Supplement: Selen

What It Is

Selenium is a trace element, a mineral the 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. Brazil nuts hold far more than any other food. How much a plant food carries depends on 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 people living on local food fall short. Most of the United States, Canada, and other countries eating a varied diet get plenty.

The body does not use the mineral on its own. It builds selenium into a small family of about 25 proteins, the selenoproteins. They do three jobs: 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. That is why low intake and high intake produce such different effects.

What It Does

Selenium's proven worth is in deficiency. Where soil selenium runs low, as in parts of China and other inland regions, ordinary diets fall below the 55-microgram target. Two diseases then appear. Keshan disease is a cardiomyopathy, a disease of the heart muscle. Kashin-Beck disease damages the joints. Selenium prevents both, and correcting the shortage across China's low-selenium regions stands among nutrition's clearest public-health wins.

The picture changes where selenium is already adequate, as it is for most people eating a varied diet. For someone already replete, most people on a varied diet, extra selenium is a different question, and the large randomized trials were built to answer it.

Each card below grades one of these findings by its own evidence, from the settled deficiency diseases to the 200-microgram supplement trials.

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 deiodinasesStrong · mixed

In plain terms

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.

In detail

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, not a finding in one group of people, so it applies across sexes and ages.

How to use it

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)Strong · no effect

In plain terms

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.

In detail

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, not 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.

How to use it

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)Strong · no effect

In plain terms

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.

In detail

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.

How to use it

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%Moderate · risk

In plain terms

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.

In detail

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.

How to use it

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, not 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 regionsModerate

In plain terms

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.

In detail

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.

How to use it

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

Li SJ 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 thyroiditisModerate

In plain terms

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.

In detail

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.

How to use it

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 needModerate · mixed

In plain terms

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 unconfirmed.

In detail

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, not 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.

How to use it

Read alongside the antibody finding, not 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, not 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)Moderate · risk

In plain terms

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.

In detail

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.

How to use it

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 outcomesEmerging

In plain terms

Your immune system needs selenium to work normally, and people who are low in it tend to fare worse. Filling a shortfall helps; there is little good evidence that piling extra selenium onto an already-adequate body boosts immunity.

In detail

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.

How to use it

This supports reaching selenium adequacy as part of normal nutrition, not taking high-dose selenium as an immune booster. Where status is 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. The logic was simple: a mineral that mops up oxidative damage ought to protect against disease. Large randomized trials in well-nourished people did not bear it out.

The largest was SELECT. It gave 35,533 men 200 micrograms of selenium a day (alone, with vitamin E, or against placebo) for years. Selenium did not reduce prostate cancer, the outcome it was designed to test, and it did not reduce other cancers either. A Cochrane review pooling the randomized trials agreed, at high certainty: selenium does not prevent cancer overall. The finding held even in people who began with low selenium.

Two results went further than a plain null. In the Nutritional Prevention of Cancer trial, 1,202 people took 200 micrograms a day for almost eight years.

More of them developed type 2 diabetes on selenium than on placebo, a hazard ratio of 1.55, about 55 percent higher. The excess fell almost entirely on those who already had high selenium at the start, where the risk more than doubled.

A later SELECT analysis split men by starting selenium: those who began low saw no change; those already high had 91 percent more high-grade prostate cancer.

The evidence sorts into three statements:

• Correcting a real deficiency, and preventing the diseases it causes: settled.
• Preventing cancer or heart disease in the well-nourished: the trials say no.
• Adding more to an already-replete body: raised type 2 diabetes, and, in men whose selenium was already high, more aggressive prostate cancer.

How It Works

Three families of selenoproteins carry most of selenium's effect. The glutathione peroxidases 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 its active form, T3. This is how selenium acts on the thyroid.

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 the two deficiency diseases, which appear where low selenium meets other triggers, such as a virus for Keshan disease.

2When you are sufficient

The selenoproteins saturate. Glutathione peroxidase and selenoprotein P reach a plateau near the 55-microgram requirement, where more selenium no longer builds more of them. Past that point there are no more enzymes for extra selenium to become, so more intake adds exposure without adding function.

