SeleniumSerum selenium
A trace mineral built into antioxidant and thyroid enzymes.
Selenium is incorporated directly into a small family of proteins, including the glutathione peroxidases that limit oxidative damage and the enzymes that convert thyroid hormone T4 into the more active T3.
It matters because the intake range between too little and too much is narrower than for most nutrients, and because soil content varies so much geographically that dietary intake differs widely between countries.
In practice serum selenium reflects recent intake more than long-term status, and it is measured infrequently outside specific clinical questions.