Selenium's value depends almost entirely on how much a person already has. Correcting a genuine shortfall supports antioxidant defenses, thyroid function, and immune resilience. Those already well supplied gain little and face higher diabetes and, in some men, aggressive prostate disease risk. Very high intake harms hair, nails, digestion, and nerves. The useful range is narrow. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Serum/Plasma Selenium | ~110–130 ng/mL | Reflects overall selenium status and guides whether to supplement |
| Plasma Selenoprotein P | Saturating range (plateau at serum selenium ~110–125 ng/mL) | Best functional marker of whether selenoprotein needs are met |
| Glutathione Peroxidase Activity | Plateau at serum selenium ~90–100 ng/mL | Confirms functional antioxidant capacity is saturated |
| Thyroid Peroxidase Antibodies | Ideally undetectable (< ~35 IU/mL) | Tracks autoimmune thyroid activity when a thyroid indication is present |
| Fasting Glucose and HbA1c | HbA1c < 5.4% | Screens for the diabetes risk signal associated with excess selenium |
Cadence: Recheck serum selenium (and thyroid antibodies if relevant) at ~3 and 6 months, then every 6–12 months if supplementation continues; glucose and HbA1c reviewed at least annually.