Selenium is an essential trace mineral and a cofactor for selenoproteins, including glutathione peroxidase and thyroid deiodinases.
Mechanism — Required for T4-to-T3 conversion. Antioxidant via glutathione peroxidase. Modulates thyroid autoimmunity (reduces TPO antibodies in Hashimoto's). Brazil nuts are the densest dietary source.
Use case — Hashimoto's adjunct, thyroid function support, regions with selenium-poor soil. Typical 100-200 mcg/day from selenomethionine or selenium-yeast forms.
Caveats — Toxicity begins around 400 mcg/day — selenosis (hair loss, brittle nails, neuropathy). Sodium selenite is the cheap, harsh form. Stick with organic forms (selenomethionine).