Selenium (selenomethionine) · mineral
Evidence tier: insufficient
A trace mineral essential for thyroid hormone conversion and antioxidant defence. Especially important in autoimmune thyroid.
Selenium is built into about 25 selenoproteins, including the glutathione peroxidases that neutralise peroxides and the deiodinases that convert thyroid hormone T4 into the active T3. That is why the thyroid marketing sounds so plausible. The catch is that these enzymes SATURATE. Once selenoprotein synthesis is maximal, which happens at ordinary dietary intakes in a replete person, additional selenium does not increase enzyme activity, it just increases body burden. This is the mechanistic reason why correcting a deficiency helps and topping up a replete person does not, and why the dose response curve turns from flat to harmful rather than continuing upward.
Selenium is an essential mineral, and that fact is doing all the persuasive work in the supplement aisle while none of the evidence work. Two things are true at once. Selenium DEFICIENCY is real and serious: it causes Keshan disease, a cardiomyopathy, and is implicated in Kashin Beck disease and myxoedematous cretinism (MedlinePlus). But deficiency is uncommon in the United States and the United Kingdom, and the question a supplement has to answer is not whether the mineral matters, it is whether swallowing MORE of it helps a person who already has enough. On that question the trials are consistently negative. The SELECT trial randomised 35,533 men and found selenium at 200 mcg daily did not prevent prostate cancer; the National Cancer Institute's own summary states that there are no clinical trials that show a benefit from taking vitamin E or selenium to reduce the risk of prostate cancer or any other cancer or heart disease. SELECT also assessed cardiovascular events in 17,488 well nourished men over 7 to 12 years and found no effect. For autoimmune thyroid disease, the marquee supplement claim, Cochrane reviewed 4 trials in 463 people at unclear to high risk of bias and concluded that the evidence to support or refute the efficacy of selenium supplementation in people with Hashimoto's thyroiditis is incomplete and not reliable to help inform clinical decision making; antibody titres moved, but Cochrane says their clinical relevance is unclear, and an antibody number is not a symptom. Male fertility evidence is confounded by the fact that selenium is almost always given with vitamin E or other antioxidants, so no clean selenium effect exists. This is the rare ingredient where more is actively worse. The dose window is narrow: the RDA is 55 mcg daily for adults and the tolerable upper intake level is 400 mcg, a ratio of about seven, and chronic excess produces selenosis, a documented poisoning syndrome of hair loss, brittle or shed nails, garlic breath, a metallic taste, rash, nausea, diarrhoea, fatigue, irritability and nerve damage. High selenium status also tracks with worse glucose control, and NIH ODS advises that individuals with high selenium status and those at risk for type 2 diabetes should avoid taking selenium supplements.
Adult RDA is 55 mcg daily (60 mcg in pregnancy, 70 mcg while breastfeeding). The tolerable upper intake level is 400 mcg daily for adults, and the trials that found harm signals used 200 mcg daily, which is inside the UL. A couple of Brazil nuts can exceed the RDA on their own. Most people in the United States and the United Kingdom already meet requirement from food, which is the central reason a supplement has so little room to help.
100 to 200 mcg daily with food. Brazil nuts are a natural source.
Toxic at high doses (>400 mcg long term). Avoid combining multiple selenium sources.
Pregnancy: caution
Breastfeeding: caution
Active compounds: selenomethionine, selenocysteine, sodium selenite
Remedies using Selenium · Browse by body system · Check it against what you take
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.