Glutathione peroxidase (GPx) is an antioxidant enzyme that neutralises hydrogen peroxide and lipid peroxides inside cells. These reactive molecules are normal byproducts of metabolism, but when they accumulate faster than the body can clear them, they damage cell membranes, proteins, and DNA. GPx is one of the body's primary defences against this process.
What makes GPx particularly useful as a test marker is that it requires selenium to function. Selenium is incorporated into the enzyme's active site as the amino acid selenocysteine, and without adequate selenium, GPx cannot be properly assembled or operate effectively. This means that measuring GPx activity inside red blood cells provides a functional assessment of selenium status, showing whether the selenium in your body is actually being put to work rather than simply measuring how much is circulating in your blood.
This test goes beyond a standard serum selenium measurement. Serum selenium tells you the concentration of selenium in plasma at the time of the draw. RBC GPx activity tells you how effectively that selenium is supporting antioxidant defence at the cellular level. The two tests complement each other, and some practitioners order both for a complete picture.
Low GPx activity may indicate selenium deficiency, increased oxidative burden, or reduced enzyme synthesis. It has been associated with cardiovascular disease, thyroid dysfunction, fatigue, and chronic illness in research settings. High GPx activity typically reflects an upregulation response to increased oxidative stress, or supplementation with selenium.
The test is most relevant for people working with integrative or functional medicine practitioners who are investigating oxidative stress, evaluating selenium sufficiency, or assessing antioxidant capacity as part of a broader metabolic workup.
To assess cellular antioxidant function, evaluate functional selenium status, contribute to an oxidative stress assessment, and inform integrative or functional medicine investigations into fatigue, metabolic health, and chronic disease risk.
