Selenium Deficiency vs Too Much Selenium: Symptoms, Testing and Safety Limits

You’re looking at a blood test print-out, or a supplement label that says “200mcg selenium”, and the questions start:

Am I low? Could I be getting too much selenium? And how do I stay in the safe middle between selenium deficiency and overload?

You’re not the only one wondering this. Selenium has a fairly narrow “sweet spot”:

• too little, and important enzymes don’t work properly
• too much, and over time you may run into problems like brittle hair and nails, and other side effects.[1][2]

This article talks you through:

• what selenium deficiency looks like
• what happens with too much selenium (selenosis)
• how doctors test selenium status
• UK RNIs, NHS advice and EFSA upper limits
• where a 200mcg supplement can fit in when used sensibly.

Table of contents

1. Walking the tightrope: why selenium is so awkwardly Goldilocks
2. What selenium actually does (in normal, boring, everyday life)
3. How common is selenium deficiency in the UK?
4. Symptoms: selenium deficiency vs too much selenium
   4.1 Early, subtle signs of low selenium
   4.2 Severe deficiency syndromes (thankfully rare in the UK)
   4.3 Mild vs severe selenosis from too much selenium
5. How do doctors test selenium status?
6. Safety limits: UK RNIs, NHS guidance and EFSA upper limits
   6.1 UK RNIs and NHS advice
   6.2 EFSA’s updated 255 µgday upper limit
   6.3 US WHO numbers – why they’re different
7. Where does a 200 µg selenium supplement fit in safely?
   7.1 Typical UK intakes and “total daily selenium”
   7.2 Who should be extra cautious with 200 µg?
8. Practical tips: getting enough without getting too much selenium
9. FAQ: long-tail questions about selenium, deficiency and safety
10. Bottom line

Walking the tightrope: why selenium is so awkwardly Goldilocks

Selenium is built into around 25 different human proteins (“selenoproteins”). These are involved in antioxidant defence, thyroid hormone activity, immune function and reproduction.[1][3] Selenium is essential – but there is a U-shaped curve:

• at low levels, there is a higher risk of problems such as weaker immunity and thyroid issues
• at high levels, there is a higher risk of side effects and, in some studies, worse health outcomes.[1][2]

Key takeaway
Think of selenium as a narrow window. Not enough can be a problem, but regularly taking high doses “just in case” is not a good idea either. There is a safe range – but it is not unlimited.

What selenium actually does (in normal, boring, everyday life)

Most of selenium’s work happens through selenoproteins, such as:

• glutathione peroxidases and thioredoxin reductases – enzymes that help protect cells from oxidative damage
• deiodinases – enzymes that activate and deactivate thyroid hormones
• other proteins involved in normal immune function and reproduction.[1][3]

According to EFSA and the NHS, selenium:[3][5]

• contributes to normal thyroid function
• contributes to the normal function of the immune system
• contributes to the protection of cells from oxidative stress
• contributes to normal spermatogenesis (normal sperm formation)

Because of this, both selenium deficiency and excess tend to affect these systems first. People may notice:

• low energy and tiredness
• more frequent infections
• thyroid changes
• fertility problems
• changes in hair and nails

In more extreme cases, there can be effects on the nervous system and the heart.[1][2][8]

How common is selenium deficiency in the UK?

The UK used to get more selenium when a lot of wheat came from North America, where soil levels are higher. As we switched to more European grain, average selenium intakes dropped.[6][7]

From UK and European data:[6][7]

• average adult intakes often sit below the UK RNI (75mcg for men, 60mcg for women)
• a 2013 UK expert report (SACN) concluded that many adults, especially women, have intakes close to or below the Lower Reference Nutrient Intake (LRNI)
• true clinical deficiency with obvious disease is rare in the UK.[7]

So in the UK, the situation looks like this:

At a glance

• Mild, long-term low selenium (showing up in blood tests rather than dramatic disease) can happen – especially in diets low in meat and fish.
• Classic severe deficiency diseases (Keshan disease, Kashin–Beck disease) are not a UK issue. They occur in areas with very low soil selenium, such as some regions of China.[8][9]

For most people here, the question is less:

“Will I get Keshan disease?”

and more:

“Could low-ish selenium be nudging my thyroid, immunity or energy in the wrong direction?”

