Selenium
Selenium
Also known as: Selenomethionine, Sodium selenite, Selenium yeast, L-selenomethionine
Selenium is an essential trace mineral that functions as an antioxidant and is critical for thyroid hormone metabolism, DNA synthesis, and reproduction.
Introduction
Selenium is an essential trace mineral discovered in 1817 by Swedish chemist Jöns Jacob Berzelius. It was initially considered toxic, but in the 1950s was recognized as essential for human health when it was linked to liver necrosis prevention in rats.
Selenium's biological activity depends on its incorporation into 25 known selenoproteins as the amino acid selenocysteine. These selenoproteins include:
- Glutathione peroxidases (GPx) - antioxidant enzymes that protect cells from oxidative damage
- Thioredoxin reductases - involved in cellular redox regulation
- Iodothyronine deiodinases - convert inactive thyroid hormone (T4) to active T3
- Selenoprotein P - transports selenium and functions as an antioxidant
The relationship between selenium and health is complex. While deficiency causes serious problems including Keshan disease (cardiomyopathy) and Kashin-Beck disease (osteoarthritis), excessive selenium is toxic. The therapeutic window is relatively narrow.
The SELECT trial (Selenium and Vitamin E Cancer Prevention Trial) showed that selenium supplementation did not prevent prostate cancer and may have increased diabetes risk in some men, complicating supplementation recommendations.
Selenium status varies dramatically by geography due to soil content. Populations in selenium-poor regions (parts of China, Russia, Europe) are at risk of deficiency, while those in selenium-rich areas may be at risk of excess.
Main Benefits
Essential component of glutathione peroxidases, protecting cells from oxidative damage and supporting antioxidant defenses.
Required for thyroid hormone metabolism via deiodinase enzymes; converts T4 to active T3; supports thyroid function.
Deficiency supplementation prevents Keshan disease (endemic cardiomyopathy) in selenium-deficient regions.
Supports immune function through selenoproteins involved in immune cell proliferation and function.
Required for normal spermatogenesis and male fertility; component of sperm mitochondrial capsule.
Mechanism of Action
Selenium exerts its biological effects through incorporation into selenoproteins:
Antioxidant Defense: Selenium is a component of glutathione peroxidases (GPx1-4 and GPx6), which reduce hydrogen peroxide and lipid hydroperoxides using glutathione. This protects cell membranes, DNA, and proteins from oxidative damage. Thioredoxin reductases (TrxR1-3) also depend on selenium and maintain cellular redox balance.
Thyroid Hormone Metabolism: Three iodothyronine deiodinases (D1, D2, D3) contain selenium. D1 and D2 convert the inactive thyroxine (T4) to active triiodothyronine (T3). D3 inactivates thyroid hormones. Selenium deficiency impairs this conversion, potentially causing hypothyroid symptoms despite normal T4 levels.
Selenium Transport: Selenoprotein P (SePP) is the primary selenium transport protein in blood and also has antioxidant properties. It delivers selenium to tissues and protects endothelial cells.
Immune Function: Selenoproteins regulate immune cell function, including T-cell proliferation, differentiation, and cytokine production. Selenium deficiency impairs immune responses and increases susceptibility to viral infections.
DNA Synthesis and Repair: Some selenoproteins are involved in DNA synthesis and repair processes, potentially affecting cancer risk.
Reproduction: GPx4 (phospholipid hydroperoxide glutathione peroxidase) is a major structural protein of the mitochondrial capsule in sperm, essential for male fertility.
Natural Sources
Selenium content in plant foods depends on soil selenium levels. Brazil nuts are exceptionally rich. Animal sources generally provide more consistent amounts.
Examples:
Brazil nuts (extremely high, vary by soil)
Seafood (tuna, shrimp, salmon, sardines)
Organ meats (liver, kidney)
Meat (beef, pork, chicken)
Eggs
Brown rice
Sunflower seeds
Mushrooms
Oats
Brazil nuts provide massive amounts; seafood and meat good sources; vegetarians/vegans in low-selenium regions may struggle; soil-dependent variability.
Deficiency Symptoms
Selenium deficiency causes Keshan disease (cardiomyopathy), Kashin-Beck disease (osteoarthritis), and increases susceptibility to infections and hypothyroidism.
Common Symptoms:
Keshan disease (cardiomyopathy, endemic in selenium-deficient China)
Kashin-Beck disease (endemic osteoarthritis)
Hypothyroidism or impaired thyroid function
Increased susceptibility to infections
Male infertility
Muscle weakness and wasting
Impaired cognitive function
Rare in North America due to diverse diet; higher risk in selenium-poor soil regions; certain populations at higher risk (Crohn's disease, vegans in low-selenium areas).
