Selenium

In humans, selenium is a trace element nutrient that functions as cofactor for reduction of antioxidant enzymes, such as glutathione peroxidases and certain forms of thioredoxin reductase found in animals and some plants (this enzyme occurs in all living organisms, but not all forms of it in plants require selenium).
The glutathione peroxidase family of enzymes (GSH-Px) catalyze reactions that remove reactive oxygen species such as hydrogen peroxide and organic hydroperoxides.
The thyroid gland and every cell that uses thyroid hormone also use selenium,[117] which is a cofactor for the three of the four known types of thyroid hormone deiodinases, which activate and then deactivate various thyroid hormones and their metabolites; the iodothyronine deiodinases are the subfamily of deiodinase enzymes that use selenium as the otherwise rare amino acid selenocysteine.
Increased dietary selenium reduces the effects of mercury toxicity.
Summary Table of Pharmacological Actions

Action	Mechanism	Examples/Notes
Antioxidant Activity	Via glutathione peroxidase, thioredoxin reductase, reduces ROS	Protects against cardiovascular disease, supports cell survival
Thyroid Regulation	Deiodinase enzymes convert T4 to T3, protects thyroid gland	Reduces thyroid autoantibodies in autoimmune thyroiditis, doses 80-200 mcg/day
Reproductive Health	Supports sperm maturation, phospholipid hydroperoxide glutathione peroxidase	Improves sperm motility, doses 100-300 mcg/day, results inconsistent
Immune Support	Enhances host defense, reduces oxidative stress in infections	Increased requirements during viral infections, e.g., HIV, hepatitis
Chemopreventive Potential	DNA repair, apoptosis, immune modulation, pro-oxidant at high doses	Increased requirements during viral infections, e.g., HIV, hepatitis
Chemopreventive Potential	DNA repair, apoptosis, immune modulation, pro-oxidant at high doses	Mixed trial results, e.g., SELECT trial showed no prostate cancer risk reduction
Bimodal Action (Dose-Dependent)	Antioxidant at low doses, pro-oxidant at high doses, induces cell death	Toxicity at >400 μg/day, potential cancer therapy at supra-nutritional levels
Compound-Specific (e.g., Ebselen)	Catalyzes hydrogen peroxide destruction, anti-oxidative stress	Experimental, not widely used clinically
Compound-Specific (Selenium Sulfide)	Anti-fungal, kills Malassezia, treats dandruff, Tinea versicolor	Used in shampoos, dermatological applications

Summary Table of Pharmacological Actions