Selenium

Selenium is a key component of glutathione peroxidase enzymes, which neutralize harmful reactive oxygen species and protect cells from oxidative damage. This is one of the most well-established functions of selenium with strong evidence from decades of biochemical research.

The thyroid gland contains the highest concentration of selenium per gram of any organ. Selenoproteins (particularly iodothyronine deiodinases) are essential for converting T4 to the active T3 thyroid hormone, making selenium critical for proper thyroid function. Evidence is strong and well-documented.

Selenium enhances both innate and adaptive immune responses, improving the activity of natural killer cells and T lymphocytes. Deficiency is associated with impaired immune function and increased susceptibility to viral infections. Clinical evidence is moderate to strong.

Selenium supports DNA repair mechanisms and may reduce cancer risk, particularly prostate, lung, and colorectal cancers, through its role in selenoprotein P and thioredoxin reductase. Evidence is mixed — the SELECT trial showed no prostate cancer benefit from supplementation, but populations with low baseline selenium may benefit.

Selenium's antioxidant properties help protect neurons from oxidative stress, and declining selenium levels have been associated with cognitive decline in aging populations. Some observational studies link higher selenium status to better cognitive performance, though interventional evidence remains limited.

Best taken once daily with a meal, typically in the morning or afternoon. Consistency in daily timing matters more than the specific time of day.

Take with food to enhance absorption and reduce the chance of stomach upset. No specific fat requirement, but a balanced meal improves tolerance and bioavailability.

Typical supplemental dose is 100-200 mcg per day for adults. The RDA is 55 mcg/day, with an upper tolerable limit of 400 mcg/day. Doses above 400 mcg/day risk toxicity (selenosis). Many people get adequate selenium from diet alone, especially in selenium-rich soil regions.

Selenomethionine is the best-absorbed organic form and is most commonly recommended. Selenium yeast provides a mix of organic selenium forms. Sodium selenite (inorganic) is also effective but may have slightly lower bioavailability. Capsule or tablet forms are most practical.

Do not exceed 400 mcg/day from all sources (food + supplements) to avoid selenosis, which causes hair loss, nail brittleness, garlic breath, nausea, and neurological symptoms. People in high-selenium regions (parts of the US Great Plains, China) should be cautious about supplementation. May interact with certain chemotherapy drugs and immunosuppressants. Those with kidney disease should consult a

Multiple clinical trials show that selenium supplementation (200 mcg/day) can reduce thyroid peroxidase (TPO) antibodies in patients with Hashimoto's thyroiditis. Evidence is moderate to strong, with several meta-analyses supporting its use as an adjunct therapy alongside standard thyroid treatment.

Selenium supplementation has been shown to improve quality of life and reduce eye symptoms in mild Graves' orbitopathy. The EUGOGO trial demonstrated significant benefit with 200 mcg sodium selenite daily for 6 months. Evidence is moderate and guideline-supported.

Selenium is concentrated in testicular tissue and is essential for sperm motility and formation. Studies suggest supplementation can improve sperm quality in men with low selenium levels, though evidence is moderate and benefits are most apparent in deficient individuals.

Low selenium status has been associated with increased cardiovascular risk in observational studies. Selenium's antioxidant role may reduce lipid peroxidation and inflammation in blood vessels. However, evidence from supplementation trials is mixed, and excess selenium may actually increase cardiovascular risk.

Selenium deficiency is linked to increased virulence of certain viruses, including Coxsackievirus (Keshan disease) and may worsen HIV progression. Supplementation in deficient populations has shown improvements in immune markers. Evidence is moderate for deficiency correction but limited for use in selenium-replete individuals.

Selenium and vitamin E work synergistically as antioxidants — selenium through glutathione peroxidase and vitamin E as a lipid-soluble free radical scavenger. Together they provide complementary protection against oxidative damage to cell membranes and intracellular components. View Vitamin E guide →

Selenium and iodine are both essential for thyroid health and work together in thyroid hormone synthesis and metabolism. Selenium deficiency can worsen the effects of iodine deficiency, and supplementing both together is more effective for thyroid support than either alone. View Iodine guide →

Zinc and selenium share complementary roles in immune function, antioxidant defense, and thyroid metabolism. Both minerals support the structural integrity of proteins and enzymes critical for immune cell signaling and inflammatory regulation. View Zinc guide →

NAC is a precursor to glutathione, the substrate upon which selenium-dependent glutathione peroxidase enzymes act. Together, they optimize the glutathione antioxidant system, providing enhanced protection against oxidative stress. View N-Acetyl Cysteine (NAC) guide →

Vitamin C works alongside selenium to regenerate other antioxidants and enhance overall antioxidant capacity. This pairing supports immune function through complementary mechanisms — vitamin C through direct immune cell support and selenium through selenoprotein activity. View Vitamin C guide →

Both selenium and levothyroxine support thyroid function — levothyroxine replaces T4 directly, while selenium is required for the deiodinase enzymes that convert T4 to active T3. Selenium supplementation can complement but not replace levothyroxine therapy in hypothyroid patients.

Both enhance the glutathione antioxidant system — NAC provides the glutathione substrate while selenium activates glutathione peroxidase enzymes. The pharmacological overlap is in hepatoprotection and antioxidant defense, with moderate clinical evidence.

Both selenium and metformin have been studied for their effects on oxidative stress and insulin sensitivity. Some research suggests selenium influences glucose metabolism through selenoprotein P, though the relationship is complex and excess selenium may paradoxically worsen insulin resistance, unlike metformin's consistent benefit.