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Selenium — The Complete Supplement Guide

A powerful antioxidant with the narrowest safe margin of any mineral — why a couple of Brazil nuts is plenty and supplements can tip you into toxicity.

Reviewed by Bodil Isaksson, Licensed Biomedical Scientist (Sweden)Last reviewed July 2026Supplement guide

SupplementsMineralsSelenium

One of the trace minerals with the narrowest safe window — the gap between “enough” and “too much” is smaller than for almost any other. For the short dietary version, see the Minerals page.

Read first. Selenium has a very narrow safe range. A couple of Brazil nuts a day already meets your needs; adding a supplement on top can cause selenosis (hair/nail loss, nerve damage). Information, not a prescription.

Key takeaways

  • Narrow safe window — the gap between enough and too much is small.
  • Adults need ≈ 55–70 µg/day.
  • Upper limit 300 µg (EFSA) / 400 µg (US) — easy to overshoot.

Function

Selenium works through ~25 "selenoproteins":

  • Antioxidant defence: the core of glutathione peroxidase, which neutralises damaging peroxides.
  • Thyroid function: the deiodinase enzymes that convert thyroid hormone T4 to active T3 are selenium-dependent.
  • Immune function: supports normal immune responses.
  • Reproduction: needed for normal sperm formation and motility.
  • Redox balance: helps recycle other antioxidants and protect cell membranes.

Different forms — and which to choose

SelenomethionineOrganic form (from selenium yeast/food); well absorbed and stored — the usual supplement form.
Sodium selenite / selenateInorganic forms; absorbed but not stored the same way.
Brazil nutsNature's richest source — but content is very variable, so a few nuts can already be a full dose (or more).

Absorption in the body

How muchHigh — often >80–90%, especially the organic selenomethionine form.
NoteSoil selenium varies hugely by region, so dietary intake (and blood levels) differ between countries.

Deficiency symptoms

SymptomsUsually subtle — impaired thyroid and immune function; classic severe deficiency (Keshan heart-muscle disease, Kashin-Beck joint disease) occurs in very low-soil regions.
Who's at riskPeople in low-selenium areas, severe GI disease, and those on long-term intravenous nutrition.

Recommended intake

Adults ≈ 55–70 µg/day

Reference intakes:

  • Adults: ~70 µg/day (EFSA) · 55 µg/day (US)

Best sources: Brazil nuts (very rich), fish and shellfish, eggs, meat, and grains (depending on soil).12

Therapeutic use

DeficiencyCorrecting low status (diet or a low-dose supplement).
ThyroidModest evidence in Hashimoto's thyroiditis (may lower antibodies) — not universally recommended.
Not for cancerThe large SELECT trial found selenium did not prevent cancer; don't take it for that.

Therapeutic doses

If needed: ~50–100 µg/day
EverydayMost people meet needs from food; a low-dose 50–100 µg supplement suffices where intake is low.
Count Brazil nutsIf you eat Brazil nuts regularly, you likely don't need a supplement at all.

Toxicity & upper limit

UL: 300 µg (EFSA) · 400 µg (US)
Upper limit300 µg/day (EFSA) or 400 µg/day (US) — one of the lowest margins of any mineral.1
SelenosisExcess causes hair and nail brittleness/loss, a garlic breath odour, GI upset, rash and, if severe, nerve damage.
Easy to overshootBrazil nuts plus a supplement can exceed the limit — pick one source.

Bottom line: selenium is essential but needs are small and the toxic threshold is close. A couple of Brazil nuts a day, fish and eggs usually cover it; only supplement (low dose) if your intake is genuinely low — and don't stack it on top of daily Brazil nuts.

← All supplementsShort dietary version

References

Reference and upper-limit values are general adult figures from official bodies; individual needs vary. Therapeutic doses describe clinical/research use, not prescriptions.

  1. US NIH Office of Dietary Supplements. Selenium — Health Professional Fact Sheet. ods.od.nih.gov
  2. EFSA Panel (NDA). Dietary Reference Values for selenium. EFSA Journal 2014;12(10):3846. efsa.onlinelibrary.wiley.com

This article summarizes nutrition science from official health authorities and peer-reviewed sources for education. It is not a substitute for individual medical advice, and the doses given are not prescriptions — always consult a professional before therapeutic supplement use, in pregnancy, in kidney disease, or alongside medication.

✓
Bodil Isaksson, Licensed Biomedical Scientist (Sweden)

At Food Health Lab we explain what the science says in plain language, and we cite our sources so you can check for yourself.

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