Selenium is an essential trace mineral your body builds into a small set of proteins that quench oxidative damage, recycle other antioxidants, and switch thyroid hormone into its active form. Where the soil is poor in it, deficiency causes real disease: Keshan disease, a cardiomyopathy of the heart muscle, and Kashin-Beck disease of the joints, both of which selenium prevents, and the supplementation programs in China's low-selenium regions are one of nutrition's clear public-health wins. Most people in the United States and other selenium-adequate places already get enough from ordinary food, and that is where the story turns. In the large trials, giving extra selenium to people who were already replete did not prevent cancer or heart disease, one trial found it raised the risk of type 2 diabetes by about half, and among men who already had high selenium it nearly doubled the risk of aggressive prostate cancer.
In Hashimoto's thyroiditis it lowers thyroid antibodies, though whether that changes how anyone feels or their need for thyroid medication is unsettled. The gap between too little and too much is narrow: at high intake selenium turns toxic, causing hair and nail loss, and one mislabeled supplement poisoned 201 people. Selenium matters most for the person who is deficient, and topping up an already-replete body carries more risk than benefit.
Findings & Outcomes
What It Is
Selenium is a trace element, a mineral your body needs in tiny amounts, about 55 micrograms a day for an adult. It comes from food: seafood, organ meats, eggs, poultry, meat, dairy, and whole grains, plus Brazil nuts, which are in a class of their own for how much they hold. How much is in a plant food depends on how much was in the soil it grew in, so selenium intake varies from region to region more than most nutrients do. Parts of China, and some other inland areas, sit on soil so low in selenium that the people living on local food fall short. Most of the United States, Canada, and countries that eat a varied diet from many sources get plenty.
Your body does not use selenium loose. It builds it into a small family of about twenty-five proteins, the selenoproteins, and those proteins do the work: they defend cells against oxidative damage, recycle other antioxidants, and turn thyroid hormone into its active form. Once you have enough selenium to fill those proteins, extra selenium has no more of them to go into, which is the reason the effects at low intake and the effects at high intake look so different.
What It Does
Selenium's story splits by where you start. For a person who is deficient, filling the gap is the difference between disease and health: severe deficiency, together with other triggers, drives the heart-muscle disease and joint disease described below, and correcting it prevents them. That is the settled, life-or-death end of the range, and it is why selenium is on every list of essential nutrients.
For a person who is already replete, which is most people eating a varied diet, adding more selenium is a different question with a different answer. The large trials asked whether extra selenium prevents cancer or heart disease in well-nourished people, and the answer came back no. In Hashimoto's thyroiditis, an autoimmune thyroid condition, selenium lowers the antibodies the immune system makes against the thyroid, a real and repeated finding, though what that means for how a person actually feels is still open.
Each card below grades one of these at the strength of its own evidence. The pattern that runs through them is the one to hold onto: the benefit lives in deficiency, and once you are replete the same doses stop helping and start carrying risk.
The Research & Studies
Everything here is based on the research we have collected and checked, sorted into groups and ordered with the strongest evidence first. Click any claim to open the studies behind it.
How it works
Selenium works through about 25 selenoproteins, including glutathione peroxidases, thioredoxin reductases and thyroid deiodinases
Your body does not use selenium on its own. It builds it into a small set of about 25 proteins that break down harmful byproducts of metabolism, keep other antioxidants working, and switch thyroid hormone into its active form. Everything selenium does runs through these proteins.
Selenoproteins incorporate selenium as the amino acid selenocysteine, encoded by a recoded UGA codon. The glutathione peroxidase family (GPx1 through GPx4) reduces hydrogen peroxide and lipid hydroperoxides; the thioredoxin reductases regenerate reduced thioredoxin and other antioxidant systems; the three iodothyronine deiodinases (DIO1, DIO2, DIO3) add or remove iodine to convert thyroid prohormone T4 to active T3 or to inactive forms. Selenoprotein P is the main transport form and a marker of status. The hierarchy of selenoprotein synthesis means some proteins are preserved and others sacrificed first when selenium is scarce, and that most reach a plateau once intake is adequate.
