Selenium benefits: what the science actually shows

Selenium is an essential mineral that your body cannot make and uses mainly to build selenoproteins — proteins with antioxidant functions and a central role in thyroid hormone metabolism. That is why selenium deficiency is linked to thyroid dysfunction and greater oxidative stress ¹.

But there is a distinction that changes everything: selenium is essential for normal body function and, at the same time, has one of the narrowest margins between what is needed and what is toxic ¹. The most consistent evidence on supplementation does not come from healthy people — it comes from people with autoimmune thyroid disease, such as Hashimoto's thyroiditis. For people with adequate levels and a varied diet, there is no proof that taking more brings any additional benefit.

What is selenium and why does your body need it?

Selenium is a trace element: your body needs it in very small amounts but cannot produce it. You get it from food — nuts (Brazil nuts are the classic example), fish, meat and eggs are the most relevant sources. In the body, it is incorporated into around 25 selenoproteins, which take part in antioxidant defence, thyroid hormone metabolism, reproductive function and neuroprotective effects ¹.

Two of these enzyme families deserve attention. Glutathione peroxidase is an antioxidant enzyme that helps neutralise unstable molecules that damage cells. Deiodinases are the enzymes that convert thyroid hormones into their active form — and they are selenoproteins ². Without enough selenium, these systems work less well. But — and this is the point most often lost on packaging — more selenium than needed does not make the system work better: selenium is essential in trace amounts but harmful in excess, and at very high doses it is toxic ¹.

What selenium does: what the science shows

The most solid evidence on selenium concerns the thyroid and antioxidant protection. In thyroid tissue, selenium contributes to the antioxidant system and is a crucial component of deiodinases; low levels reduce glutathione peroxidase activity, which increases oxidative stress and damage to thyroid tissue ². In regions with mild selenium deficiency, serum levels correlate inversely with autoimmune thyroiditis ².

In antioxidant defence, selenium is an integral part of glutathione peroxidase. This is a real benefit, but it is a maintenance benefit: your body needs selenium for this system to work, and if you are deficient, the system is compromised. There is no evidence that extra doses beyond what is adequate measurably increase protection.

On the immune system, the reading has to be careful. Selenium is needed for the immune response and selenoproteins take part in regulating immune cell functions ³. However, a review that brought together nine randomised human trials concluded that the effects on immunity are limited and that the selenium doses with a measurable influence on immune function remain uncertain ⁴. In other words: correcting a deficiency helps; in people with normal levels, the effect is not clear.

Selenium and Hashimoto's thyroiditis: the strongest evidence

This is where the research is most consistent — and it is important to say from the outset that this is a specific population, with a diagnosed disease, not healthy people.

A meta-analysis of 21 randomised trials, with 1,610 participants with Hashimoto's thyroiditis, showed that selenium supplementation significantly reduced anti-thyroid peroxidase antibodies (TPOAb) at 3 months and at 6 months, and anti-thyroglobulin antibodies (TgAb) at 3 months. A reduction in TSH at 6 months was also observed, with improvements in well-being or mood. In this analysis, in patients with Hashimoto's thyroiditis, selenomethionine was more effective than sodium selenite and selenium yeast ⁵.

Another network meta-analysis, which compared different supplements in patients with Hashimoto's, also found significant reductions in TPOAb and TgAb with selenium compared with placebo ⁶. And a meta-analysis of randomised trials assessed thyroid function, antibodies, ultrasound findings and adverse events in this population, but the results are not detailed here ⁷. One individual randomised trial, with 43 patients receiving selenium yeast for 6 months and 47 untreated, found significant decreases in TPOAb, TgAb and TSH, accompanied by increases in selenium, GPx3 and selenoprotein P1 ⁸.

What this means in practice: in people with Hashimoto's, selenium supplementation appears to reduce antibodies and, in some cases, improve TSH and well-being. It does not mean that selenium treats the disease, nor that these results carry over to people without thyroid disease. If you have Hashimoto's, this is a decision to discuss with your doctor — not a self-prescription.

