Selenium Supplements: Forms, Dosage, and What to Know

What Selenium Does in Your Body

Selenium is incorporated into selenoproteins, a family of at least 25 proteins that carry out essential functions. The most important ones fall into a few categories:

• • Glutathione peroxidases (GPx): These enzymes neutralize hydrogen peroxide and lipid hydroperoxides, protecting cells from oxidative damage. They require selenium at their active site to function. Without adequate selenium, GPx activity drops and oxidative stress increases.
• • Thyroid hormone metabolism: The deiodinase enzymes that convert inactive thyroid hormone (T4) into active T3 are selenium-dependent. Low selenium can impair this conversion, potentially affecting energy, metabolism, and body temperature regulation even when thyroid hormone production itself is normal.
• • Immune function: Selenium influences both innate and adaptive immune responses. Adequate selenium status supports the proliferation and function of T cells and natural killer cells. Selenium deficiency has been associated with increased susceptibility to viral infections and slower immune recovery.
• • Reproductive health: Selenium is concentrated in testicular tissue and is essential for normal sperm development and motility. Selenoprotein P also plays a role in protecting sperm from oxidative damage during maturation.

The RDA for selenium is 55mcg per day for adults. This amount is set to maximize glutathione peroxidase activity, which plateaus at relatively modest selenium intakes.

Forms of Selenium Supplements

Selenium supplements come in several forms that differ in absorption, storage, and how they are used by your body.

What the Research Says

Selenium has been studied for several health outcomes. The evidence is strongest for thyroid function and immune support, while results for cancer prevention have been mixed.

• • Thyroid health: The thyroid gland has the highest selenium concentration per gram of tissue in the body. A 2010 Cochrane review found that selenium supplementation reduced thyroid peroxidase antibodies in people with autoimmune thyroiditis (Hashimoto's), though the clinical significance of antibody reduction alone remains debated. Selenium deficiency combined with iodine deficiency worsens hypothyroidism more than iodine deficiency alone.
• • Immune response: A 2019 systematic review found that selenium supplementation improved certain markers of immune function, including enhanced T cell proliferation and natural killer cell activity, particularly in people with low baseline selenium levels. In populations with adequate selenium, supplementation showed minimal additional immune benefit.
• • Cancer prevention: The 1996 NPC trial found that 200mcg/day of selenomethionine reduced prostate, lung, and colorectal cancer incidence in people with low baseline selenium. However, the larger SELECT trial (2009) using 200mcg/day of selenium yeast in men with adequate baseline selenium found no cancer prevention benefit and a non-statistically-significant trend toward increased type 2 diabetes risk. Current evidence suggests selenium may only reduce cancer risk in people who are deficient, not in those who already have adequate levels.
• • Heart health: The relationship between selenium and cardiovascular disease follows a U-shaped curve. Both low and high selenium levels have been associated with increased cardiovascular risk in observational studies. A 2013 meta-analysis of 12 randomized trials found no significant effect of selenium supplementation on cardiovascular events. Supplementation in people who already have adequate selenium does not appear to provide cardiovascular benefit.

Most selenium research points to the same conclusion: supplementation benefits people who are deficient but offers little to those who already get enough from food.

Dosage and Safety

Selenium has a relatively narrow margin between the amount you need and the amount that can cause problems. This makes dosage more important than with many other supplements.

Who May Benefit From Selenium Supplements

Most people in the U.S. get enough selenium from food. But certain groups are at higher risk for low levels:

• • People with autoimmune thyroid conditions: Research suggests selenium supplementation may reduce thyroid antibody levels in people with Hashimoto's thyroiditis, though whether this translates to improved thyroid function or reduced disease progression varies by individual. It is often recommended by endocrinologists as an adjunct to thyroid hormone replacement.
• • People living in low-selenium regions: Soil selenium levels vary dramatically by geography. Parts of China, Russia, and Eastern Europe have selenium-poor soil, and populations in these areas have historically shown higher rates of selenium deficiency diseases like Keshan disease and Kashin-Beck disease. Most U.S. soil has adequate selenium, though some areas in the Pacific Northwest and parts of the Great Lakes region are lower.
• • People on dialysis: Hemodialysis can deplete selenium over time, and several studies have found that dialysis patients have lower selenium levels than the general population. Supplementation in this group should be supervised by a nephrologist.
• • People with HIV: Selenium deficiency is common in people living with HIV and has been associated with faster disease progression. Some clinical trials have shown that selenium supplementation can slow viral load increase and improve CD4 counts in selenium-deficient HIV-positive individuals.
• • People with GI conditions affecting absorption: Crohn's disease, celiac disease, and other conditions that impair nutrient absorption in the small intestine can reduce selenium uptake. People who have had bariatric surgery may also be at risk.

If you eat a varied diet that includes protein sources like meat, seafood, eggs, or Brazil nuts, you are likely getting enough selenium without a supplement.

What to Look For When Buying

Side Effects and Interactions

Selenium supplements are generally safe at recommended doses, but the narrow therapeutic window means there are important considerations:

• • Selenosis: Chronic intake above 400mcg per day can cause selenium toxicity. Symptoms include hair loss, nail brittleness and discoloration, garlic breath, nausea, diarrhea, skin rashes, fatigue, and nervous system irritability. Symptoms are reversible when intake is reduced, but severe acute toxicity requires medical attention.
• • Diabetes risk: Several observational studies and the SELECT trial have raised concerns about a possible link between high selenium intake and increased type 2 diabetes risk, particularly in people who already have adequate selenium levels. The mechanism is not fully understood but may involve selenium's effects on insulin signaling. This is another reason not to supplement if you are not deficient.
• • Drug interactions: Selenium may interact with certain chemotherapy drugs, as its antioxidant activity could theoretically interfere with treatments that rely on oxidative mechanisms to kill cancer cells. People undergoing cancer treatment should discuss selenium supplementation with their oncologist before starting.
• • Statin interaction: Some research suggests that combining selenium with statins may reduce the cholesterol-lowering effects of the medication, though the evidence is limited. If you take a statin, mention selenium supplementation to your doctor.