Who this may not transfer to:Selenoprotein biochemistry is a property of human cells generally rather than a finding in one group of people, so it applies across sexes and ages.
This mechanism is the reason the effects at low and high intake differ so sharply: below adequacy the proteins cannot be made and function falls; above adequacy the proteins are saturated and extra selenium has nowhere useful to go. It describes how selenium works, not a claim that any particular supplement helps.
The study · 1
Rayman MP, Selenium and human health · Lancet 2012;379(9822):1256-1268
Counts once: this finding and 1 other here come from the same source, so they are one body of evidence, not separate confirmations.
Cancer Risk And Outcome
Selenium did not prevent prostate cancer in 35,533 men (SELECT)
In the largest trial ever run on selenium and cancer, giving more than 35,000 men a daily selenium supplement for years did not lower their chance of prostate cancer at all, and did not prevent other cancers either.
SELECT (Lippman et al., 2009) used a 2x2 factorial design with 200 micrograms a day of L-selenomethionine and/or 400 IU of vitamin E in relatively healthy men from the US, Canada and Puerto Rico. Selenium produced no reduction in prostate cancer or overall cancer. The vitamin E arm later showed a statistically significant 17 percent increase in prostate cancer on extended follow-up, a finding about vitamin E rather than selenium. Selenium was also associated with a non-significant increase in type 2 diabetes, which aligned with an earlier trial's signal.
Who this may not transfer to:Prostate cancer is a male-specific outcome, so this trial was in men by design; the parallel cancer-prevention and diabetes findings from mixed-sex trials are graded separately on this page.
This is strong evidence that a selenium supplement does not prevent prostate cancer in well-nourished men, so the antioxidant-for-cancer rationale does not justify taking it. The men in SELECT were largely selenium-replete to begin with, which is the context in which the null, and the risk signals, apply.
The study · 1
Lippman SM et al., Effect of Selenium and Vitamin E on Risk of Prostate Cancer and Other Cancers: SELECT · JAMA 2009;301(1):39-51
Selenium supplements do not prevent cancer overall (Cochrane review)
Pooling the high-quality trials together, selenium supplements do not lower the risk of cancer, even in people who started with low selenium. The review also flagged more type 2 diabetes and more skin, hair and nail problems with supplementation.
Vinceti et al. (2018 Cochrane update) concluded that randomized trial evidence shows no benefit of selenium in preventing cancer overall or prostate cancer, in contrast with earlier observational associations. The review noted harms at supplemental doses around 200 micrograms a day: increased type 2 diabetes, increased non-melanoma skin cancer, advanced prostate cancer among those with the highest baseline exposure, and dermatological abnormalities. It graded the cancer-prevention nulls as high certainty.
Who this may not transfer to:The pooled trials included both sexes; prostate-specific findings necessarily reflect the male participants, while overall cancer and diabetes findings span mixed populations.
This settles the cancer-prevention question against selenium supplements for the general population, low-status included, and pairs the null with real harms, which is why routine supplementation to prevent cancer is not supported.
The study · 1
Vinceti M et al., Selenium for preventing cancer (Cochrane systematic review) · Cochrane Database Syst Rev 2018;1:CD005195
Counts once: this finding and 1 other here come from the same source, so they are one body of evidence, not separate confirmations.
In men already high in selenium, supplements raised high-grade prostate cancer 91%
When SELECT researchers sorted men by how much selenium they already had, the supplement did nothing for men who started low, but nearly doubled the risk of aggressive prostate cancer in men who were already high. The harm depended on already having plenty.
Kristal et al. (2014) used a case-cohort design within SELECT with 1,739 total and 489 high-grade prostate cancer cases and a 3,117-man random subcohort, estimating hazard ratios by quintile of baseline toenail selenium. Selenium supplementation increased high-grade prostate cancer risk by 91 percent among men above roughly the 60th percentile of baseline selenium, with no effect below it. The authors concluded that men should avoid selenium supplementation at doses exceeding recommended dietary intakes.
