Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal found naturally in the soil, water, and some foods, and it exists in several physical forms including a gray metallic allotrope and a red powder

. Selenium is essential in small amounts for many animals, including humans, as it plays a critical role in various bodily functions. Biologically, selenium is a trace mineral vital for metabolism and acts as a powerful antioxidant. It helps protect cells from oxidative damage caused by aging, environmental factors, and lifestyle choices, which are linked to diseases like cancer, heart disease, and cognitive decline

. Selenium supports thyroid health by aiding hormone regulation, contributes to immune system function, and is involved in DNA production and reproduction

. It is also a component of key antioxidant enzymes such as glutathione peroxidase, which help neutralize harmful reactive oxygen species in the body

. While selenium is necessary in small amounts, excessive intake can be toxic, causing symptoms like fatigue, hair loss, and nausea

. It is found naturally in foods, with its content varying depending on soil selenium levels, and is available as a dietary supplement

. In summary, selenium is an essential trace mineral and antioxidant important for thyroid function, immune defense, and protection against cellular damage, but it must be consumed in balanced amounts to avoid toxicity