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Minerals

Selenium

Also known as: Selenomethionine, Selenite, Brazil nut selenium

The thyroid-and-immunity trace mineral — essential in small amounts, toxic in large ones, easy to overdo with Brazil nuts.

  • By IngredientWiki Editorial Team
  • Fact-checked by IngredientWiki Editorial Team
  • Updated 2026-09-26
  • 2 min read

In short

Selenium is an essential trace mineral that works through selenoproteins — a family of enzymes handling thyroid hormone activation, antioxidant defense (glutathione peroxidase), and immune function. Most Americans get enough from food (Brazil nuts, seafood, meat), and trials of selenium supplementation have repeatedly disappointed — it did not prevent cancer, cardiovascular disease, or improve thyroid function in selenium-sufficient people. Deficiency is real in low-soil regions, but so is toxicity: the safe window is narrow.

  • Essential trace mineral working via selenoproteins
  • Key roles: thyroid hormone activation, antioxidant enzymes, immunity
  • US diets are usually sufficient — supplementation trials disappointed
  • Toxic at high doses: upper limit 400 mcg daily for adults

What is Selenium?

Selenium's reputation ran ahead of its evidence. After observational studies linked higher selenium to lower cancer risk, it became a supplement staple — until large randomized trials (like the famous SELECT prostate cancer trial) failed to find benefit and even flagged a possible risk. Selenium is genuinely essential: your thyroid cannot activate its hormones without it, and immune cells depend on selenoproteins. But for the selenium-sufficient majority, food beats pills, and the margin to toxicity is closer than most supplements.

Where it comes from

Natural sources

  • Brazil nuts — the richest source (1 nut can supply a full day's needs)
  • Seafood: tuna, sardines, shrimp
  • Organ meats and muscle meats
  • Eggs, dairy, and grains (soil-dependent)

How it's made

Supplements use selenomethionine (organic, from yeast) or sodium selenite (inorganic).

What it does in products

  • Cofactor for glutathione peroxidase (antioxidant defense)
  • Required for thyroid hormone (T4→T3) activation
  • Supports immune cell function
  • Sperm formation and male fertility

Benefits

Deficiency correction

strong evidence

In selenium-deficient people (low-soil regions, malabsorption, long-term IV nutrition), supplementation normalizes thyroid function and immunity — the clear-cut benefit.

Thyroid support in autoimmune thyroiditis

moderate evidence

Trials of selenium in Hashimoto's show modest drops in thyroid antibodies, but without consistent changes in thyroid function or symptoms.

Cancer prevention

moderate evidence

Large trials (SELECT, NPC) found no prostate, lung, or total cancer reduction — and a possible diabetes signal in one. Observational hopes did not survive testing.

Cardiovascular protection

moderate evidence

Randomized evidence is null; selenium supplementation is not supported for heart-disease prevention.

Side effects & safety

Garlic breath and metallic taste (uncommon)Early sign of excess selenium intake.
Hair loss, brittle nails, nausea at high doses (rare)Chronic excess above 400 mcg daily; selenium toxicity (selenosis) is serious.

Safety considerations

  • Adequate selenium matters in pregnancy, but requirements are modest and easily met with food; supplements beyond a prenatal are usually unnecessary.
  • No major drug interactions.
  • Not for: Existing high selenium intake (frequent Brazil nuts or multiple supplements)

How it's used

In supplements

RDA: 55 mcg daily for adults; upper limit 400 mcg. Most people need no supplement — test or assess Brazil nut intake first. If supplementing, 55–100 mcg daily is a sensible cap.

Regulatory status

  • Dietary supplement under DSHEA; selenium is an FDA-recognized essential nutrient.
  • GRAS (generally recognized as safe) for food use.

Popular products containing Selenium

  • Selenium 100 mcgNOW Foods · Supplement
  • Selenium (yeast-free)Pure Encapsulations · Supplement
  • Selenium 200 mcgNature's Bounty · Supplement

Product names and brands are shown for reference and belong to their respective owners.

The Brazil nut math

A single Brazil nut can contain 68–91 mcg of selenium — above the 55 mcg RDA — and soil variability means some nuts hold far more. Regular Brazil nut eaters easily exceed the 400 mcg upper limit without realizing it, producing the classic signs: garlic breath, brittle nails, hair loss. Either eat one or two nuts occasionally as your selenium source, or none at all if you already supplement.

Frequently asked questions

Should I take selenium supplements?

Only with a demonstrated deficiency or a specific medical recommendation. Large prevention trials failed, and most US diets provide enough. If you eat Brazil nuts regularly, you likely already exceed needs.

Does selenium help the thyroid?

It is required for thyroid hormone activation, so deficiency impairs function — but in selenium-sufficient people, supplementing does not improve thyroid function. In Hashimoto's, it may modestly lower antibodies without clear symptom change.

What is the best form of selenium?

Selenomethionine (the organic form in yeast and food) is better absorbed and retained than inorganic selenite. If a supplement is warranted, selenomethionine at 55–100 mcg is the standard choice.

The bottom line

Selenium is an essential mineral whose supplement story ended in disappointment: food sources suffice for most people, prevention trials failed, and the toxicity window is narrow. Eat a few Brazil nuts occasionally, skip the pill unless tested deficient.

References

  1. Selenium — Health Professional Fact Sheet. NIH Office of Dietary Supplements. https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/
  2. SELECT trial: selenium and vitamin E for prostate cancer. PubMed (National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/?term=SELECT+trial+selenium+vitamin+E+prostate
  3. Selenium compound records. PubChem (National Library of Medicine). https://pubchem.ncbi.nlm.nih.gov/#query=selenium
  4. Selenium and thyroid autoimmunity — literature search. PubMed (National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/?term=selenium+hashimoto+thyroid+randomized