Selenium
Trace element for antioxidation, thyroid function, and immune defense
Also known as
Definition
Selenium is an essential trace element incorporated into selenoproteins (e.g., glutathione peroxidases, thioredoxin reductases, deiodinases). It protects cells from oxidative stress, regulates thyroid hormone activation (T4 → T3), and is essential for the immune system. Selenium status varies greatly by region: soils in Germany and Central Europe are selenium-poor, making deficiency common.
| Parameter | Value |
|---|---|
| Unit | µmol/l (µg/l) |
| Reference Range | 0.89–1.65 µmol/l (70–130 µg/l) |
Agree on your personal target range with your doctor.
Conversion: 1 µmol/l = 79 µg/l. US labs report in µg/l or ng/ml (normal: 70–150 µg/l).
↓ What a low value means
Selenium deficiency (< 0.89 µmol/l) occurs in selenium-poor regions (Central Europe), with vegan diets, kidney disease, or chronic bowel inflammation. Consequences: increased thyroid risk (Hashimoto's, hypothyroidism), weakened immune system, elevated oxidative stress, muscle pain (Keshan disease with extreme deficiency), elevated TPO antibodies.
↑ What a high value means
Selenium toxicity results from excessive supplementation or very high consumption of Brazil nuts. Symptoms: selenosis with hair loss, nail changes, garlic breath odor, nerve damage, nausea. In 2023 EFSA set a new tolerable upper intake level for selenium, lower for adults than before; it is based on hair loss as an early sign of selenosis (EFSA overview of tolerable upper intake levels, version 11, August 2025). It is a safety limit, not an intake recommendation.
✓ What can influence this value
The selenium value is influenced by the selenium content of the soil in the region of origin (low in Central Europe), diet (e.g., Brazil nuts with highly variable selenium content, tuna, salmon, beef, poultry, eggs), vegan diets, kidney disease, and chronic bowel inflammation. Selenium-containing supplements raise the value; the organic form selenomethionine is absorbed better than inorganic compounds. The assessment is made by the treating doctor.
When to test
Reasons for a measurement can include thyroid disease (especially Hashimoto's), a vegan diet, chronic inflammatory conditions, or hair loss. Selenoprotein P is a more sensitive marker than serum selenium. Long-term status is better reflected by whole-blood selenium.
Frequently asked questions
What role does selenium play in Hashimoto's thyroiditis? +
Multiple randomized trials show that TPO antibody titers in Hashimoto's fall and thyroid inflammation decreases with selenium. Whether and how treatment is given is decided by your doctor.
What role do Brazil nuts play in the selenium level? +
Brazil nuts have the highest selenium content of any food, with wide variation by origin (Brazil > Bolivia). Even a few nuts can affect the selenium level markedly; high consumption can lead to excessive selenium intake.
What is the difference between selenomethionine and sodium selenite? +
Selenomethionine (organic) is better absorbed and stored in muscles. Sodium selenite (inorganic) is less well absorbed but acts faster on selenoprotein levels and was used in some Hashimoto's studies.
Is selenium really important for the thyroid? +
Yes. Selenium is essential for the deiodinases that convert thyroxine (T4) into active triiodothyronine (T3). Without adequate selenium, T3 remains low despite normal TSH. Selenoproteins also protect the thyroid from oxidative stress, which plays a role in autoimmune thyroiditis.
Selenium
Sources
Last updated: May 28, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.