Selenium

What Does Selenium Do?

• Antioxidant defence via glutathione peroxidases (GPx): Four of the 25 human selenoproteins are glutathione peroxidases — enzymes that reduce hydrogen peroxide and lipid hydroperoxides to water and lipid alcohols respectively, preventing oxidative damage to cell membranes, DNA, and proteins. GPx activity is directly proportional to selenium status, making adequate intake essential for cellular antioxidant defence, particularly in tissues with high metabolic activity (liver, kidney, testis).
• Thyroid hormone metabolism: Three iodothyronine deiodinase enzymes (DIO1, DIO2, DIO3) are selenoproteins that convert the precursor thyroid hormone T4 (thyroxine) to the active form T3 (triiodothyronine) and regulate T3 inactivation. Selenium deficiency impairs T4 to T3 conversion and is thought to worsen the effects of iodine deficiency, worsening hypothyroidism. The thyroid contains the highest selenium concentration of any organ, reflecting the critical role of selenoproteins in protecting it from oxidative damage during hormone synthesis.
• Immune function: Selenium modulates both innate and adaptive immunity. Adequate selenium enhances natural killer cell activity, T-cell proliferation, and antibody production. Deficiency is associated with impaired viral immunity — notably, studies of HIV-positive individuals and selenoprotein knockout models show that selenium deficiency facilitates viral replication and mutation to more virulent strains. This has motivated ongoing research into selenium supplementation in immunocompromised populations.
• Male reproductive function: Selenoprotein P (SELENOP) and the sperm-specific selenoprotein SELENOT are essential for sperm maturation and motility. Infertile men with low sperm motility often have reduced selenium status. Selenium supplementation has shown improvements in sperm motility and sperm count in clinical trials of infertile men with documented deficiency. Additionally, testicular mitochondria-associated capsule (mitochondria-rich selenoprotein) is required for normal spermatozoa structural integrity.
• Thyroid protection against autoimmunity: Adequate selenium reduces circulating thyroid peroxidase antibodies (TPO-Ab) in Hashimoto's thyroiditis — the most common autoimmune thyroid disease. Several RCTs show 50–60% reduction in TPO-Ab with 200 mcg/day selenomethionine supplementation. The proposed mechanism is enhanced GPx activity protecting thyrocytes from the oxidative stress that triggers autoimmune cascades. This is one of the most clinically actionable selenium findings in evidence-based medicine.

Signs of Selenium Deficiency

Selenium deficiency is defined as serum selenium below 70–75 mcg/L (or plasma GPx activity below half maximal). Geographic selenium deficiency (from low-selenium soils) affects large populations in China, New Zealand, Finland (pre-fortification), and parts of sub-Saharan Africa. Classic deficiency syndromes:

• Keshan disease — a fatal cardiomyopathy (heart muscle disease) first described in children and women of reproductive age in selenium-poor regions of China; characterised by cardiac enlargement, arrhythmias, and heart failure; the endemic selenium deficiency was confirmed when supplementation eliminated new cases
• Kashin-Beck disease — a chronic osteoarthropathy causing joint deformity, growth retardation, and skeletal muscle atrophy in children; occurs in selenium- and iodine-deficient areas of China, Tibet, and Siberia
• Hypothyroidism and goitre worsening — selenium deficiency impairs T4→T3 conversion and reduces thyroid antioxidant protection, exacerbating the effects of iodine deficiency
• Impaired immune response — increased susceptibility to viral infections; animals fed selenium-deficient diets show markedly increased vulnerability to viral mutations producing more virulent strains
• Male infertility — reduced sperm motility and count associated with inadequate selenoprotein function in spermatozoa
• Muscle weakness — nonspecific myopathy reported in selenium-deficient individuals and in people on long-term total parenteral nutrition without selenium supplementation

Upper Limit: The Tolerable Upper Intake Level (UL) for selenium is 400 mcg/day for adults from all sources (food + supplements). Chronic intake above this causes selenosis: garlic breath odour, brittle nails, hair loss, neuropathy, and gastrointestinal problems. Brazil nuts are the most common source of inadvertent overconsumption — a handful daily can easily exceed the UL given that single nuts can contain up to 288 mcg.

How to Get the Most from Selenium

Selenium absorption is highly efficient — 50–90% of dietary selenium is absorbed regardless of food form or intake level, unlike many other minerals. However, the bioavailability of selenium varies significantly by chemical form:

Common Selenium Questions