An essential trace mineral the body uses in a small but important set of antioxidant enzymes — particularly those that protect the thyroid and recycle vitamin E. Selenium has a narrow therapeutic window: too little causes problems, too much also causes problems. Two Brazil nuts can deliver a full daily dose.

Role in the body Component of selenoproteins including glutathione peroxidases (antioxidant) and iodothyronine deiodinases (thyroid hormone activation)

Recommended daily intake

Adults · 55 mcg
Pregnancy · 60 mcg
Lactation · 70 mcg

Upper intake limit

400 mcg/day for adults

Signs of deficiency

Keshan disease (a cardiomyopathy seen historically in selenium-poor regions of China)
Impaired immune function
Increased risk of certain cancers
Worsened thyroid function in iodine-deficient settings
Muscle weakness

Cofactor for glutathione peroxidase, one of the body's main antioxidant defense enzymes
Critical for thyroid hormone activation — selenium-dependent enzymes convert T4 into the active T3
Supports immune function and the body's defense against viral infection
May reduce thyroid antibody levels in autoimmune thyroid disease
Brazil nuts are exceptionally rich — two nuts often provide more than the daily requirement
Found also in seafood, organ meats, eggs, and grains grown in selenium-rich soil

Active in Selenocysteine (Sec, U), Selenomethionine, Se²⁻ (selenide).

Excess selenium causes selenium toxicity (selenosis) — symptoms include hair loss, brittle nails, garlic breath, fatigue, and nerve damage

Brazil nut content varies enormously by region — eating a handful daily can deliver toxic levels over time

Use caution with thyroid medication — selenium affects thyroid hormone activation

Use caution with anticoagulants

Selenium supplementation has shown both protective and harmful effects in cancer trials, depending on baseline status — never supplement at high doses without testing

Skip selenium supplements if Brazil nuts are part of regular eating

The gap between requirement and toxicity is one of the smallest of any nutrient

NIH ODS — Selenium · MedlinePlus — Selenium · Britannica — Selenium

Works with

Vitamin E Selenium (glutathione peroxidase) and vitamin E (tocopherols) are synergistic antioxidants that partially compensate for mild deficiency of each other. Iodine Selenium is required for deiodinase enzymes (DIO1, DIO2) that convert T4 to active T3; deficiency impairs thyroid hormone metabolism.

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