Selenium
MineralsSelenium is an essential trace element that your body builds into proteins called selenoproteins.
Also known as: Se, serum selenium
They help protect your cells from oxidative damage, support your immune system and are needed to turn the thyroid hormone T4 into its active form, T3. You get selenium mainly from fish, eggs, meat and whole grains, and Brazil nuts are so rich in it that eating several every day can push your intake too high. Blood selenium mostly reflects your recent intake and also drops with inflammation, so your result is read together with your hs-CRP and albumin values. Both too little and too much matter: high doses from supplements can cause hair loss, brittle nails and stomach upset.
Why this matters
Low selenium can impair thyroid function, weaken the immune response, and increase oxidative stress, with severe deficiency linked to heart-muscle and joint disorders. Very high selenium from over-supplementation can be toxic, causing brittle hair and nails, gastrointestinal upset, and neurological symptoms. Maintaining adequate selenium supports thyroid health, antioxidant defenses, and immunity.
How this connects to other biomarkers
- Check hs-CRP and Albumin before interpreting a low result: inflammation triggers a negative acute-phase response that drives serum Selenium down independent of true selenium status, and low Albumin — selenium's main carrier — compounds this. A low Selenium with elevated hs-CRP and/or low Albumin likely reflects acute or chronic inflammation rather than a dietary deficiency.
- Selenium is required to convert thyroid hormone T4 into its active form T3, so low selenium can blunt thyroid action even when TSH and T4 look normal.
- As a core component of antioxidant (glutathione peroxidase) enzymes, low selenium reduces antioxidant capacity.
- Both deficiency and excess matter: chronic over-supplementation (e.g. too many Brazil nuts or supplements) can push selenium into the toxic range (selenosis).
How often should I test Selenium?
There is no fixed schedule for testing selenium in healthy adults. A first measurement is useful if you follow a restricted diet, have a digestive condition that affects absorption, or take selenium or a multivitamin with selenium every day. If you start or stop a supplement, retest after about 3 months, since blood selenium follows recent intake.
At baseline / for screening: Once as a baseline, especially with a restricted diet, an absorption problem or regular selenium supplements.
When monitoring an intervention or change: About 3 months after starting, stopping or changing a selenium supplement.
Note: Inflammation and infection lower blood selenium for a while, so a low value during or just after an illness may not reflect your real supply. Skip selenium supplements on the day of your blood draw.
Sources
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