditis report an association between low selenium status and higher TPO antibody levels. Whether the cited sources support exactly this statement is open: Wichman et al. (2016), PubMed, DOI, and Zuo et al. (2021), PubMed, DOI. According to its title, Wichman et al. examines the effect of supplementation on thyroid autoantibodies. That does not directly support the described association in cohorts.
• Vitamin D (25-hydroxy): Vitamin D and selenium both contribute to immune regulation and thyroid function. With absorption problems, both deficiencies can occur together. Measuring them together could therefore be informative; a clinical benefit of this test combination is not established here, however.

When a repeat selenium test can show changes

After a lasting change in intake, plasma selenium needs time to settle at a new level. A period of 8–12 weeks is given, with reference to studies on repleting selenium status. The literature cited is Hurst et al. (2010), PubMed, DOI. After a change in diet or starting a supplement, a repeat test after 8–12 weeks could therefore show whether levels have changed. Whether this interval fits an individual case is a clinical question.

For healthy adults without an absorption problem or confirmed deficiency, an annual check is described as sufficient. That does not mean annual selenium tests are generally necessary, though; the benefit and the right interval for routine checks are unresolved here. The differing figure of one to two tests per year in the FAQ should not be read as an established recommendation either.

To make results as comparable as possible, tests should be done at the same lab using the same method. Serum and plasma values can differ and are not directly interchangeable.

Where possible, avoid having blood drawn during or shortly after an acute illness. During the acute-phase response, selenium is redistributed. This can lower the serum value and produce a misleadingly low result. A repeat test under stable conditions without acute inflammation can provide a more meaningful baseline.

When selenium results should be discussed with a doctor

A result outside the reference range first needs to be put in context. For values above 150 µg/L, the suggestion is to review supplements and multivitamins and to retest after making changes. This applies especially if brittle hair or nails, garlic-like breath or digestive changes are also present.

A persistently low value should be discussed with a doctor before you change your intake on your own. This matters especially with absorption problems, autoimmune thyroid disease, dialysis or pregnancy.

If a value is low during an acute illness or alongside elevated hs-CRP, a temporary drop caused by the acute-phase response is a possibility. In that case there need not be a true deficiency. A repeat test under stable conditions helps clarify this. The trend across two or more blood draws usually gives a better basis for decisions than a single result.

Selenium is essential for life, but too much of it can do harm. When it comes to judging your supply, a blood value is most useful alongside other findings: the trend in thyroid values, inflammation markers and levels of related nutrients.

Frequently asked questions

What does a selenium blood test measure?

A serum or plasma selenium test measures how much selenium is circulating in the blood. The usual unit is micrograms per liter, abbreviated µg/L or mcg/L. The value reflects recent dietary intake and the amount available to build selenoproteins. These include enzymes that protect cells from oxidative damage and activate thyroid hormones. The test gives a useful snapshot but does not capture all the stores in the tissues.

How does selenium support thyroid function?

The body builds selenium into deiodinases. These enzymes convert thyroxine (T4) into the more potent triiodothyronine (T3). If selenium supply is inadequate, this conversion may run less efficiently. That could affect how thyroid hormones act throughout the body. Selenium also helps protect the thyroid from the oxidative stress produced during hormone synthesis.

What is a healthy selenium level in the blood?

Many labs give roughly 70 to 150 µg/L as the reference range for serum selenium. The exact limits depend on the method and the population studied. For the activity of glutathione peroxidase, an important antioxidant enzyme, a plateau is given at about 90 µg/L. By that account, values in the lower part of the reference range may also be compatible with adequate enzyme activity. Whether this threshold can be attributed precisely to the cited literature remains open: Hurst et al. (2010), PubMed, DOI. Because reference ranges vary between labs, your specific result should be interpreted by a doctor.

What causes low selenium levels?

The most common cause given is too little intake from the diet, especially in regions with selenium-poor soils. With celiac disease, inflammatory bowel disease or after bariatric surgery, absorption in the gut may also be impaired.

Infections or systemic inflammation can also lower levels temporarily even though the body’s overall stores are sufficient. That is why selenium is called a negative acute-phase marker: its blood level can drop during such a response without a true deficiency being present.

