Selenium is an essential trace mineral, found in foods like brazil nuts, beef and egg yolks. Selenium serves as a required cofactor in enzymes that activate thyroid hormones and have antioxidant roles. This test measures selenium in the serum, the cell free portion of the blood, and is a good indicator of short term selenium intake. Low serum selenium can lead to a greater risk of hypothyroidism, infection, cognitive issues, arthritis, and several other issues. High levels suggest an excessive dietary intake.