Who this may not transfer to:Prostate cancer is a male-specific outcome; this case-cohort analysis was within the all-male SELECT trial.
The practical rule is to correct a deficiency, not to top up a replete body. Because this harm fell on men who were already high in selenium, the sensible step before any supplement is checking status rather than assuming more is safer.
The study · 1
Kristal AR et al., Baseline Selenium Status and Effects of Selenium and Vitamin E Supplementation on Prostate Cancer Risk · J Natl Cancer Inst 2014;106(3):djt456
Heart And Vascular
Selenium prevents Keshan disease, the deficiency cardiomyopathy of low-selenium regions
In parts of China where the soil holds almost no selenium, people developed a serious heart-muscle disease called Keshan disease. Giving those populations selenium largely stopped it, one of the clearest examples of a mineral deficiency causing disease and its correction preventing it.
Keshan disease resembles dilated cardiomyopathy, with heart enlargement and multifocal myocardial necrosis and fibrosis, and historically struck children and women of childbearing age in China's selenium-poor regions, with cases also reported in Korea and Japan. From the 1970s, comprehensive prevention including sodium selenite supplementation and dietary improvement markedly reduced incidence in endemic areas. Selenium deficiency is the leading etiological hypothesis; a competing view emphasizes coxsackievirus infection or fungal toxins, and the current understanding is that low selenium sets the stage and other factors trigger the disease. The effect of selenium supplementation on the prognosis of established disease is less settled than its role in prevention.
Who this may not transfer to:Keshan disease affected both sexes in endemic regions, with a historical predominance among children and women of childbearing age; the deficiency mechanism applies across sexes.
This is the settled benefit of selenium, and it belongs to genuine deficiency in low-selenium regions. It does not imply that people already replete gain heart protection from extra selenium; the large trials in well-nourished people found no cardiovascular benefit.
The study · 1
Shi Y et al., Recent Advances on Selenium Nutrition and Keshan Disease · Int Heart J 2024;65(2):173-179
Thyroid
Selenium lowers thyroid peroxidase antibodies in Hashimoto's thyroiditis
In Hashimoto's, an autoimmune thyroid disease, taking selenium lowers the level of antibodies the immune system makes against the thyroid. This antibody drop is a real and repeated finding in the trials.
Toulis et al. (2010) pooled four randomized trials and found selenium reduced TPOAb titers at 3 months (random-effects weighted mean difference -271.09, 95% CI -421.98 to -120.19). The 2024 review by Huwiler et al. of randomized controlled trials found selenium decreased TSH in Hashimoto's patients not on levothyroxine, and reduced TPOAb and the oxidative-stress marker malondialdehyde, and judged it safe in this use. TPOAb is a marker of autoimmune activity, not a direct measure of thyroid function or symptoms.
Who this may not transfer to:Hashimoto's thyroiditis is far more common in women, and trial populations were predominantly female; the antibody effect is reported in mixed but female-skewed samples.
The antibody reduction is the reason selenium is discussed for Hashimoto's, but it is a change in a laboratory marker. Whether lowering TPOAb changes how a person feels or the course of the disease is the separate, unsettled question the next card addresses, so this belongs alongside standard thyroid care decided with a clinician.
The studies · 2
Toulis KA et al., Selenium supplementation in the treatment of Hashimoto's thyroiditis: a systematic review and meta-analysis · Thyroid 2010;20(10):1163-1173
Huwiler VV et al., Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials · Thyroid 2024;34(3):295-313
Counts once: this finding and 1 other here come from the same source, so they are one body of evidence, not separate confirmations.
The antibody drop has not been shown to change symptoms, disease course or levothyroxine need
Selenium clearly lowers a thyroid antibody in Hashimoto's, but the trials have not shown that this makes people feel better, slows the disease, or reduces the need for thyroid medication. The marker moves; whether anything a patient notices changes is still unproven.
Meta-analyses consistently report reductions in TPOAb and, in patients not on levothyroxine, in TSH, but the trials are generally short, heterogeneous, and focused on biochemical markers rather than on how patients feel or fare over years. The 2024 Huwiler review explicitly identified insufficient evidence on patient-reported outcomes and long-term clinical endpoints. So the biochemical effect is better established than any clinical one, and selenium is not a substitute for standard management of hypothyroidism.
