Selenium
Trace mineralsYour intake
What each level of selenium does
Approximate dose-response bands. Individual response varies — these are starting points, not prescriptions.
- Severely lowYOU ARE HERE0 mcg – 18.15 mcg
Well below target. Risk of deficiency symptoms tied to thyroid · antioxidant.
- Insufficient18.15 mcg – 55 mcg
Below the recommended daily target. Long-term adequacy not assured.
- Adequate55 mcg – 82.5 mcg
Daily target met. Standard nutritional support for thyroid · antioxidant.
- Therapeutic82.5 mcg – 110 mcg
Common for specific health goals. Check the evidence for your situation before sustaining this level.
- High110 mcg – 400 mcg
Approaching the tolerable upper limit. Monitor and consider clinical guidance.
- Over upper limit400 mcg – +
Above the tolerable upper limit. Risk of adverse effects — back off or consult a clinician.
Overview
Trace mineral incorporated as selenocysteine into ~25 selenoproteins (glutathione peroxidases, thioredoxin reductases, iodothyronine deiodinases). Soil content varies enormously by region — Brazil nuts are the highest food source by far. The therapeutic window is narrow: deficiency drives Keshan cardiomyopathy; excess drives selenosis.
Functions
- ●Forms selenocysteine, the 21st amino acid, in selenoproteins
- ●Cofactor for glutathione peroxidases (antioxidant)
- ●Cofactor for iodothyronine deiodinases (thyroid hormone activation: T4 → T3)
- ●Cofactor for thioredoxin reductase (redox signalling)
- ●Required for sperm motility (selenoprotein P)
Mechanism
Glutathione peroxidase uses GSH to reduce hydrogen peroxide and lipid peroxides — the second major antioxidant system after vitamin E. Iodothyronine deiodinases (DIO1, DIO2, DIO3) remove iodine atoms from thyroid hormones, converting T4 to active T3 in peripheral tissues. Selenium and iodine deficiency together cause especially severe cretinism (myxedematous form).
Benefits
- ●Prevents Keshan disease (selenium-deficient region cardiomyopathy)
- ●Adequate status supports thyroid hormone metabolism (especially Hashimoto's)
- ●May reduce autoimmune thyroiditis antibody titres at 200 mcg/day
- ●SELECT trial found no prostate cancer benefit (and possibly increased risk in selenium-replete men)
Deficiency
Region-specific. Common in low-selenium soil belts (parts of China, Tibet, Russia). Rare in the US except in TPN without selenium or severe GI disease.
- ●Cardiomyopathy (Keshan disease)
- ●Osteoarthropathy (Kashin-Beck)
- ●Impaired thyroid function
- ●Reduced immunity, slower viral mutation control (Coxsackie, HIV)
- ●Infertility (especially male)
- ●Low-selenium-soil regions
- ●TPN without selenium
- ●Severe GI disorders
- ●Hemodialysis patients
Excess
Selenosis from chronic intake >400 mcg/day. Single doses of contaminated supplements have caused acute toxicity. Brazil nut over-consumption is the realistic everyday risk.
- ●Garlic breath
- ●Brittle hair and nails, hair loss
- ●Skin rash, nail discolouration
- ●GI upset, fatigue
- ●Neurological symptoms (severe)
- ●Possible increased type 2 diabetes risk in selenium-replete adults
Forms
- SelenomethionineOrganic, well-absorbed, builds tissue stores; preferred supplement form
- Selenium-enriched yeastMixed selenocompounds; the SELECT trial form
- Sodium selenite, sodium selenateInorganic; well-absorbed but less retained
- MethylselenocysteineInvestigational; possible anti-cancer mechanism
Food sources
- Brazil nuts · 1 nut70–90 mcg
- Yellowfin tuna (cooked) · 3 oz90 mcg
- Cooked sardines · 3 oz45 mcg
- Cooked beef · 3 oz30 mcg
- Cooked chicken breast · 3 oz20 mcg
- Eggs (whole) · 1 large15 mcg
Supplement forms
Selenomethionine (organic, well-absorbed, builds tissue stores) is preferred over sodium selenite or sodium selenate. Two Brazil nuts most days will hit the RDA without a supplement. Don't stack multiple selenium-containing supplements with daily Brazil-nut intake — selenosis is real.
Bioavailability
~80–90% of dietary selenium is absorbed regardless of form, but selenomethionine is incorporated into general protein pools (long-term stores) while inorganic selenite goes directly to selenoprotein synthesis.
Longevity relevance
U-shaped: deficiency drives cardiomyopathy and thyroid dysfunction; excess associates with type 2 diabetes and (in SELECT) possibly cancer. Aim for adequacy through food; routine supplementation is unnecessary outside specific clinical contexts (Hashimoto's protocols, low-selenium regions).
Relationships
- Vitamin E · Together cover lipid + aqueous antioxidant defence; deficiency of either potentiates the other's role
- Iodine · Both needed for thyroid hormone synthesis and activation
- Glutathione (GSH) · Substrate for glutathione peroxidase; selenium is the cofactor
- Heavy metals (mercury, cadmium, arsenic) · Bind selenium and reduce selenoprotein synthesis; heavy seafood eaters need adequate selenium
- Sulfate (very high) · Competes for absorption
References
About Selenium
Glutathione peroxidase, thyroid hormone metabolism.
- Role
- Thyroid · antioxidant
- Daily target
- 55 mcg (DV)
- Upper limit
- 400 mcg
- Also called
- selenium, selenomethionine, l-selenomethionine, sodium selenate
The mechanisms and systems this nutrient feeds. Click any to drill into what runs on it.
★ = load-bearing / primary cofactor. Track these in My Journey.
Top food sources of Selenium
Whole foods that contribute meaningfully (≥10% DV per 100 g serving). Click any food to see its full nutrient profile and what else it brings to the table.