Brittle nails that snap the second they get long. Hair coming out in the shower in clumps you didn't used to see. A thyroid test that came back "normal" while you still feel like you're wading through fog. If any of that sounds familiar, there's a good chance nobody has ever mentioned selenium to you, even though it's one of the few nutrients with genuinely strong, government-recognised evidence behind it.

Selenium is a trace mineral your body needs in tiny amounts, but those tiny amounts do a lot of heavy lifting: it helps your thyroid convert its main hormone into the active form your cells actually use, it helps protect your cells from oxidative stress, and it has an officially authorised UK and EU health claim for maintaining normal hair and nails.^[3][4] This article breaks down what selenium actually does, why it matters more than most people realise for hormonal health, how much you need, and why more is not automatically better, not even close. We'll also be upfront about the one thing most selenium content skips: the safe upper limit, and what happens if you go over it.

What does selenium actually do in the body?

Short answer: Selenium builds selenoproteins that regulate thyroid hormone, defend cells from oxidative damage, and support immune function and normal hair and nails.

Selenium doesn't work on its own. It gets built into a family of around 25 "selenoproteins," and those selenoproteins are what actually do the work.^[1] Two groups matter most for this conversation:

• Deiodinase enzymes, which convert thyroxine (T4), the hormone your thyroid produces in bulk, into triiodothyronine (T3), the active form your cells actually use to run your metabolism.^[1][5]
• Glutathione peroxidases, a group of antioxidant enzymes that neutralise the free radicals your cells produce as a normal by-product of, well, being alive.^[1]

Selenium also contributes to normal immune function and is a structural component of proteins involved in male fertility, which we'll get to.^[3][7] None of this is fringe or emerging science. Selenium's role in thyroid hormone metabolism has been documented since the early 1990s and is now considered one of the more settled areas of micronutrient research.^[1][5][6]

What makes selenium worth a proper conversation, rather than a one-line mention in a "hair vitamin" listicle, is how tightly it's woven into systems this audience already cares about. Thyroid function, oxidative stress, hair and nail structure, and immune resilience aren't four separate topics. They're four outputs of the same small set of selenoproteins, which is exactly why a deficiency in one area so often shows up alongside symptoms in another.

Does selenium affect thyroid function?

Short answer: Yes. Selenium is essential for the enzymes that convert inactive thyroid hormone (T4) into active T3, and it contributes to normal thyroid function.

Here's where it gets genuinely relevant to this audience specifically. Thyroid issues and PCOS overlap far more than most people are told, and the overlap almost never comes up in the same conversation. Women with PCOS have a higher rate of subclinical hypothyroidism than the general population, and both conditions share overlapping symptoms, fatigue, weight changes, irregular cycles, hair thinning, which makes it genuinely hard to know which system is misbehaving without proper testing.^[9]

Selenium's job here is specific and well defined: the deiodinase enzymes it's built into are what convert T4 into the active T3 hormone your cells actually use.^[1][5][6] Without adequate selenium, that conversion step doesn't run as efficiently, regardless of how much T4 your thyroid is producing. The research on this is actually pretty clear, selenium's role in thyroid hormone metabolism is one of the better-established relationships in endocrine nutrition, backed by decades of biochemistry going back to work on the deiodinase enzymes themselves.^[5][6]

What selenium is not, and what we won't pretend it is, is a treatment for a diagnosed thyroid condition like Hashimoto's or hypothyroidism. If you have a diagnosed thyroid condition, that's a conversation for your GP or endocrinologist, and any supplement should sit alongside your treatment plan, not instead of it. The authorised claim is that selenium "contributes to normal thyroid function," which describes its normal physiological role, not a therapeutic effect on a diagnosed disease.^[3][4] The British Thyroid Foundation makes the same distinction: selenium supports thyroid function as part of normal nutrition, but excess supplementation isn't recommended and shouldn't replace medical treatment.^[9]

Is selenium good for hair and nails?

Short answer: Selenium has an officially authorised UK and EU health claim for maintaining normal hair and nails, and deficiency is a documented cause of hair and nail changes.

This is one of only a handful of nutrients that has cleared the bar for an authorised "maintenance of normal hair" and "maintenance of normal nails" claim under UK and EU food law.^[3][4] That bar is high. Regulators reviewed the available evidence specifically on selenium and hair and nail structure before approving it, which is more scrutiny than most trending "hair vitamin" ingredients ever get.