Who this may not transfer to:Trial populations were predominantly female, reflecting the sex distribution of Hashimoto's; the limitation in clinical-outcome evidence applies across the studied groups.
Read alongside the antibody finding rather than on its own. A person considering selenium for Hashimoto's should understand that the reliably demonstrated effect is on a lab marker, and should weigh a defined course with their clinician rather than expect symptom relief or a lower medication dose that the evidence does not yet support.
The study · 1
Huwiler VV et al., Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials · Thyroid 2024;34(3):295-313
Counts once: this finding and 1 other here come from the same source, so they are one body of evidence, not separate confirmations.
Blood Sugar
Selenium 200 mcg/day raised type 2 diabetes risk about 55% (NPC trial)
In one long trial, people taking a daily 200-microgram selenium supplement developed type 2 diabetes about 55 percent more often than people taking a dummy pill, and the extra risk was worst in those who already had high selenium to begin with.
Stranges et al. (2007) reported this as a secondary analysis of the NPC trial, conducted in a low-selenium area of the eastern US among people without diabetes at baseline. Overall hazard ratio for type 2 diabetes was 1.55 (95% CI 1.03 to 2.33); in the highest tertile of baseline plasma selenium it was 2.70. The 2018 Cochrane review found the diabetes signal persisted across the wider trial evidence, though the single-trial nature and uncertain mechanism warrant caution in interpretation.
Who this may not transfer to:The trial enrolled both men and women; the diabetes signal has been examined in mixed-sex pooled analyses without a clear sex difference reported.
This is a reason not to take selenium supplements for general health, particularly if you are already selenium-replete, since the harm was greatest in exactly those people. It does not argue against reaching dietary adequacy, which is where selenium's established benefits sit.
The studies · 2
Stranges S et al., Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial · Ann Intern Med 2007;147(4):217-223
Vinceti M et al., Selenium for preventing cancer (Cochrane systematic review) · Cochrane Database Syst Rev 2018;1:CD005195
Counts once: this finding and 1 other here come from the same source, so they are one body of evidence, not separate confirmations.
Immune Function
Adequate selenium supports normal immune function; low status tracks with worse outcomes
Your immune system needs selenium to work normally, and people who are low in it tend to fare worse. Filling a genuine shortfall helps; there is little good evidence that piling extra selenium onto an already-adequate body boosts immunity.
Rayman (2012) summarizes associations between low selenium status and increased mortality, poorer immune function, and impaired antiviral responses, alongside proposed benefits of higher status for immunity and reproduction. Much of this is observational and subject to reverse causation, since illness itself lowers selenium status. The immune role is mechanistically grounded in selenoprotein function, but a clinical benefit of supplementing replete people has not been reliably shown.
Who this may not transfer to:The immune associations are drawn from mixed-sex populations; the selenoprotein basis for immune function applies across sexes.
This supports reaching selenium adequacy as part of normal nutrition, not taking high-dose selenium as an immune booster. Where status is genuinely low, correction is reasonable; where it is adequate, the immune case for more is thin.
The study · 1
Rayman MP, Selenium and human health · Lancet 2012;379(9822):1256-1268
Counts once: this finding and 1 other here come from the same source, so they are one body of evidence, not separate confirmations.
The Supplement Question
For years selenium had a reputation as a cancer-preventing antioxidant, built on early trials and on the logic that a mineral which mops up oxidative damage ought to protect against disease. When that idea was put to large randomized trials in well-nourished people, it did not hold up, and the trials turned up risks that the antioxidant story had not predicted.
The largest was SELECT, which gave 35,533 men 200 micrograms of selenium a day, vitamin E, both, or a placebo, and followed them for years. Selenium did not reduce prostate cancer, the outcome it was designed to test, and it did not reduce other cancers either. The Cochrane review, pooling the randomized trials of selenium and cancer, reached the same place at high certainty: selenium supplements do not prevent cancer overall, and the finding held even in people with low selenium to start with.
