Selenium — Thyroid + Antioxidant Cofactor
Selenium cofactors thyroid T4→T3 conversion and glutathione synthesis. Soil-driven deficiency is common. Two Brazil nuts a day covers the daily requirement for most adults.
Selenium is an essential trace mineral that acts as a cofactor in thyroid hormone conversion (T4 → T3) and in glutathione antioxidant pathways — deficiency is geographically common because soil selenium varies 100-fold across regions, yet two Brazil nuts a day cover the daily requirement for most adults without any supplementation.
The biomarker
- Name: Serum Selenium
- Units: µg/L (some labs report µg/dL — divide by 10 to convert)
- Standard reference range: 70–150 µg/L (varies by lab)
- Optimal range: 120–160 µg/L
How to read your result
| Value (µg/L) | Interpretation |
|---|---|
| < 70 | Low — investigate dietary intake; thyroid conversion and antioxidant capacity are impaired below this level |
| 70–120 | Adequate — below optimal but not deficient; dietary improvement is worthwhile |
| 120–160 | Optimal — supports thyroid T4→T3 conversion and glutathione synthesis at full capacity |
| ≥ 200 | High — risk of selenosis (hair loss, nail brittleness, neurological symptoms); stop selenium supplements |
What moves the needle
- Brazil nuts — the most concentrated food source. One or two Brazil nuts per day (5–10 g) provides the full daily selenium requirement for most adults. The selenium content varies with soil, but two nuts is a reliable practical target.
- Other food sources. Tuna, sardines, shrimp, and eggs provide meaningful selenium with lower variability than Brazil nuts. Meat and poultry also contribute.
- Supplementation — rarely necessary. If blood levels are confirmed low, supplementation under clinician guidance can correct them. Self-supplementing selenium is a narrow therapeutic window — the gap between adequacy and toxicity is smaller than for most minerals. Avoid stacking multiple products that each contain selenium (multivitamin + thyroid support + antioxidant complex).
- Geographic soil variability. Selenium-poor regions include parts of Europe, China, and New Zealand. If you live in or emigrated from a low-selenium region and eat locally, testing is especially worthwhile.
Why ask for it
- Selenium deficiency impairs thyroid hormone activation — T4 cannot convert to the active T3 form without selenoenzymes (deiodinases). If your TSH is borderline elevated and your diet is low in selenium, correcting selenium status is a dietary step worth taking before more invasive interventions.
- Glutathione peroxidase, the body's primary intracellular antioxidant enzyme, is selenium-dependent. Suboptimal selenium reduces antioxidant capacity at the cellular level.
- A single test is sufficient to establish baseline; retest only if the first result was abnormal and you have made dietary changes.
- Selenium is rarely included in standard metabolic panels — request it specifically, especially if you are investigating thyroid function (useful companion to the TSH / Free T3 / T4 panel).
Related protocols
- TSH, Free T3 and T4 — Thyroid Panel
- Magnesium RBC — Cellular Magnesium Status Explained
- Zinc — Immune + Hormone Cofactor
Sources
AgeGen lab guides are educational only. We do not provide medical diagnosis, prescribe brands, or recommend specific doses. Talk to a licensed clinician before changing your supplement or medication routine.
