Selenium
Trace MineralAlso known as: Selenomethionine · Sodium selenite · Se
An essential trace mineral and component of selenoproteins including glutathione peroxidase and thyroid deiodinases. Important for thyroid function and antioxidant defense. Narrow therapeutic window — excess is toxic.
How expert claims hold up
68 of 86 claims assessed18 of 68 assessed claims supported or partially supported by published research
Expert Consensus
Dose divergence: Experts recommend different amounts (50–200micrograms, 50 to 200micrograms, 1 to 2brazil nuts). Check the Stack & Timing tab for study-backed dosing ranges.
Evidence Summary
The research on selenium supplementation spans multiple health domains including thyroid function, immune response, male fertility, metabolic health, and cancer prevention. Across 15 studies ranging from individual RCTs to large systematic reviews and meta-analyses, the overall picture is mixed: selenium appears to play a meaningful biological role in several systems, but translating that into clear supplementation benefits for most people is not straightforward. The evidence is strongest in specific clinical contexts — particularly thyroid-related conditions like Graves' orbitopathy and Hashimoto's thyroiditis — while evidence in other areas such as diabetes, athletic performance, and cancer prevention is either inconsistent or points toward potential harm at higher doses.
Read full evidence summary →Top studies
The Effect of Nutrients and Dietary Supplements on Sperm Quality Parameters: A Systematic Review and Meta-Analysis of Randomized Clinical Trials.
The Effect of Nutrients and Dietary Supplements on Sperm Quality Parameters: A Systematic Review and Meta-Analysis of Randomized Clinical Trials.
The efficacy and safety of selenium supplementation versus placebo in the treatment of Graves' orbitopathy: A systematic review and meta-analysis of randomised controlled trials.
The efficacy and safety of selenium supplementation versus placebo in the treatment of Graves' orbitopathy: A systematic review and meta-analysis of randomised controlled trials.
Expert Mentions
All 86 mentions
“people might be taking 10 different supplements and they like they say, they all might have selenium in them, correct? And then you're getting toxic doses of selenium 100%. And you don't know that cuz you're not adding it all up.”
Taking multiple supplements can result in toxic doses of selenium because people don't add up the total selenium content across all their products.
“Testing blood selenium or selenium via red blood cell testing before supplementing is worthwhile given the narrow therapeutic window.”
Testing blood selenium or selenium via red blood cell testing before supplementing is worthwhile given selenium's narrow therapeutic window.
None of the 10 provided studies directly examine or evaluate the practice of baseline blood or red blood cell selenium testing prior to supplementation. While the studies address selenium supplementat…
Safety, interactions & who should avoid Selenium
caution_warranted
Selenium has a narrow margin between adequate and toxic intake levels, and the SELECT trial raised concerns that supplementation at higher doses may cause harm rather than benefit in certain populations. Supplementation should be approached cautiously, particularly in individuals who are not selenium-deficient, and doses should stay within established safe upper limits.
Selenium has a narrow therapeutic window; the tolerable upper intake level is approximately 400 mcg/day for adults, and chronic excess intake can lead to selenosis (symptoms include hair loss, nail brittleness, gastrointestinal disturbance, and neurological effects). Supplementation should be approached cautiously in those with adequate dietary selenium intake, as excess may be harmful, particularly regarding glycemic and cardiovascular outcomes suggested by some meta-analyses.
Who should avoid it
Individuals with existing high selenium status or those living in selenium-replete geographic areas should use caution, as supplementation may push levels into a potentially harmful range. People with diabetes or impaired glycemic regulation should consult a healthcare provider, as some meta-analysis data suggest high-dose selenium may negatively affect glycemic control. Pregnant or breastfeeding individuals should not exceed established recommended intakes without medical supervision.
Known interactions
- ·May interact with anticoagulants (e.g., warfarin) — selenium may affect platelet function
- ·May interact with chemotherapy agents — antioxidant supplementation during cancer treatment is debated
- ·High-dose selenium may interact with niacin and statins in some lipid-lowering regimens
- ·Concurrent iodine supplementation may influence thyroid hormone dynamics in selenium-replete individuals
Pregnancy & breastfeeding
Our sources specifically flag pregnancy or breastfeeding considerations for Selenium — see the cautions above.
We don’t assign pregnancy-safety ratings. Many supplements lack adequate safety data in pregnancy and breastfeeding, and the absence of a warning here does not mean a supplement is safe to take. Don’t start, stop, or continue any supplement while pregnant or nursing without your OB-GYN or midwife.
Read: Supplements during pregnancy & breastfeeding →This is educational information only. Consult a healthcare provider before starting any supplement.
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Key findings
- ·Selenium supplementation shows the most consistent benefit in thyroid-related autoimmune conditions: multiple RCTs and a meta-analysis support modest improvements in Graves' orbitopathy, and a network meta-analysis found selenium among effective supplements for Hashimoto's thyroiditis.
- ·The large SELECT trial found no cancer prevention benefit from selenium supplementation and raised concerns about possible harm at higher doses, directly contradicting earlier observational findings that inspired the trial.
- ·Meta-analyses examining sperm quality and male fertility suggest some positive effects of selenium (often combined with other nutrients), but the magnitude of benefit and clinical relevance remain uncertain.
Evidence gaps
- ·Most studies do not report participants' baseline selenium status, making it difficult to determine whether benefits are limited to people who are actually deficient versus those with adequate selenium levels — a critical distinction for supplementation recommendations.
- ·Long-term safety data across diverse populations is lacking; the narrow window between potentially beneficial and harmful selenium doses (the therapeutic index) is not well characterized across different supplemental forms and dosing regimens.
- ·Evidence in several areas — including ME/CFS fatigue, photoprotection, HIV, and athletic performance — is drawn from broader systematic reviews where selenium is one of many supplements examined, leaving selenium-specific conclusions weak or inconclusive.