Two results went further than a plain null. In the Nutritional Prevention of Cancer trial, which gave 1,202 people 200 micrograms a day for almost eight years, type 2 diabetes developed in more of the people taking selenium than the people taking placebo, a hazard ratio of 1.55, about 55 percent higher, and the excess was concentrated in those who already had high selenium at the start, where the risk more than doubled. A later analysis of SELECT looked at the men by their baseline selenium status and found that selenium supplements did nothing for men who started low, but among men who already had high selenium they raised the risk of aggressive, high-grade prostate cancer by 91 percent.
The evidence sorts into three statements:
- Selenium correcting a real deficiency, and preventing the diseases that deficiency causes: settled.
- Extra selenium preventing cancer or heart disease in well-nourished people: the large trials say no.
- Extra selenium in people who are already replete: a raised risk of type 2 diabetes, and of aggressive prostate cancer in men who were already high.
The dividing line is selenium status, not dose alone. The same 200 micrograms a day that protects a deficient population raises the risk of diabetes and aggressive prostate cancer in people who already had plenty, which is why knowing where you stand matters more for selenium than for most nutrients.
How It Works
Selenium works entirely through the selenoproteins it is built into, and three families of them carry most of the effect. The glutathione peroxidases use selenium to break down hydrogen peroxide and lipid peroxides, the reactive byproducts of normal metabolism, before they damage cell membranes. The thioredoxin reductases keep the cell's other antioxidant systems recharged. The iodothyronine deiodinases convert the thyroid's storage hormone, T4, into the active form, T3, which is the direct link between selenium and the thyroid.
This machinery explains the shape of every finding on this page. When selenium is scarce, the selenoproteins cannot be made, antioxidant defense and thyroid conversion falter, and tissue is left open to damage. When selenium is adequate, the proteins are full and working. When selenium is far too high, it stops being a controlled ingredient and starts disrupting the very proteins, hair and nail keratin among them, that depend on ordinary sulfur chemistry.
Anatomy of the Practice
1When you are deficient
Below the level needed to fill the selenoproteins, glutathione peroxidase and the other selenium enzymes fall off, antioxidant defense weakens, and the deiodinases make less active thyroid hormone. In selenium-poor regions this is the backdrop to Keshan disease of the heart and Kashin-Beck disease of the joints, both of which appear where deficiency meets other triggers and both of which selenium prevents.
2When you are sufficient
The selenoproteins saturate. Glutathione peroxidase and selenoprotein P reach a plateau where more selenium no longer builds more of them. Past that point, extra selenium is not doing the antioxidant job people imagine, because there are no more enzymes for it to become, which is why topping up a replete body adds exposure without adding function.
3When you have far too much
At sustained high intake selenium is incorporated in place of sulfur into keratin and other proteins, disrupting hair and nail structure and, at extreme doses, nerve function. This is selenosis, and the gap between the amount that fills the enzymes and the amount that starts to harm is narrower than for almost any other nutrient.
Ways to Do It
For most people selenium is a nutrient to get from food, not a supplement to add. The higher-dose uses belong to specific situations, and because the risk rises with excess, they are worth checking rather than guessing.
A varied diet covers the roughly 55 micrograms a day an adult needs. Seafood, eggs, poultry, meat, dairy, and whole grains are all good sources, and in most of the world the food supply carries enough selenium that a deficiency is unlikely without an unusual diet or a low-selenium region. This is the route for almost everyone, and it is free.
Brazil nuts are the richest food source by a wide margin, and also the most variable: a single nut can carry anywhere from about 70 to several hundred micrograms depending on where it grew. One a day is a generous top-up. A daily handful can push you past the safe upper range, so they are best used as an occasional food rather than a routine supplement.
Because the benefit sits only in deficiency and the risk rises with excess, knowing where you stand matters more here than for most nutrients. A direct-to-consumer blood test for selenium, which you can order yourself, tells you whether you are actually low before you decide to add any. Most people eating a varied diet will find they do not need to.