What signs can point to too little or too much selenium?

Persistently low selenium status does not always cause symptoms. It can, however, be linked to weaker antioxidant defense and less favorable conversion of thyroid hormones.

An excess with toxic effects is called selenosis. Possible effects include brittle hair and nails, a metallic or garlic-like breath odor and digestive complaints. With higher exposure, neurological changes can occur. A tolerable upper intake level of 400 micrograms per day is given for adults. Whether this limit applies to Germany or Europe is unclear; it is not an intake recommendation.

Can you raise your selenium level through diet alone?

For most people with a mild shortfall, dietary changes can improve selenium status. A period of 8 to 12 weeks is given for a noticeable improvement; this figure has not been independently verified here. Good sources include fish and seafood, eggs, meat, whole grains and Brazil nuts. With Brazil nuts in particular, though, the content varies widely. People who eat them regularly may exceed the daily upper limit.

A varied diet with enough protein generally supplies enough selenium, so supplements are not needed. Whether it is enough in an individual case also depends on where the food comes from and on possible absorption problems.

How often should selenium be measured?

After a lasting change in intake, a period of 8 to 12 weeks is usually given for serum selenium to reach a new steady state. That does not establish a generally suitable monitoring interval. For most healthy adults who want to keep an eye on their nutritional status, testing once or twice a year is described as helpful. The benefit of this frequency has not been independently clinically tested, however, and the figure also differs from the annual check given in the main text.

With a condition that impairs absorption, with parenteral nutrition or after a major change in diet, more frequent checks under medical supervision may be appropriate.

Are there interactions between selenium, thyroid medication and iodine status?

Selenium and iodine work together in thyroid hormone metabolism. If iodine is lacking, better selenium status cannot fix that bottleneck. Conversely, more iodine does not make up for an inadequate selenium supply.

In people taking thyroid hormone replacement therapy, selenium status probably does not directly change how the medication works. An inadequate supply may, however, affect how well the thyroid itself functions. Which combined tests and which decisions about supplementation make sense is best worked out with a doctor who knows your thyroid history.

References

Note: The assignment of some statements to the references is partly open; supporting sources are missing or not yet clearly assigned. The linked records have not been independently verified.

• Lippman SM, Klein EA, Goodman PJ, Lucia MS, Thompson IM, Ford LG, Parnes HL, Minasian LM, Gaziano JM, Gaziano JM, Hartline JA, Parsons JK, Bearden JD, 3rd, Crawford ED, Goodman GE, Claudio J, Winquist E, Cook ED, Karp DD, Walther P, … Coltman CA, Jr. (2009). Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA, 301(1), 39-51. https://doi.org/10.1001/jama.2008.864. PubMed
• Stranges S, Marshall JR, Natarajan R, Donahue RP, Trevisan M, Combs GF, Cappuccio FP, Ceriello A, Reid ME (2007). Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Annals of internal medicine, 147(4), 217-23. https://doi.org/10.7326/0003-4819-147-4-200708210-00175. PubMed
• Hurst R, Armah CN, Dainty JR, Hart DJ, Teucher B, Goldson AJ, Broadley MR, Motley AK, Fairweather-Tait SJ (2010). Establishing optimal selenium status: results of a randomized, double-blind, placebo-controlled trial. The American journal of clinical nutrition, 91(4), 923-31. https://doi.org/10.3945/ajcn.2009.28169. PubMed
• Wichman J, Winther KH, Bonnema SJ, Hegedüs L (2016). Selenium Supplementation Significantly Reduces Thyroid Autoantibody Levels in Patients with Chronic Autoimmune Thyroiditis: A Systematic Review and Meta-Analysis. Thyroid, 26(12), 1681-1692. https://doi.org/10.1089/thy.2016.0256. PubMed
• Zuo Y, Li Y, Gu X, Lei Z (2021). The correlation between selenium levels and autoimmune thyroid disease: a systematic review and meta-analysis. Annals of palliative medicine, 10(4), 4398-4408. https://doi.org/10.21037/apm-21-449. PubMed