What it does
Selenium was discovered in 1817 by Swedish chemist Jöns Jacob Berzelius and named after Selene, the Greek word for moon. In the body it is incorporated as the unusual amino acid selenocysteine into about 25 selenoproteins. The best known is glutathione peroxidase, which breaks down peroxides formed by reactive oxygen species and works with vitamin C and vitamin E as part of the antioxidant defence. Selenium is also needed by the deiodinase enzymes that convert the thyroid hormone T4 into active T3, which is why the thyroid has one of the highest selenium concentrations of any tissue. It also supports sperm formation and immune cell function.
How much and from what
Most guidelines set adult needs at around 55–60 micrograms a day (60 μg in Korea's 2020 reference intakes) with a tolerable upper limit of 400 μg. A single Brazil nut can hold 50–90 μg or more, so one or two a day is plenty; tuna, mackerel and other fish, oysters and shrimp, eggs, meat and whole grains are also good sources. Plant content depends heavily on soil selenium. In Keshan County in China's Heilongjiang province, extremely low soil levels caused an endemic heart-muscle disease, Keshan disease, which fell sharply with selenium supplementation. Deficiency is rare in people eating a varied diet.
Supplements and toxicity
Hopes that selenium might prevent cancer led to the SELECT trial of more than 35,000 men, reported in 2009. Selenium did not reduce prostate cancer, and there was a hint of a slightly higher risk of type 2 diabetes. The current view is that people who already get enough gain little from extra supplements. Taking more than the upper limit for long periods can cause selenosis, with garlic-smelling breath, brittle nails and hair loss. If you combine a multivitamin with a zinc or other mineral product, add up the selenium in each label.
This article is general health information and is not a substitute for diagnosis or treatment. If symptoms persist, see a doctor.
FAQ
- How many Brazil nuts a day is safe?
- One or two cover the daily requirement because each nut is so rich. Eating many every day can push intake past the 400 μg upper limit.
- Is selenium good for the thyroid?
- The thyroid does need selenium to activate its hormones, but in people who are not deficient, the benefit of supplements is uncertain. If you have a thyroid condition, decide with your doctor.
Sources reviewed: 2026-10-26
References
- Rayman, M. P. (2012). Selenium and human health. The Lancet, 379(9822), 1256–1268.
- Lippman, S. M., et al. (2009). Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the SELECT trial. JAMA, 301(1), 39–51.
- National Institutes of Health, Office of Dietary Supplements. (2024). Selenium: Fact Sheet for Health Professionals.
- 한국영양학회. (2020). 2020 한국인 영양소 섭취기준. 보건복지부.