Selenium for hair and selenium benefits for skin both trace back to the same underlying mechanism: selenoproteins support the antioxidant environment your hair follicles and nail matrix need to keep producing healthy keratin. When selenium status drops too low, brittle nails, hair thinning and hair loss are recognised signs, your body is trying to tell you something is off, even if that something is a mineral most people have never heard of, rather than anything more dramatic.^[1]

Here's the twist most "just take more selenium" content leaves out: the exact same symptoms, hair loss and nail changes, are also documented signs of taking too much selenium. A 2024 case report published in the Journal of Cosmetic Dermatology described a patient who developed hair loss and nail dystrophy after taking a high-dose selenium supplement, with symptoms resolving once the supplement was stopped.^[8] That's not a reason to avoid selenium. It's a reason to get your dose right rather than assuming more equals better, which is exactly the mindset this whole industry could use a lot more of.

What are the antioxidant and immune benefits of selenium?

Short answer: Selenium contributes to the protection of cells from oxidative stress and supports the normal function of the immune system.

Oxidative stress is one of those terms that gets thrown around a lot, so here's the plain version: it's the wear and tear your cells accumulate from normal metabolism, environmental exposure, and inflammation, and it's been studied fairly extensively in the context of PCOS and endometriosis specifically, where markers of oxidative stress tend to run higher than average.^[1] Selenium's antioxidant selenoproteins, the glutathione peroxidase family, are part of the system your body uses to keep that damage in check.^[1]

Selenium also has an authorised claim for contributing to the normal function of the immune system.^[3][4] This isn't a "boost your immunity" claim, and we're not going to pretend it is. It means selenium is one of the nutrients your immune system needs to function normally, the same way it needs protein or vitamin D. Deficiency, not supplementation on top of an already adequate diet, is what tends to compromise that function.

It's worth pausing on why oxidative stress gets so much airtime in PCOS and endometriosis research specifically. Chronic low-grade inflammation and elevated oxidative markers show up repeatedly in studies of both conditions, and while selenium is only one piece of the antioxidant picture, alongside vitamin C, vitamin E and zinc, it's one of the few with an actual regulatory role, since selenoproteins are enzymes, not just passive "mop-up" molecules.^[1] That distinction matters. An enzyme keeps working cycle after cycle. A one-off antioxidant molecule gets used up.

Does selenium play a role in fertility?

Short answer: Selenium contributes to normal spermatogenesis in men, and selenoproteins are structurally involved in healthy sperm formation.

If you're in the trying-to-conceive part of this audience, selenium's most well-documented fertility connection is actually on the male side, which is worth knowing if you're navigating fertility as a couple rather than researching only your own body. Selenium has an authorised EU claim for contributing to normal spermatogenesis, and one particular selenoprotein, phospholipid hydroperoxide glutathione peroxidase, is a structural component of mature sperm, meaning it's literally built into sperm cell architecture, not just floating around supporting it from a distance.^[1][3][7]

Research on selenium and female fertility specifically is much thinner and less conclusive, so we're not going to overstate it here. What we can say honestly is that adequate selenium status supports the broader antioxidant and thyroid systems that fertility depends on, and thyroid function in particular is closely tied to ovulatory cycles. If fertility is your main focus right now, selenium is one piece of a much bigger picture, not a standalone fix.

How much selenium do you need?

Short answer: UK guidance sets 75 micrograms a day for men and 60 micrograms a day for women, most people can get this from food.

UK Reference Nutrient Intakes put selenium needs at 75 micrograms a day for men and 60 micrograms a day for women aged 19 to 64.^[2] In the US, the Recommended Dietary Allowance is set slightly lower, at 55 micrograms a day for adults of any sex, rising to 60 during pregnancy and 70 during breastfeeding.^[1] The gap between those two figures is a good reminder that "the right amount" isn't one magic number, it's a range, and most people sitting anywhere within it are doing fine.

If you eat fish, meat, eggs or nuts regularly, there's a decent chance you're already hitting your target without thinking about it.^[2] Selenium deficiency is genuinely uncommon in the UK, largely because it takes so little to meet the requirement, but it isn't nonexistent, and it shows up more often in people with limited diets, certain digestive conditions that affect absorption, or those living in areas with selenium-poor soil (which affects how much ends up in locally grown crops, not something most people think to check).

