Selenium is a trace mineral you get from food, and your body needs only a very small amount of it. Men aged 19 to 64 need 75µg a day and women need 60µg, and most people get that from a varied diet.1 Selenium contributes to the normal function of the immune system, and to the normal thyroid function.2
Selenium at a glance
What is selenium?
Selenium is a mineral rather than a vitamin, which trips a lot of people up, and it's a trace mineral at that, meaning you need it in amounts small enough to be measured in micrograms.1
Your body can't make it, so all of it comes from what you eat. Selenium starts in the soil, gets taken up by plants, and works its way into the food chain from there, which means the amount in any given food depends on where it was grown or what the animal was fed.4
In food it mostly turns up as selenomethionine, an organic form, while supplements tend to use the inorganic forms selenate and selenite. Your body absorbs all of them well, at over 90%.4
Selenium benefits: what it does in your body
Almost everything selenium does, it does through proteins called selenoproteins. Humans make around 25 of them, and they’re the reason a mineral you need in microgram amounts turns up in so many different places at once.⁴
Start with your thyroid. The hormone it produces arrives in an inactive form, and the enzymes that convert it into the active one are selenoproteins, which is why they can’t work without selenium.⁴
Other selenoproteins work as antioxidant enzymes, clearing up the unstable molecules your body throws off as a by-product of everyday metabolism. When those build up faster than you can clear them, the result is what’s called oxidative stress.
Selenoproteins also turn up throughout immune tissue, as well as your hair and nails. Selenium also contributes to normal spermatogenesis, the process by which your body makes sperm.2
How much selenium do you need a day?
The NHS says you should be able to get all the selenium you need from a varied and balanced diet that includes meat, fish, or nuts.1 If you've ever compared a supplement label with those figures and found they don't match, there's a reason. Labels use the nutrient reference value, a single EU-derived figure used for labelling across the board, and for selenium that's 55µg. So a product showing "100% NRV" is giving you 55µg, which sits a little under the NHS figure for men and a little over the one for women.1
How much is too much?
The NHS says taking 350µg a day or less from supplements is unlikely to cause any harm.1 EFSA sets the upper limit at 255µg a day for adults, but that's the total from everything you eat and take, not supplements alone.3
The practical version: count selenium across everything you're taking, not just the one bottle.
Where to get selenium
The NHS names Brazil nuts, fish, meat, and eggs as the good sources,1 and you can add seeds, mushrooms, seafood, and poultry to that list.4
Brazil nuts deserve their own sentence, because a single nut can contain more than a day's worth of selenium, and the amount varies enormously from nut to nut depending on where it grew. That makes them an easy way to top up and an easy way to overdo it, which is why the usual advice is to treat them as an occasional food rather than a daily habit.
Selenium and your thyroid
Your thyroid holds more selenium per gram than any other tissue in your body, and there's a reason for that.
Thyroid hormone doesn't arrive ready to use. Your thyroid makes mostly T4, which is the inactive version, and enzymes called deiodinases convert it into T3, the active one. Those enzymes are selenoproteins, so they need selenium to work.4 Selenium contributes to the normal thyroid function, so if you have a thyroid condition, or you take levothyroxine, speak to your GP or pharmacist before starting any supplement.
Read more about the overlooked symptoms of hypothyroidism, or check how your thyroid is working with a thyroid blood test.
Selenium and fertility
Selenium contributes to normal spermatogenesis.2 Spermatogenesis is simply the process by which your body makes sperm, and selenoproteins are involved in it.4 If you're trying to conceive, speak to your GP. That conversation covers far more ground than any single nutrient does. Read more about testing for sexual function and fertility with a blood test.
Do you need a selenium supplement?
Probably not, if you eat meat, fish, or nuts. The NHS position is that a varied and balanced diet including those should cover it.1 Intake tends to be lower on vegan and restricted diets, simply because most of the richest sources are animal foods or Brazil nuts.
If you do take one, the forms you'll see are selenomethionine, sodium selenite, and selenium yeast. All are well absorbed, at over 90%, so the choice matters less than the total amount you're taking.4 Stay within the dose on the label, and count selenium across everything you take, because it turns up in multivitamins as well as in standalone products.
Numan supplements
Four of our supplements contain selenium, each at 55µg per daily dose. You can see the full range in the Numan supplement range.
Food supplements should not be used as a substitute for a varied, balanced diet and healthy lifestyle.
Common questions
Is selenium a vitamin or a mineral?
Selenium is a mineral, not a vitamin. It's a trace mineral, meaning you need only small amounts, measured in micrograms. Men aged 19 to 64 need 75µg a day and women need 60µg, according to the NHS.1
How much selenium should I have a day?
The NHS says men aged 19 to 64 need 75µg a day and women 60µg. Most people get that from a varied diet including fish, meat, eggs, or the occasional Brazil nut. If you take a supplement, the NHS says 350µg a day or less is unlikely to cause harm.1
What are the symptoms of having too much selenium?
Too much selenium causes selenosis, which in its mildest form means losing hair and nails.1 Higher intakes have also been linked with diarrhoea, tiredness, joint pain, and brittle or discoloured nails.4
What foods are highest in selenium?
Brazil nuts by a distance, then fish and seafood, meat, eggs, and seeds.1,4 A single Brazil nut can hold more than a day's worth, and the amount varies a lot between nuts.
Is selenium the same as selenium sulfide shampoo?
No, and it's a fair question given the shared name. Selenium sulfide is a medicine applied to the scalp, used for dandruff, seborrheic dermatitis, and a fungal skin condition called tinea versicolor.4 Intact skin doesn't absorb it.4 Dietary selenium is the mineral you get from food or supplements, and that's what this page is about.
Can I take selenium with levothyroxine?
Speak to your GP or pharmacist before taking any supplement alongside thyroid medicine.
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