Selenium, Serum or Plasma

Selenium is a mineral your body needs in small amounts to protect cells from damage, support your thyroid gland, and keep your immune system working properly. This test measures how much selenium is currently in your blood. Too little selenium—often due to a poor diet or difficulty absorbing nutrients—can affect your heart, muscles, and thyroid. Too much selenium, usually from over-supplementation, can cause hair loss, nail problems, and other symptoms. Most people who eat a varied diet get enough selenium without needing supplements.

When elevated: Elevated serum selenium suggests excessive intake or toxicity (selenosis), which may be associated with hair loss, nail brittleness, peripheral neuropathy, gastrointestinal disturbance, and in severe cases, hepatotoxicity or cardiovascular effects. Chronic high-level exposure has been associated with increased risk of type 2 diabetes in some epidemiological studies, though causality is not firmly established. When low: Low serum selenium indicates deficiency, which is associated with Keshan disease (an endemic cardiomyopathy), Kashin-Beck disease (an osteoarthropathy), impaired thyroid hormone metabolism, compromised antioxidant capacity, and increased susceptibility to oxidative stress-related conditions. Severe deficiency in critically ill or parenterally nourished patients may contribute to immune dysfunction and poor clinical outcomes.

Selenium supports antioxidant defense during intense training, which generates oxidative stress, and aids thyroid function critical for metabolic rate and endurance capacity. Athletes with high sweat losses or restrictive diets may have suboptimal intake; adequate selenium also supports immune resilience during heavy training blocks.