Can you take too much selenium?

Short answer: Yes. The safe upper limit is 400 micrograms a day from all sources, and UK guidance advises no more than 350 micrograms a day from supplements alone.

This is the part of the selenium conversation that gets skipped far too often, and it's exactly the kind of thing hormonal literacy isn't complicated, it's just rarely taught, should cover. Selenium has one of the narrower safety margins of any commonly supplemented mineral. The tolerable upper intake level for adults is 400 micrograms a day from food and supplements combined, and the UK's Department of Health and Social Care advises that taking 350 micrograms a day or less from supplements specifically is unlikely to cause harm.^[1][2]

Go meaningfully over that and you're looking at selenosis, with early warning signs including a garlic-like odour on the breath, a metallic taste in the mouth, nausea, fatigue and irritability.^[1] And here's the part that circles right back to the hair and nails conversation: the same brittle nails and hair loss that can signal a deficiency can also signal you've gone too far the other way.^[1][8] That 2024 case report we mentioned earlier is a real, published example of exactly this happening from supplement use.^[8]

The practical takeaway isn't to fear selenium, it's to be sensible about it. Check the dose on anything you're taking, avoid stacking multiple selenium-containing supplements at once without realising it, and don't assume that because a nutrient is "natural" or trace-level, the ceiling doesn't apply to you. If you're eating a normal varied diet that already includes fish, eggs, meat or nuts, factor that in before adding a high-dose selenium supplement on top.

What foods are high in selenium?

Short answer: Brazil nuts are by far the richest source, followed by fish, meat, eggs and other seafood.

If you only remember one food source, make it this one: Brazil nuts. A single ounce, roughly six to eight nuts, can contain around 544 micrograms of selenium, which is well over the daily upper limit in one small handful.^[1] That's genuinely useful to know both ways, it means a few Brazil nuts a week can comfortably cover your selenium needs, and it means eating a large handful daily, every day, isn't the flex some wellness corners of the internet make it sound like.

Food	Approx. selenium
Brazil nuts (1 oz, roughly 6 to 8 nuts)	544 mcg
Yellowfin tuna (3 oz, cooked)	92 mcg
Sardines, canned (3 oz)	45 mcg
Shrimp (3 oz)	42 mcg
Pork chop (3 oz, cooked)	37 mcg

Figures per NIH Office of Dietary Supplements.^[1] Selenium content in plant foods varies with soil levels, so this table focuses on more consistent animal and nut sources.

Eggs, chicken, wholemeal bread and mushrooms all contribute smaller but still useful amounts.^[2] If you're vegetarian or vegan, selenium is one of the easier micronutrients to get right, a few Brazil nuts and some wholegrains generally does it, but it's still worth keeping an eye on if your diet is limited for any reason.

Does the form of selenium in a supplement matter?

Short answer: Yes. Organic forms like L-selenomethionine are generally better absorbed and retained than inorganic forms like sodium selenite.

Not all selenium supplements are built the same way. Selenium shows up in two broad categories: inorganic forms, like sodium selenite or sodium selenate, and organic forms, like selenomethionine, which is the form selenium naturally takes when it's incorporated into plant and animal proteins.^[1] Organic forms tend to be absorbed more efficiently and retained in the body for longer, which is one reason formulators tend to reach for them over the cheaper inorganic alternatives.

This is a genuinely useful thing to check on a supplement label rather than skip past. Two products can both say "selenium 100mcg" and behave quite differently in your body depending on which form is behind that number. It's a small detail, but reading past the front-of-pack claim to the actual ingredient list is a habit worth building for every supplement you take, not just this one.

Who's most likely to need to pay attention to their selenium intake?

Short answer: People with restricted diets, digestive conditions affecting absorption, or an undiagnosed thyroid issue alongside PCOS or another hormonal condition.

If you've been diagnosed with PCOS, endometriosis, PMDD or you're in perimenopause, thyroid function is probably not the first thing on your radar, and that's exactly the problem. Thyroid symptoms and hormonal condition symptoms overlap so heavily that it's genuinely common for one to be missed while the other gets all the attention.^[9] Fatigue gets blamed on PCOS. Hair thinning gets blamed on stress. Brain fog gets blamed on "just getting older." Sometimes that's exactly right. Sometimes there's a thyroid piece nobody's checked.

