Selenium
Content by: OpenSupplement Editorial Team | Medical review: pending | Last updated: April 13, 2026
TL;DR
Trace mineral essential for thyroid health and glutathione production
Low evidence for preventing cognitive decline and macular degeneration
Has a very 'narrow therapeutic window' (easy to take too much)
Potent antioxidant via selenoproteins
Cheap: ~$5-8/month
Best for: Thyroid health (Hashimoto's), general antioxidant support
What it is
Selenium is a trace mineral that the body uses to create 'selenoproteins.' These are among the most powerful antioxidants we have, specifically responsible for recycling glutathione. It is also critical for the enzyme that converts thyroid hormone from T4 (inactive) to T3 (active).
Most people get enough from food—just one Brazil nut contains more than the daily requirement. However, in areas with 'selenium-poor' soil, deficiency can lead to heart and immune issues.
What the research says
Brain evidence is observational. Evidence. Low selenium levels in the blood have been linked to faster cognitive decline in the elderly. Mechanism. By reducing oxidative stress in the brain, selenium may protect neurons. However, a major RCT (the SELECT trial) showed no benefit for cognitive function in healthy men. It's an 'adequate levels only' nutrient. [1][2][3]
Selenium is part of the 'antioxidant eye' theory. Evidence. While it's often included in vision formulas, the evidence from the AREDS trials shows that other antioxidants (Vitamin C, E, Zinc) are far more important. Selenium's role in AMD is likely minor and indirect.
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Selenium on Amazon
Safety
Caution required. The 'Tolerable Upper Limit' is 400 mcg daily. Chronic intake above this can cause selenosis, characterized by hair loss, brittle nails, and a metallic taste in the mouth. Do not 'double up' on selenium.
Interactions
- Statins: Some evidence suggests high selenium might interfere with the heart-protective benefits of statins and niacin, though this is debated.
Dosing
Standard Dose: 500-200 mcg daily. Form: Selenomethionine is the most bioavailable form.
Cost
Cheap. $5-8 for a multi-month supply.
The bottom line
Selenium is a 'Goldilocks' mineral. You need just enough, but not too much. If you have thyroid issues, it's worth a look; otherwise, just eat a few Brazil nuts a week.
References
- Kryscio RJ, Abner EL, Caban-Holt A, et al. Association of Antioxidant Supplement Use and Dementia in the Prevention of Alzheimer's Disease by Vitamin E and Selenium Trial (PREADVISE). JAMA Neurol. 2017;74(5):567-573.
- Berr C, Balansard B, Arnaud J, et al. Cognitive decline is associated with systemic oxidative stress: the EVA study. Etude du Vieillissement Artériel. J Am Geriatr Soc. 2000;48(10):1285-1291.
- NIH Office of Dietary Supplements. Selenium Fact Sheet for Health Professionals.
Sources for this page include published meta-analyses, systematic reviews, and NIH dietary supplement fact sheets. All claims reflect the evidence as of early 2026.
This is not medical advice. Consult your healthcare provider before starting any supplement, especially if you take medications.