3When you have far too much

At sustained intake many times the 55-microgram requirement, selenium is taken up in place of sulfur into keratin and other structural proteins, and at extreme doses it impairs nerve function. This is selenosis.

Ways to Do It

For most people a varied diet already covers the day's selenium. The higher-dose uses belong to specific situations, and because the risk rises with excess, check your level first.

1

Get it from everyday foodFreeEasy

A varied diet covers the roughly 55 micrograms a day an adult needs. In most of the world the food supply carries enough selenium that deficiency is unlikely without an unusual diet or a low-selenium region. This is the route for almost everyone.

2

One Brazil nut, treated as a food$Easy

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 carry you past the 400-microgram upper limit.

3

Test before you supplement$$Easy

Because the benefit sits only in deficiency and the risk rises with excess, knowing where you stand matters more for selenium than for most nutrients. A blood test you can order yourself, without a clinician, tells you whether you are actually deficient before you add any. A varied diet supplies enough for most people, so the test often comes back replete.

4

A modest supplement, if you are low$Easy

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. The aim is the roughly 55-microgram requirement, since the trials found no reward for pushing a replete body higher.

5

For Hashimoto's, alongside standard care$Easy

The thyroid-antibody trials used about 200 micrograms a day. Whether that changes symptoms or medication need has not been shown; discuss it with your thyroid clinician (see Go Deeper).

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 deficiency is the thing to correct.
• 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 actually fall short on.

The Chinese Medicine View

Chinese medicine has no concept of selenium. What the tradition has is centuries of experience with the foods that happen to be rich in it, and a way of thinking about who a rich food suits.

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.

The tradition's instinct is that the right amount depends on the person. Rich, warming, nourishing foods suit someone depleted, dry, or worn down. For someone already damp, sluggish, or phlegm-laden, the same richness feeds the accumulation. A food that is medicine for one constitution is a burden to another.

Cautions For This Practice

Extra restraint

Everything 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; the tolerable upper limit is 400. That gap is small compared with most nutrients. Sustained intake above the limit causes selenosis. The signs: hair loss, brittle or discolored nails, a garlic odor on the breath, stomach upset, fatigue, irritability, and at high exposure nerve damage. Selenium is essential, and toxic in excess.

Before adding a supplement, test first

Most people on a varied diet are not deficient. Before adding a supplement, a selenium blood test shows whether you fall below the requirement, so you are correcting a real shortfall and not a guessed one.

Contaminated or mislabeled products

In 2008 a liquid supplement contained 200 times its labeled selenium and poisoned 201 people across ten states. They had taken a median of about 42,000 micrograms a day. Many were left with hair loss, nail damage, fatigue, and joint pain, some lasting months. Buy from a reputable manufacturer, and be wary of very high per-dose amounts. 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

In pregnancy, reaching adequacy from food is safe and sensible, and a real deficiency is worth correcting. A high-dose or long-term supplement, or use for a thyroid condition, is worth talking through with a clinician. Selenium status, your diet, and your other conditions all bear on whether it helps 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?

Most people in the United States and other selenium-adequate places already get the roughly 55 micrograms a day they need from a varied diet. In the large trials, adding more to a well-nourished body brought no benefit and some added risk. A supplement makes sense for someone who is low (a poor diet, or a low-selenium region) and a selenium blood test settles which group you are in.

Are Brazil nuts a good way to get selenium?

Yes, in small amounts. The catch is that you cannot know the dose: two nuts from different regions can differ several-fold in selenium, so a Brazil nut is a food, not a measured supplement. One a day is plenty; a daily handful is where people overshoot.

Does selenium help the thyroid or Hashimoto's?

In Hashimoto's thyroiditis, selenium at the doses used in the trials lowers the thyroid antibodies, a finding repeated across several trials. The trials did not measure whether patients felt better, needed less medication, or had a milder disease course. So it is a reasonable thing to discuss with your thyroid clinician, alongside standard care.

Can you get too much selenium?

Yes. The usual cause is a high-dose supplement taken for months, or a mislabeled product; food rarely does it. The earliest signs people notice are hair shedding, brittle nails, and a garlic smell on the breath. If those appear after you start a supplement, stop and get your selenium level checked.