Selenium, skin and hair: where the evidence is weakest

It is in the areas that most attract people looking for beauty supplements that the evidence is weakest. There are associations between low selenium levels and problems such as hair loss or brittle nails, but association is not causation: people with selenium deficiency usually have deficiencies in other nutrients too, and it is hard to isolate the effect of a single mineral.

There is one randomised trial that assessed an oral supplement with hydrolysed marine collagen, amino acids, iron and selenium in 83 people with hair loss (androgenetic alopecia or telogen effluvium), over 12 weeks, in combination with pharmacological treatments ⁹. It is a study of a multi-ingredient formula in people with a diagnosed condition — it does not allow the conclusion that selenium, on its own, improves hair in healthy people. Saying this openly is more useful than promising results that science has not yet confirmed.

If your goal is caring for your skin, collagen has its own body of evidence, with results on hydration and elasticity in meta-analyses of randomised trials ¹⁰. You can explore that route in collagen and in the beauty collection.

Who really benefits from a selenium supplement?

If you eat a varied diet and live in a region where the soil is not poor in selenium, the likelihood of deficiency is low. The groups in which deficiency is most recognised include people with intestinal diseases that affect absorption, dialysis patients, people with HIV and populations living in areas with very selenium-poor soil ¹¹. In Europe, some countries have poor soils, but the diet usually compensates.

For most healthy adults, selenium is not a supplement whose absence you would notice. Supplementation makes sense if deficiency is suspected — for example, if a blood test shows low levels — or if the diet is restrictive enough to exclude animal sources and nuts. If you do not fit these cases, selenium does not work like a switch: there is no visible benefit in taking more than your body needs.

How much selenium per day and what is the safe limit?

In adults, the recommended daily amount in Europe is 55 micrograms for men and women, rising to around 60–70 micrograms during pregnancy and lactation ¹². The upper intake level considered safe is 300 micrograms per day for adults. Above that, the risk of selenosis — poisoning that causes hair loss, brittle nails, nausea and a garlicky breath odour — increases. These references are detailed in selenium: recommended daily dose and safe limits.

A practical note: a single Brazil nut can contain more than 70 micrograms of selenium, and some far exceed that value. If you eat nuts, fish and meat regularly, you are probably already within the recommended values — and that is why adding a supplement to a rich diet can bring you close to the upper limit without you realising it.

Selenium in gummies: what changes and what does not

Gummies are a convenient way to take supplements, especially for people who find tablets hard to swallow. With selenium, the form does not significantly alter absorption: the mineral is absorbed in the small intestine and the gummy matrix does not interfere with that. What changes is the dose per serving and the convenience. You can compare formats in selenium gummies or tablets.

If you choose gummies, check the label to know exactly how much selenium you are taking. Collagen Gummies provide 18 micrograms of selenium per serving of 2 gummies (33% of the reference value), plus 1,000 mg of hydrolysed collagen, 24 mg of vitamin C and 15 micrograms of biotin. Immunity & Defence Gummies with Echinacea provide 55 micrograms of selenium per serving of 2 gummies (100% of the reference value), together with 80 mg of vitamin C, 5 mg of zinc, 5 micrograms of vitamin D3, 100 mg of echinacea extract and Lactobacillus rhamnosus. In either case, the stated dose is 2 gummies per day and should not be exceeded.

A note on dose: gummies are easy to eat too many of, and selenium is one of the minerals where excess is a real risk. Respect the dose stated on the pack and do not combine several selenium sources without adding up the doses. If you are unsure about the total you are taking in, see is selenium bad for you?.

Where popular claims outrun the evidence

Selenium is often sold as a powerful antioxidant that combats ageing, protects against cancer and improves skin. The antioxidant part is true — selenium is incorporated into selenoproteins that support antioxidant defence systems ¹ — but the translation into concrete beauty or longevity benefits is not proven. A review on selenium and ageing concluded that supplementation does not necessarily help prevent ageing and related diseases, and that low selenium levels appear to be what is beneficial ¹³.

