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Selenium supplement
Essential Trace Mineral

Selenium: Benefits, Dosage, Forms & Research

Essential Trace Mineral

This content is for informational purposes only and does not constitute medical advice. Consult your healthcare provider before starting any supplement. Full disclaimer

TL;DR — Quick Answer

Selenium supports thyroid function, immune defense, and antioxidant protection through selenoproteins. The recommended dose is 55-200mcg daily, with selenomethionine being the best-absorbed form. Brazil nuts are the richest food source — just 1-2 nuts daily provide adequate selenium.

Key Facts

What it is
An essential trace mineral incorporated into 25 selenoproteins with antioxidant and thyroid functions
Primary benefits
  • Supports thyroid hormone conversion (T4 to T3)
  • Powerful antioxidant via glutathione peroxidase
  • Enhances immune function
  • May reduce autoimmune thyroid antibodies
Typical dosage
55-200mcg daily
Evidence level
Moderate
Safety profile
Generally Safe

What the Research Says

Selenium research highlights the importance of baseline status. The Nutritional Prevention of Cancer (NPC) trial (Clark et al., 1996, n=1,312) initially showed a 52% reduction in prostate cancer with 200mcg selenium yeast daily, but this was a secondary endpoint. The subsequent SELECT trial (Lippman et al., 2009, n=35,533) found no cancer prevention benefit with selenomethionine in a selenium-replete population, and even suggested possible diabetes risk at 200mcg/day. For thyroid autoimmunity, a systematic review by Wichman et al. (2016, 16 studies) confirmed selenium supplementation reduces TPO antibody levels in Hashimoto's thyroiditis, though clinical significance remains debated.

Benefits of Selenium

  • Thyroid support — selenium is essential for the deiodinase enzymes that convert T4 to active T3; a 2010 Cochrane review found selenium supplementation reduced TPO antibodies in Hashimoto's thyroiditis
  • Antioxidant defense — selenoproteins including glutathione peroxidase and thioredoxin reductase protect cells from oxidative damage
  • Immune enhancement — selenium deficiency impairs both innate and adaptive immunity; supplementation in deficient populations improves immune cell function and vaccine response
  • Cancer research — the SELECT trial (n=35,533) found no benefit for prostate cancer prevention, but observational data suggests adequate selenium status is associated with lower cancer risk
  • Mood support — a 1991 Benton & Cook RCT found 100mcg selenium daily improved mood scores in participants with low baseline selenium
Did you know?

Selenium research highlights the importance of baseline status.

Forms of Selenium

FormBioavailabilityBest For
SelenomethionineHigh (90%+)General supplementation — best absorbed and retained in body tissues
Selenium YeastHighFood-form selenium — contains multiple selenocompounds similar to dietary selenium
Sodium SeleniteModerateInorganic form — used in many multivitamins but lower retention than organic forms

Dosage Recommendations

General recommendation: 55-200mcg daily

Timing: With food for best absorption • Take with food for best absorption.

Dosage by Condition

ConditionRecommended DoseEvidence
General antioxidant support55-100mcg dailyModerate
Thyroid support (Hashimoto's)200mcg dailyModerate
Immune function100-200mcg dailyModerate

Upper limit: 400mcg/day (NIH UL); chronic intake above this causes selenosis

Side Effects and Safety

Safety profile: Generally Safe

Potential Side Effects

  • Garlic breath odor at higher doses
  • Nausea and diarrhea (uncommon at recommended doses)
  • Selenosis at chronic doses >400mcg/day: hair loss, nail brittleness, fatigue, irritability
  • Possible increased type 2 diabetes risk at high doses (>200mcg/day) per SELECT trial data

Drug & Supplement Interactions

  • May enhance the anticoagulant effect of warfarin
  • Cisplatin and other chemotherapy drugs — selenium may alter drug metabolism
  • Statins — selenium may have additive cholesterol-lowering effects
  • Vitamin C taken simultaneously may reduce selenium absorption — separate by 30 minutes
Check Selenium interactions with other supplements →
BenefitsDosage GuideSide EffectsTypes & FormsResearchFAQ

Related Conditions

Immune Function

Related Supplements

ZincVitamin EIodine

Frequently Asked Questions

Can I get enough selenium from Brazil nuts?

Yes. Brazil nuts are the richest food source of selenium, containing 68-91mcg per nut on average. Just 1-2 Brazil nuts daily provides adequate selenium for most adults. A 2008 Thomson et al. study confirmed that 2 Brazil nuts daily was as effective as 100mcg selenomethionine supplement for raising selenium status.

Does selenium help with thyroid problems?

Selenium is essential for thyroid hormone metabolism. For Hashimoto's thyroiditis specifically, multiple studies show 200mcg daily reduces TPO antibodies. However, evidence that this translates to improved thyroid function or reduced medication needs is less consistent. Selenium is most beneficial for thyroid patients who are selenium-deficient.

How much selenium is too much?

The upper limit is 400mcg/day. Chronic intake above this causes selenosis, characterized by garlic breath, hair loss, nail changes, fatigue, and gastrointestinal problems. Acute toxicity is possible above 1,000mcg. If eating Brazil nuts, limit to 2-3 per day as selenium content varies widely.

References

  1. (). Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. DOI
  2. (). Selenium supplementation significantly reduces thyroid autoantibody levels in patients with chronic autoimmune thyroiditis: a systematic review and meta-analysis. Thyroid. DOI
  3. (). Brazil nuts: an effective way to improve selenium status. American Journal of Clinical Nutrition. DOI