What it's actually good for
Selenium is an essential trace mineral your body uses in genuinely small amounts — micrograms, not milligrams — but the selenoproteins it builds do outsized work. Glutathione peroxidase and thioredoxin reductase form your enzymatic antioxidant defense; the deiodinase enzymes that convert inactive T4 thyroid hormone into active T3 depend on it too. That deficiency is not hypothetical: Keshan disease, an endemic dilated cardiomyopathy that killed thousands in selenium-poor regions of China through the 1960s and 70s, was traced to soil selenium levels so low that dietary intake couldn't meet enzymatic demand, and it largely disappeared once selenium supplementation programs started. Most of North America has adequate soil selenium; parts of China, Europe, and New Zealand do not — "selenium deficiency" is a real, geographically clustered condition, not a marketing premise.
The clearest case for supplementing is correcting that kind of shortfall, or addressing thyroid autoimmunity specifically. Outside those two situations, selenium is a poor fit for more-is-better logic — the toxic dose isn't that far above the therapeutic one.
What the research says
Thyroid autoimmunity (Grade B). A blinded, placebo-controlled RCT by Gärtner et al. (2002) gave 200 mcg/day of sodium selenite to 70 women with autoimmune (Hashimoto's) thyroiditis and elevated TPO antibodies. After three months, antibody titers dropped to 63.6% of baseline in the selenium group versus 88% with placebo, and roughly a quarter of treated patients normalized their antibody levels versus about 6% on placebo. Worth being precise about what this trial did and didn't show: TSH and T4 — actual thyroid function — did not differ significantly between groups. The effect was on autoimmune antibody titers, not on clinical thyroid status. That's a real, replicated finding, but it's not the same as "selenium fixes your thyroid."
Antioxidant defense (Grade B). Selenium is a required cofactor for glutathione peroxidase, and selenium-deficient cells lose the bulk of their GPx activity in controlled studies. The catch is the same one that shows up across antioxidant supplements generally: benefit tracks baseline status. People who are low in selenium see measurable improvement in oxidative stress markers when they supplement; people who already have adequate intake mostly don't, because you can't push an enzyme system past its saturation point by adding more of one cofactor.
Cancer prevention — tested and negative. The SELECT trial, one of the largest supplement RCTs ever run, gave 200 mcg/day of selenium to over 35,000 selenium-replete men for years, expecting a prostate cancer prevention effect based on earlier observational signals. It found none (Lippman et al., 2009), and picked up a nonsignificant but unwelcome signal toward higher type 2 diabetes risk in the selenium group before stopping early for futility. It's the strongest evidence in selenium's file, and it argues against supplementing when you're already getting enough.
How much, and which form
The RDA is 55 mcg/day. Clinical trials, including the thyroid studies above, generally use 200 mcg/day, and most supplement products fall in the 100-200 mcg range. Selenomethionine (the organic form, also used in SELECT) is well-absorbed and stored in body proteins; sodium selenite is an inorganic alternative with adequate but different absorption kinetics; selenium yeast provides a mixed profile of organic forms. None has a decisively better track record — selenomethionine is simply the more common, better-characterized default.
Safety & interactions
This is not a mineral to megadose. The upper limit is 400 mcg/day, and chronic intake above that risks selenosis — garlic-smelling breath, hair loss, brittle nails, GI upset, and in severe cases neurological symptoms. The SELECT trial's diabetes signal, even at a standard 200 mcg/day dose in replete men, is a reminder that "more antioxidant" isn't a safe default assumption. Selenium may also interact with cisplatin and other chemotherapy agents, and high-dose vitamin C taken simultaneously may blunt absorption. This is informational, not medical advice — check with a clinician before starting, particularly if you're on chemotherapy or already selenium-replete from diet.
How we picked the brand
A selenium product earns a spot when it uses a well-absorbed form (selenomethionine or selenium yeast), states the dose clearly in micrograms, comes from a GMP-certified facility, and has third-party testing behind it.