Selenium
Selenium is an essential trace mineral critical for thyroid hormone metabolism, antioxidant defense, and immune function. Evidence supports supplementation for thyroid autoimmunity and in selenium-deficient populations, but the SELECT trial definitively showed no cancer prevention benefit and potential harm at high doses.
Quick Verdict
Essential for thyroid function and beneficial in deficiency, but the SELECT trial ended hopes for cancer prevention and excess intake carries real risks.
If you have Hashimoto's thyroiditis or live in a selenium-poor region, 100-200mcg/day is well-supported. Do not take high doses hoping to prevent cancer — the evidence is clearly negative. Check your status before supplementing if you eat a varied diet.
At a Glance
Dose
100-200mcg/day selenium as selenomethionine
When to Take
With food
Best Form
Selenomethionine
Evidence
56% avg
What It Is
Selenium is an essential trace mineral incorporated into 25 selenoproteins that perform critical functions in thyroid hormone metabolism, antioxidant defense, DNA synthesis, and immune regulation. The most important selenoproteins include glutathione peroxidases (antioxidant), thioredoxin reductases (redox regulation), and iodothyronine deiodinases (thyroid hormone activation). Selenium intake varies dramatically by geography due to soil selenium content, with deficiency common in parts of China, Europe, and New Zealand. Brazil nuts are the richest dietary source, with a single nut providing 70-90mcg.
How It Works
Selenium exerts its biological effects through incorporation into selenoproteins as the amino acid selenocysteine. Glutathione peroxidases reduce hydrogen peroxide and lipid hydroperoxides, protecting cells from oxidative damage. Iodothyronine deiodinases convert inactive T4 thyroid hormone to active T3, making selenium essential for normal thyroid function. In immunity, selenium supports natural killer cell activity, T-cell proliferation, and antibody production. For autoimmune thyroiditis, selenium may reduce thyroid peroxidase antibody levels by decreasing oxidative stress in thyroid tissue. The therapeutic window is narrow — deficiency impairs these functions, but excess selenium (selenosis) is toxic.
Evidence by Goal
thyroid function
Good evidence that selenium supplementation reduces thyroid peroxidase antibodies in Hashimoto's thyroiditis. Essential for T4-to-T3 conversion. Benefits most pronounced in selenium-deficient populations.
immune function
Moderate evidence for enhanced immune response, particularly in deficient individuals. Supports NK cell activity and T-cell function, but supplementation in selenium-replete individuals shows limited additional benefit.
Dosage & Usage
Standard Dose
100-200mcg/day selenium as selenomethionine
Loading Protocol
No loading phase; do not exceed 400mcg/day (upper tolerable limit)
Timing
With food; any time of day
Best Form
Selenomethionine (best studied, highest bioavailability) or selenium-enriched yeast
📅 What to Expect
Week 1-4
Selenium-dependent enzymes (glutathione peroxidase) normalizing if previously deficient.
Week 4-8
Thyroid function optimizing. Antioxidant defense at full capacity.
Month 2-3
Thyroid antibodies may decrease in autoimmune thyroiditis (Hashimoto's).
Month 3+
Maintenance phase. Do not exceed 200mcg/day — toxicity risk at higher doses.
🎯 How to Know It's Working
Thyroid function supported if deficient
Immune markers improve
Antioxidant status better on bloodwork
If not working: If no change after 4 weeks, you likely get enough from diet (2 Brazil nuts = daily selenium). Don't exceed 200mcg/day — toxicity risk.
🏷️ Buying Guide
✓ Look For
- • Selenomethionine form (best absorbed)
- • 200mcg per serving (clinical dose)
- • Third-party tested for purity
✗ Avoid
- • Sodium selenite (inorganic, less bioavailable)
- • Doses above 400mcg/day (toxicity risk)
🥗 Get It From Food
🛡️ Safety Profile
Common Side Effects
- • garlic breath at higher doses
- • mild GI upset
- • metallic taste
⚠️ Contraindications
- 🚫 selenium-replete individuals taking >200mcg/day (no benefit, potential harm)
💊 Drug Interactions
- • cisplatin (selenium may reduce nephrotoxicity but also potentially reduce efficacy)
- • statins (additive selenium depletion)
- • niacin (selenium may reduce HDL-raising effect)
🔴 Serious Risks
- • selenosis at chronic doses >400mcg/day (hair loss, nail brittleness, neurological symptoms, GI distress)
- • possible increased type 2 diabetes risk at high doses (SELECT trial)
Long-Term Safety
Safe at 100-200mcg/day for years based on clinical trial data. The SELECT trial found 200mcg/day selenium did not prevent cancer and may have slightly increased type 2 diabetes risk. Do not exceed 400mcg/day from all sources. The toxicity margin is narrower than many minerals.
👥 Who Benefits Most
Best For
- ✓ individuals with Hashimoto's thyroiditis
- ✓ those in selenium-poor soil regions (parts of Europe, China, NZ)
- ✓ people with documented selenium deficiency
- ✓ those with suboptimal thyroid function
Less Effective For
- — individuals with adequate selenium status from diet
- — those seeking cancer prevention (SELECT trial was negative)
- — people already consuming Brazil nuts regularly
📚 Key Research
3 peer-reviewed studies supporting this supplement's effects.
Selenium and human health
Rayman MP · The Lancet (2012)
Landmark review establishing selenium's U-shaped dose-response relationship: both deficiency and excess are harmful. Optimal status varies by population, and supplementation benefits are primarily in deficient individuals.
Selenium supplementation for Hashimoto's thyroiditis: summary of a Cochrane systematic review
Wichman J, Winther KH, Bonnema SJ, et al. · Thyroid (2016)
Meta-analysis found selenium supplementation significantly reduced thyroid peroxidase antibody levels in Hashimoto's thyroiditis patients at 3, 6, and 12 months, though effects on thyroid hormone levels and clinical outcomes were less consistent.
Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT)
Lippman SM, Klein EA, Goodman PJ, et al. · JAMA (2009)
Large RCT (35,000+ men) found selenium supplementation (200mcg/day) did not reduce prostate cancer risk and showed non-significant trends toward increased type 2 diabetes, definitively disproving the cancer prevention hypothesis.
💰 Cost & Forms
Estimated Monthly Cost
$5-15
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