Selenium Testing: The Overlooked Antioxidant Mineral
Content last reviewed 2026-09-12 · Educational information — not medical advice
Quick answer
Selenium supports thyroid function, antioxidant enzymes (selenoproteins), and immunity. Deficiency follows soil-poor regions, GI disease, and restrictive diets; excess (rare, from unregulated supplements) causes garlic breath and toxicity. Testing clarifies ambiguous deficiency presentations.
Selenium's biological jobs
Selenium integrates into selenoproteins: glutathione peroxidases (antioxidant defense), iodothyronine deiodinases (thyroid hormone activation — T4→T3 conversion), and selenoprotein P (transport). Keshan disease (cardiomyopathy) marked severe deficiency historically; borderline insufficiency underlies subtle fatigue, hair/muscle complaints, and impaired immune responsiveness.
Who should test
- GI absorption disorders — IBD, celiac, short-bowel, bariatric surgery
- Restrictive diets — prolonged parenteral nutrition, vegan-raw extremes
- Hypothyroid patients unresponsive to levothyroxine (selenium + iodine both needed for hormone conversion)
- Endemic low-soil regions (northern China, parts of Europe) with failure-to-thrive syndromes
- Supplement-overdosing users — confirm rather than assume
Interpretation
Serum selenium <70 µg/L suggests insufficiency; <40 µg/L frank deficiency. Whole-blood or platelet assays reflect longer-term status. Excess >150-200 µg/L from unregulated high-dose supplements warrants discontinuation (selenosis: garlic breath, hair loss, brittle nails, GI upset).
Supplementation wisdom
Daily need ~55 µg adults (RDA). Brazil nuts (one-two nuts daily) supply it biochemically. Supplements beyond ~100-200 mcg/day add nothing and accumulate toward toxicity — 'more is better' fails here too. Fun fact: selenium-supplement risks once feared (diabetes signal in SELECT trial) still shape cautious dosing rhetoric.
Related guides
Frequently asked questions
Does selenium cure thyroid disease?
It supports T4→T3 conversion and may slow autoimmune thyroid-progression slightly, but doesn't fix hypothermia; TSH-driven levothyroxine remains the therapy.
Could my chronic fatigue be low selenium?
Possible contributing factor in at-risk groups only — test before supplementing; most fatigue causes lie elsewhere.