Selenium is an essential trace element that plays a crucial role in antioxidant defense and thyroid hormone metabolism. This test measures selenium levels in whole blood to evaluate nutritional status and detect deficiency or toxicity. Selenium is a component of several important enzymes including glutathione peroxidase, which protects cells from oxidative damage, and deiodinases, which are essential for thyroid hormone activation. Deficiency can cause muscle weakness, cardiomyopathy, and impaired immune function. Selenium toxicity, which can occur from industrial exposure or excessive supplementation, may cause hair loss, nail changes, skin problems, and neurological symptoms. This test is particularly valuable for monitoring patients on long-term parenteral nutrition, evaluating unexplained muscle weakness or heart problems, and assessing exposure in occupational settings.