Selenium
The essential trace mineral for thyroid function, antioxidant defense, and immune health
Also known as: Se • Selenomethionine • Sodium Selenite
Overview
Selenium is an essential trace mineral that plays critical roles in thyroid hormone metabolism, antioxidant defense, immune function, and reproductive health. It is incorporated into selenoproteins — a family of 25 proteins that include glutathione peroxidases (antioxidant defense), thioredoxin reductases (redox regulation), and iodothyronine deiodinases (thyroid hormone conversion). The thyroid gland contains more selenium per gram than any other tissue, and selenium deficiency significantly impairs thyroid function. Selenium also supports immune function by enhancing the activity of natural killer cells and T-cells. The relationship between selenium and health follows a U-shaped curve — both deficiency and excess are harmful, making proper dosing important. Soil selenium content varies dramatically by region, creating geographic patterns of deficiency.
Key Benefits
Thyroid Function
Selenium is essential for converting T4 (inactive) to T3 (active thyroid hormone) via deiodinase enzymes. Supplementation improves thyroid function in selenium-deficient individuals and may reduce thyroid antibodies in Hashimoto's thyroiditis.
Antioxidant Defense
Selenium is a cofactor for glutathione peroxidase, one of the body's most important antioxidant enzyme systems. It protects cells from oxidative damage, particularly in the thyroid, liver, and immune cells.
Immune Function
Selenium enhances both innate and adaptive immunity. Deficiency impairs immune cell function and increases susceptibility to viral infections. Adequate selenium status is associated with better outcomes from viral infections.
Cancer Prevention (Controversial)
The SELECT trial showed no cancer prevention benefit from selenium supplementation in selenium-replete men. However, supplementation may reduce cancer risk in selenium-deficient populations. The relationship is complex and dose-dependent.
Dosage & How to Take
55-200mcg daily from food and supplements combined
| Purpose | Dose | Notes |
|---|---|---|
| General health (RDA) | 55mcg daily | Minimum to prevent deficiency |
| Thyroid support | 100-200mcg daily | Especially important for Hashimoto's |
| Immune support | 100-200mcg daily | During cold/flu season |
| Upper safe limit | 400mcg daily (max) | From all sources — do NOT exceed |
Best Time to Take
With any meal. No specific timing requirements.
With or Without Food
Take with food for best absorption and to reduce GI upset
Forms & Bioavailability
| Form | Absorption | Best For | Notes |
|---|---|---|---|
| Selenomethionine | Excellent (90%+) | General supplementation | Organic form, well-absorbed, stored in body proteins |
| Selenium Yeast | Excellent | Whole-food form | Contains multiple selenium species — mimics food sources |
| Sodium Selenite | Good (50-60%) | Budget option | Inorganic form, pro-oxidant at high doses |
| Methylselenocysteine | Good | Cancer research | Used in some cancer prevention studies |
Side Effects & Safety
Common
- Garlic breath (at higher doses)
- Mild GI upset
Rare
- Selenosis (toxicity) — hair loss, nail brittleness, nausea, fatigue (above 400mcg/day)
- Metallic taste
Contraindications
- Do not exceed 400mcg/day from all sources
- Skin cancer history (SELECT trial showed possible increased risk)
Interactions
May reduce selenium absorption if taken simultaneously
Selenium may enhance statin effects on cholesterol
May affect efficacy — consult oncologist
Scientific Research
Thyroid Antibodies in Hashimoto's
200mcg/day selenomethionine reduced thyroid peroxidase antibodies by 21% in Hashimoto's patients
SELECT Cancer Prevention Trial
200mcg/day selenium did not reduce prostate cancer risk in selenium-replete men; may have increased diabetes risk
Immune Function
Selenium supplementation enhanced immune cell proliferation and cytokine production in elderly subjects
Food Sources
Frequently Asked Questions
Medical Disclaimer
This content is for educational and informational purposes only and is not intended as medical advice. Always consult with a qualified healthcare professional before starting any supplement regimen, especially if you have pre-existing health conditions or are taking medications. The statements on this page have not been evaluated by the Food and Drug Administration.