Keshan disease can be fatal; Kashin-Beck causes disability; thyroid dysfunction affects metabolism; significant impact when present.
Recommended Daily Intake
RDA: adults 55 mcg/day. Upper Limit (UL) is 400 mcg/day due to toxicity risk.
Reference Values:
| Adult men | 55 mcg/day |
| Adult women | 55 mcg/day |
| Pregnancy | 60 mcg/day |
| Lactation | 70 mcg/day |
| Upper Limit (UL) | 400 mcg/day |
Sources for RDI/AI:
Brazil nuts contain extremely variable selenium (50-90 mcg per nut). Eating just 4-5 Brazil nuts can exceed UL. Limit to 1-2 nuts per day.
Effectiveness for Specific Focuses
Essential for glutathione peroxidases; key component of antioxidant defense system; protects against oxidative stress.
Required for converting T4 to T3; deficiency causes thyroid dysfunction; critical for thyroid hormone metabolism and metabolic rate.
Supports immune cell function; deficiency increases infection risk; selenoproteins regulate immune responses.
Required for spermatogenesis; component of sperm structure; deficiency linked to male infertility.
Antioxidant role may support healthy aging; SELECT trial complicated cancer prevention picture; mixed evidence.
Safety Information
Potential Side Effects
Garlic breath odor
Metallic taste
Hair and nail loss/brittleness
Nausea and diarrhea
Fatigue and irritability
Nervous system abnormalities (high doses)
Contraindications
Existing selenium toxicity
History of skin cancer (some concern from SELECT trial)
Overdose Information
Narrow therapeutic window; UL 400 mcg/day; toxicity (selenosis) occurs above UL; Brazil nuts can cause toxicity.
Garlic breath, metallic taste, hair/nail loss, skin rashes, nausea, diarrhea, fatigue, irritability, nervous system damage.
Documented Overdose Symptoms:
Garlic breath
Hair loss
Nail changes
Gastrointestinal distress
Nervous system effects
Toxicity Thresholds: UL: 400 mcg/day. Selenosis can occur above 400-900 mcg/day. Acute toxicity from gram doses.
Narrow therapeutic window. Brazil nuts vary enormously in selenium content. One ounce of high-selenium Brazil nuts can provide 500+ mcg.
Interactions
Drug Interactions:
Anticoagulants (may increase bleeding risk at high doses)
Chemotherapy (theoretical interactions)
High doses may affect blood clotting; caution with chemotherapy; generally safe at RDA levels.
Other Supplement Interactions:
Vitamin E - complex relationship; SELECT trial showed potential negative interaction at high doses
Iodine - synergistic for thyroid function
Vitamin C - may enhance absorption
High-dose vitamin E may interact negatively; caution with combined antioxidant supplements at high doses.
Do not exceed 400 mcg/day. Limit Brazil nut consumption to 1-2 nuts daily. People with hypothyroidism should ensure adequate but not excessive selenium. Discuss with healthcare provider before supplementing if history of skin cancer.
Forms and Bioavailability
Selenium exists in organic (selenomethionine) and inorganic (selenite, selenate) forms with different absorption characteristics.
Selenomethionine
Organic form bound to methionine; best absorbed and retained in body; incorporated into body proteins.
~90% absorbed; incorporated into general proteins like methionine; better retention than inorganic forms.
Preferred form for supplementation. Yeast-based selenomethionine provides food-form selenium with excellent bioavailability.
Selenium Yeast
Yeast grown in selenium-rich medium; contains selenomethionine as primary form; food-based.
Excellent bioavailability; food matrix may enhance benefits; most studied form for cancer prevention (SELECT used this).
Provides selenium in food-like matrix. The form used in SELECT trial. Good choice for general supplementation.
Sodium Selenite
Inorganic form; well-absorbed but less retained than organic forms; directly used for selenoprotein synthesis.
~50% absorbed; not incorporated into general proteins; directly available for selenoprotein synthesis.
Less preferred than selenomethionine but effective. May be preferred for specific therapeutic situations.
Warnings & Suitability
Did You Know...?
A single Brazil nut can contain 50-90 mcg of selenium - enough to meet or exceed the daily requirement. Just 4-5 nuts could potentially cause toxicity.
The name "selenium" comes from the Greek moon goddess Selene because the element was discovered in association with tellurium (named after Earth).
Keshan disease was named after Keshan County in China where it was first identified as being linked to selenium deficiency.
Selenium is the only mineral that is incorporated into proteins as a specific amino acid (selenocysteine) via its own genetic code (UGA codon).
General Scientific Sources
Tags
Content Verification
Last Medical Review: 2/13/2026
Reviewed by: Editorial Team