As for cancer, the relationship is complex and does not boil down to more is better: the effect of increased selenium intake on cancer risk remains unclear, and high serum selenium levels are associated with an increased risk of type 2 diabetes, in a non-linear relationship ¹⁴. You also hear that selenium strengthens the immune system. There is a basis for this — selenium is needed for the immune response ³ — but, as we have seen, human trials show limited effects and the doses with a measurable impact remain uncertain ⁴.

For vitamins and minerals, the health claims authorised in the European Union are restricted and specific; for selenium, the permitted claims refer to the normal functioning of the immune system, the protection of cells from oxidative stress and the normal functioning of the thyroid ¹⁵. None of this amounts to promising aesthetic results or preventing disease.

What to do with this information

If you are considering a selenium supplement, the first step is not to buy — it is to assess your diet. If you eat fish, meat, eggs and nuts regularly, you probably already have enough selenium. If you follow a very restrictive diet, have a condition that affects nutrient absorption, or a test showed low levels, it makes sense to talk to a doctor or dietitian about the right dose for you.

For most people, selenium is not a supplement whose absence you would notice. The most solid benefits — thyroid and antioxidant — are maintenance, not transformation, and the strongest data come from people with autoimmune thyroid disease, not healthy adults. The beauty benefits are plausible but not yet proven.

If you decide to try it, choose a product you trust, check the dose on the label and do not combine several selenium sources without adding up the doses. Pregnancy, breastfeeding, children, ongoing medication or a diagnosed disease are situations in which you should talk to a health professional before taking any supplement. And remember: selenium is not a magic potion. It is an essential nutrient your body needs, but one it already gets — in most cases — from what you eat.

If you have a condition that affects nutrient absorption, are on dialysis or have HIV, or if a test showed low selenium levels, talk to a doctor or dietitian before supplementing — the right dose depends on your case. And do not combine several selenium sources without adding up the doses: excess is a real risk.

Frequently asked questions

What does selenium do in the body?

Selenium is incorporated into around 25 selenoproteins, which take part in antioxidant defence, thyroid hormone metabolism, reproductive function and neuroprotective effects. It is essential for the normal functioning of the thyroid and for the protection of cells from oxidative stress. The body does not produce it, so it depends on your diet.

Does selenium help with Hashimoto's thyroiditis?

This is the area with the most consistent evidence. Meta-analyses of randomised trials in patients with Hashimoto's show reductions in TPOAb and TgAb antibodies and, in some cases, in TSH, with improvements in well-being. These results apply to people with a diagnosed disease, not healthy adults, and the decision should be discussed with your doctor.

How much selenium should I take per day?

In adults, the recommended daily amount in Europe is 55 micrograms, rising to around 60–70 micrograms during pregnancy and lactation. The upper intake level considered safe is 300 micrograms per day. Most people reach the recommended values with a varied diet, without a supplement.

Does selenium improve skin and hair?

There are no robust clinical trials showing that selenium supplementation improves skin or hair in healthy people. There are associations between low selenium levels and hair loss or brittle nails, but association is not causation. There is one trial of a formula that included selenium, collagen and other nutrients in people with hair loss, but it does not allow the effect of selenium to be isolated.

Can selenium be bad for you?

Yes, in excess. Selenium has one of the narrowest margins between what is needed and what is toxic. Above 300 micrograms per day in adults, the risk of selenosis increases, with hair loss, brittle nails, nausea and a garlicky breath odour. Do not combine several selenium sources without adding up the doses.

Who should talk to a health professional before taking selenium?

Pregnant and breastfeeding women, children, people with a diagnosed disease (including thyroid disease) and anyone taking medication should seek advice from a health professional before supplementing. People with intestinal diseases that affect absorption, those on dialysis or those on a very restrictive diet should also seek individual guidance.