Selenium builds the antioxidant enzymes that protect cells from oxidative damage and powers the conversion of thyroid hormone T4 into active T3. Both deficiency and excess carry real risks, a narrow healthy window.
Standard range versus optimal range
| Framework | Selenium value | Source |
|---|---|---|
| Typical serum range | about 70-120 µg/L | Common laboratory range |
| Adult daily requirement | 55 µg | NIH Office of Dietary Supplements |
| Upper intake limit | 400 µg per day | NIH Office of Dietary Supplements |
When it runs high
Excess selenium, selenosis, announces itself with a garlic breath odor, brittle nails, hair loss and stomach upset. It follows overzealous supplementation, the classic case being people doubling up on Brazil nuts plus pills.
- Excessive supplementation
- Very high Brazil nut intake, one nut can exceed the daily need
- Industrial exposure
When it runs low
Low selenium weakens antioxidant defense and thyroid hormone activation. Severe deficiency causes a heart muscle disease, Keshan disease, and worsens autoimmune thyroiditis activity in susceptible people.
- Low-selenium soil regions, parts of Europe and China
- Long-term parenteral nutrition without supplementation
- Severe digestive disease
- Kidney dialysis losses
The U-curve mineral
Selenium breaks the more-is-better rule harder than almost any nutrient. Deficiency collapses glutathione peroxidase, the enzyme army that neutralizes peroxides, while excess is toxic, and the healthy window between them is unusually narrow, 55 µg needed, 400 µg the ceiling. The NIH notes that Brazil nuts are so concentrated that a single nut can contain over 90 µg, making the casual handful-plus-supplement habit the commonest route to selenosis.
The thyroid connection
The thyroid holds more selenium per gram than any other organ, because making and activating thyroid hormone generates oxidative sparks that selenium enzymes must contain. The deiodinase enzymes converting T4 to active T3 are selenoproteins themselves. In autoimmune thyroiditis, supplementation trials lowered antibody levels, though clear clinical benefit remains debated, so it sits in the reasonable-to-try category under medical guidance rather than a proven treatment.
Soil decides more than diet plans
Regional soil content silently sets population status, high in most of North America, marginal across much of Europe, frankly low in parts of China where Keshan heart disease was first described. This geography explains why supplementation trials benefit the deficient but not the replete, and why measuring before supplementing is the rational sequence for this particular mineral.
Related calculators
Related biomarkers TSH · Zinc · Free T4