For someone with a poor or very restricted diet, or from a low-selenium region, a small supplement around the daily requirement, well under the 400-microgram upper limit, covers the gap. Selenomethionine and selenium-enriched yeast are the common forms. This is about reaching adequacy, not exceeding it, since the trials found no reward for pushing a replete body higher.
The thyroid-antibody trials used about 200 micrograms a day. The drop in antibodies is real and repeated, but whether it changes symptoms, the course of the disease, or the need for thyroid medication has not been shown, so this belongs in a conversation alongside standard thyroid care rather than as something to self-start indefinitely.
Go Deeper
- Thyroid health: where selenium fits among the things that affect the thyroid, and how the antibody findings sit next to standard care for Hashimoto's.
- Type 2 diabetes: the condition the selenium trials raised a signal for, and what actually moves the risk.
- Zinc: another trace mineral where the benefit is about reaching adequacy rather than piling on more.
- Vitamin D: the same pattern of a real deficiency worth correcting and disappointing trials of routine high-dose supplements in people who were already sufficient.
- Multivitamin: where a modest selenium dose usually sits, and whether a broad daily supplement earns its place.
- Iodine: the other trace mineral the thyroid depends on, with the same lesson that adequacy helps and excess harms.
- Micronutrient inadequacy: the wider picture of which nutrients people fall short on, and why correcting a real deficiency beats piling on more.
The Chinese Medicine View
Chinese medicine has no concept of selenium, and it would be an invention to claim otherwise. What the tradition has is a long reading of the foods that happen to be rich in it, and a way of thinking about who a rich food suits. Those lenses can be laid over the modern mineral as interpretation, held apart from the trial data above.
The best selenium foods are ones the tradition knows well. Seafood and marine foods are read as nourishing to Kidney essence, the deep reserves associated with constitution and aging, and to Yin and Blood. Organ meats such as kidney and liver are used as tonics for the organs they share a name with, a food-as-medicine logic that predates any nutrient analysis. Eggs are seen as gently nourishing to Blood and Yin. None of this maps onto selenoproteins or glutathione peroxidase in any literal way, and the tradition never isolated a trace element from a food.
Where the two views touch is on the idea that the right amount depends on the person. In this tradition, rich, nourishing, and warming foods suit someone depleted, dry, or worn down, and burden someone already damp, sluggish, or phlegm-laden, for whom more richness feeds the accumulation. That instinct, that a food which is medicine for one constitution is a load for another, sits beside a modern reading in which selenium helps the deficient and harms the replete. The parallel is in the reasoning, not in the mechanism, and it is offered as a way of thinking rather than a claim about the mineral.
Cautions For This Practice
Extra restraintEverything to be aware of is here, in one place. This practice suits most healthy people; a few situations call for real care.
High-dose selenium causes selenosis: hair and nail loss, fatigue and GI upset
MacFarquhar et al. (2010) investigated the outbreak: a liquid dietary supplement contained roughly 200 times its labeled selenium. The median estimated dose was 41,749 micrograms a day. Beyond the frequent symptoms, mean initial serum selenium was 751 micrograms/L against a reference of 125 or below. Classic chronic selenosis also includes a garlic odor on the breath and, at extreme exposure, peripheral neuropathy. The tolerable upper intake level for adults is 400 micrograms a day, so the safe margin above the 55-microgram requirement is narrow.MacFarquhar JK et al., Acute Selenium Toxicity Associated With a Dietary Supplement
Selenium has a narrow safe range: harm at both low and high intake
Rayman (2012) describes selenium's pleiotropic effects through selenoproteins, from antioxidant and anti-inflammatory activity to thyroid hormone production, and the evidence that both low and high status carry risk. Low status is linked with poorer outcomes; benefits of higher status apply where intake was inadequate, while supplementing the already-replete can raise the risk of type 2 diabetes and, in some analyses, other harms. Selenoprotein synthesis plateaus once needs are met, which is the biochemical basis for why more is not better past adequacy.Rayman MP, Selenium and human health
The window between too little and too much is narrow
An adult needs about 55 micrograms a day, and the tolerable upper limit is 400 micrograms a day. That gap is small compared with most nutrients. Sustained intake above the upper limit causes selenosis: hair loss, brittle or discolored nails, a garlic odor on the breath, gastrointestinal upset, fatigue, irritability, and at high enough exposure nerve damage. Selenium is essential, and it is also toxic in excess, so more is not better once you are replete.