This isn't a suggestion to self-diagnose a thyroid problem off the back of a blog post, it's a nudge to ask your GP for a full thyroid panel if you've got PCOS, endometriosis or perimenopause symptoms and something still doesn't add up. Selenium won't fix an undiagnosed thyroid condition, and it was never designed to. What it does is support the normal physiological processes, thyroid hormone conversion, antioxidant defence, hair and nail maintenance, that a lot of women in this exact situation are quietly running short on without ever being told to check.^[1][3][4]

Frequently Asked Questions

Is selenium good for thyroid health?
Yes. Selenium is essential for the enzymes that convert inactive thyroid hormone (T4) into the active form (T3), and it contributes to normal thyroid function as part of a balanced diet.^[1][3][5]

How much selenium should I take a day?
UK guidance sets 75 micrograms a day for men and 60 micrograms a day for women. Most people get this from food, and supplement doses are usually formulated well below the 350 microgram supplement safety guidance.^[1][2]

Can selenium help with hair loss?
Selenium has an authorised claim for maintaining normal hair, and deficiency is a documented cause of hair changes. That said, hair loss has many possible causes, so it's worth checking with your GP if it's significant or sudden, since correcting an actual deficiency is different from supplementing on top of an already adequate intake.^[1][3][4]

What are the signs of selenium deficiency?
Documented signs include hair loss, brittle nails, fatigue and, in more severe or prolonged deficiency, muscle weakness. Deficiency is uncommon in the UK for people eating a varied diet.^[1][2]

What are the signs of too much selenium?
Early signs of excess intake include a garlic-like odour on the breath, a metallic taste in the mouth, nausea, fatigue, irritability, and, notably, the same hair loss and nail changes seen in deficiency.^[1][8]

Can I get enough selenium from food alone?
For most people eating fish, meat, eggs or nuts regularly, yes. A small handful of Brazil nuts a week can meet most adults' selenium needs on its own.^[1][2]

Is selenium linked to PCOS or endometriosis?
Selenium isn't a treatment for either condition, but its role in antioxidant defence is relevant given the oxidative stress patterns seen in both, and thyroid issues (which selenium supports) are more common alongside PCOS than most people realise.^[1][9]

Should I take selenium if I'm pregnant or breastfeeding?
Requirements shift slightly during pregnancy and breastfeeding. Always check with your GP or midwife before starting any new supplement while pregnant, breastfeeding, taking medication or managing a diagnosed health condition.^[1]

References

1. National Institutes of Health, Office of Dietary Supplements. Selenium: Fact Sheet for Health Professionals. ods.od.nih.gov
2. NHS. Vitamins and minerals: Others (Selenium). nhs.uk
3. EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Scientific Opinion on the substantiation of health claims related to selenium. EFSA Journal 2010;8(10):1727. efsa.europa.eu
4. Commission Regulation (EU) No 432/2012 establishing a list of permitted health claims made on foods. legislation.gov.uk
5. Köhrle J. Selenium and the control of thyroid hormone metabolism. Thyroid. 2005;15(8):841-853. pubmed.ncbi.nlm.nih.gov
6. Köhrle J. Selenium, Iodine and Iron, Essential Trace Elements for Thyroid Hormone Synthesis and Metabolism. Int J Mol Sci. 2023;24(4):3393. mdpi.com
7. Boitani C, Puglisi R. Selenium, a key element in spermatogenesis and male fertility. Adv Exp Med Biol. 2008;636:65-73. pubmed.ncbi.nlm.nih.gov
8. Civas M, et al. Selenium in the supplement as the probable cause of hair loss and nail dystrophy. J Cosmet Dermatol. 2024. onlinelibrary.wiley.com
9. British Thyroid Foundation. Selenium supplements and thyroid. btf-thyroid.org

Further Reading

• What's PCOS? A guide to polycystic ovary syndrome
• PCOS and thyroid issues: what you need to know about hypothyroidism
• Hair, Skin & Nails supplement

This article is for general information only and does not replace advice from your GP or a qualified healthcare professional. Food supplements should not be used as a substitute for a varied and balanced diet. If you're pregnant, breastfeeding, taking medication or managing a diagnosed health condition, check with a healthcare professional before starting a new supplement.

References