A trace mineral with a narrow sweet spot. Getting too little and getting too much both cause harm.
Mechanisms, studies and evidence grades for each claim above.
Selenium works as selenocysteine, built directly into about 25 human selenoproteins. The best-known are the glutathione peroxidases, which use glutathione to neutralize hydrogen peroxide and lipid peroxides, and thioredoxin reductases, which regenerate the cell's other major reducing system.1 This puts selenium directly downstream of the glutathione story on this site — see Glutathione — and upstream of lipid-peroxide handling relevant to the linoleic acid argument. (Strong — biochemistry)
↑ back to summaryKeshan disease is an endemic cardiomyopathy (heart-muscle failure) that was widespread in a belt of selenium-poor soil across China from the 1950s to 1970s. A systematic review of community supplementation trials found that giving selenium to residents of endemic areas significantly reduced incidence, and concluded that selenium deficiency meets modern epidemiological criteria as a cause.2 This is the clearest demonstration of what selenium does — and it applies to deficient populations, not to people already eating adequate amounts. (Strong)
↑ back to summaryThe thyroid holds more selenium per gram than any other organ. The deiodinase enzymes that convert T4 into active T3 are selenoproteins, and the gland relies on glutathione peroxidase to protect itself from the hydrogen peroxide it generates while making hormone.1
A 2024 meta-analysis of 35 randomized trials in Hashimoto thyroiditis found selenium lowered TSH in patients not yet on thyroid hormone (SMD −0.21), and lowered thyroid peroxidase antibodies (TPOAb, SMD −0.96, 29 cohorts, 2,358 people) whether or not patients were on hormone replacement. It also lowered a marker of oxidative damage (MDA). Adverse effects were no different from control, and the overall certainty of evidence was rated moderate.3 What's missing is evidence that lower antibodies translate into fewer people progressing to overt hypothyroidism. (Moderate)
↑ back to summarySelenium has an unusually narrow gap between adequate and excessive intake, and the trial record shows harm appearing where people were already replete:1
The practical reading: selenium is worth ensuring, not maximizing. Status depends heavily on the soil where food was grown, so a food-first intake around the RDA is the target, and supplements above ~100 mcg make most sense for a specific reason (like Hashimoto's) or a known low-selenium diet.
↑ back to summaryRDA 55 mcg/day for adults; tolerable upper intake 400 mcg/day.7
Brazil nuts. Their selenium content varies widely with the soil they grew in, which makes them an imprecise dose. But in a 12-week randomized trial in New Zealanders (a low-selenium population), two Brazil nuts a day — which actually delivered an average of 53 mcg, ranging from roughly 20 to 84 — raised plasma selenium and glutathione peroxidase activity as effectively as 100 mcg of selenomethionine.8 Two nuts works; a handful risks overshooting.