Supplementing when you are already replete
Most people eating a varied diet are not deficient in selenium, and the trials show that adding more then does not help and can hurt. Extra selenium raised the risk of type 2 diabetes by about half in one trial, with the excess concentrated in people who already had high selenium, and a later analysis found it raised aggressive prostate cancer by 91 percent in men who were already high. If you are considering a supplement, checking whether you are actually low, with a test you can order yourself, is the step that tells you whether there is anything to gain.
Brazil nuts vary enormously
Brazil nuts are the one food that can single-handedly overshoot. A single nut can hold anywhere from about 70 to several hundred micrograms of selenium depending on its soil, so a small daily handful can carry you past the upper limit without any supplement involved. Enjoy them as an occasional food, and do not treat a daily fistful as a health habit.
Contaminated or mislabeled products
In 2008 a liquid supplement that contained 200 times its labeled selenium poisoned 201 people across ten states, who took a median of about 42,000 micrograms a day and were left with hair loss, nail damage, fatigue, and joint pain, some lasting months. Buy from a reputable manufacturer, be wary of very high per-dose amounts, and stop and seek advice if you notice hair or nail changes or a persistent garlic taste after starting a product.
If you are pregnant, on thyroid medication, or considering a high dose
Reaching adequacy from food is safe and sensible in pregnancy, and a deficiency is worth correcting. A high-dose or long-term supplement, or use for a thyroid condition, is worth talking through with a clinician, since selenium status, your diet, and your other conditions all bear on whether adding it helps you or exposes you.
Start slow, be smart, read the research, and consult a professional if you have any concerns. This is here to inform your choice, not make it for you.
Common Questions
Should I take a selenium supplement?
For most people in the United States and other selenium-adequate places, no. A varied diet already supplies the roughly 55 micrograms a day you need, and the large trials found that adding more to a well-nourished body did not prevent cancer or heart disease and raised the risk of type 2 diabetes and, in already-high men, aggressive prostate cancer. Selenium supplements make sense for someone who is low, from a poor diet or a low-selenium region, and the way to know is a test you can order yourself rather than a guess.
Are Brazil nuts a good way to get selenium?
They are the richest food source, and that is exactly why to go easy. A single Brazil nut can carry a full day's selenium or several times it, depending on the soil it grew in, so one a day is a generous top-up and a daily handful can push you past the safe upper limit. Treat them as an occasional food, not a supplement you eat by the fistful.
Does selenium help the thyroid or Hashimoto's?
In Hashimoto's thyroiditis, selenium lowers the antibodies the immune system makes against the thyroid, and that finding has repeated across trials. What is not established is whether the antibody drop changes anything a person notices: symptoms, the long-run course of the disease, and the need for thyroid medication have not been shown to improve. It is reasonable to discuss with the clinician managing your thyroid, alongside standard care, rather than to start on your own and continue indefinitely.
Can you get too much selenium?
Yes, and the margin is narrow. The upper limit for adults is 400 micrograms a day, not far above the 55 you need. Sustained intake above that causes selenosis: hair loss, brittle or discolored nails, a garlic smell on the breath, stomach upset, fatigue, and at high enough doses nerve damage. Because the safe range is small, selenium is a nutrient where reaching enough matters and going higher does not help.
Explore Related
Other pages this one connects to, by the evidence they share, the outcomes they touch, and the ground they cover.
All 13 sources on this page independently checked and cross-referenced.
Thomas Dehli, Founder & Editor, Sacred Lotus
Sacred Lotus has published Chinese medicine reference material since 2001. Integrative pages are held to the same standard as the herb and formula library: cite the source, grade the claim at its real strength, and say where the research has not looked. This page is educational and it is not medical advice. Last reviewed and updated August 16, 2026.
Evidence strength
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