Symptoms: selenium deficiency vs too much selenium

Early, subtle signs of low selenium

Mild selenium deficiency can be hard to spot because symptoms are non-specific.[8][9] Possible signs include:

• tiredness or low energy
• getting infections more often
• muscle weakness
• reduced exercise tolerance
• worsening of existing thyroid problems if iodine is also low

More severe or long-lasting low selenium has been linked with:[8][9]

• heart muscle problems (a type of nutritional cardiomyopathy)
• worse outcomes in some people with heart failure
• reduced fertility
• changes in mood or thinking

Did you know?
In some European heart-failure studies, people with lower selenium levels tended to have worse symptoms and outcomes – but research on whether supplements help is still ongoing.[9]

Severe deficiency syndromes (thankfully rare in the UK)

In places with very low soil selenium and limited diets, severe deficiency has caused classic syndromes:

• Keshan disease – a heart muscle disease, mainly in children and women of child-bearing age, described in parts of China[8][9]
• Kashin–Beck disease – a joint and bone disorder affecting growth[8]

In these areas, selenium intake can be around 10mcg/day. Even around 20mcg/day can protect against Keshan disease.[8]

This level of profound selenium deficiency is not seen in the general UK population. But it reminds us that very low long-term selenium is clearly harmful for the heart, bones and thyroid.

Mild vs severe selenosis from too much selenium

Taking too much selenium for a long time can cause selenosis. The NIH Office of Dietary Supplements and EFSA describe typical chronic toxicity symptoms such as:[8][4]

• hair loss and brittle nails
• a “garlic” smell on the breath
• metallic taste in the mouth
• skin rashes
• nausea, diarrhoea and stomach pain
• tiredness, irritability and nervous system symptoms[8]

Very high intakes – usually from badly mis-labelled supplements, not from food – can be dangerous.

One well-known outbreak in the USA happened when a liquid supplement contained about 200 times the amount of selenium shown on the label. Instead of micrograms, people were taking tens of milligrams. Many developed severe gut symptoms, neurological problems, hair loss, nail changes and abnormal liver tests.[12][13]

TL;DR on “overdoing it”

• Normal UK foods, even if you eat plenty of fish, are very unlikely to give you too much selenium.
• Selenosis usually comes from regular high-dose supplements on top of a good diet, or from mislabelled products.[8][12][13]

How do doctors test selenium status?

In the UK, selenium testing is usually done in hospital laboratories. It is not a standard “check-up” test at your GP surgery.

Common hospital tests include:[10][11]

• serum or plasma selenium (micromol/L)
  • North West London Pathology gives adult reference ranges of about 0.75–1.46micromol/L for 19–65-year-olds[11]
  • South Tees Hospitals report a similar adult range of roughly 0.66–1.57micromol/L[10]

Other measures, like red blood cell selenium, or selenium in hair or nails, are used more in research and environmental studies than in everyday NHS practice.[9]

Doctors might order selenium tests if:[10][11]

• someone is on long-term parenteral nutrition (intravenous feeding) or a very restricted diet
• there is suspected selenium deficiency, for example in unexplained cardiomyopathy in someone on TPN
• there is concern about too much selenium after taking a high-dose or mis-formulated supplement

Key point
Selenium testing can be helpful in specific situations, but it is not a routine “wellness” test in the UK. If you’re worried, speak to your GP or a relevant specialist instead of ordering private tests or changing supplements on your own.

Safety limits: UK RNIs, NHS guidance and EFSA upper limits

UK RNIs and NHS advice

In the UK, the Reference Nutrient Intake (RNI) for selenium is:[5][6]

The NHS puts it simply: 75mcg/day for men and 60mcg/day for women, and points to meat, fish, eggs and Brazil nuts as good sources.[5]

For upper intakes, the NHS says that taking 350 mcg/day or less from supplements is unlikely to cause harm in adults.[5] This assumes you are not also eating very high-selenium foods every single day.

EFSA’s updated 255mcg/day upper limit

The European Food Safety Authority (EFSA) has recently reviewed selenium using systematic studies of toxicity, especially hair and nail changes:[4][19]

• EFSA now sets a Tolerable Upper Intake Level (UL) of 255 mcg/day of selenium for adults, including pregnant and breastfeeding women[4][19]
• for children and teenagers, ULs are lower and adjusted for body size[4]

This UL covers total selenium intake from food plus supplements over the long term. It is a safety ceiling, not a target.

EFSA has also set an Adequate Intake (AI) for adults at 70 mcg/day, linked to the intake needed to saturate selenoprotein P, an important selenium-transport protein.[3]

So for adults in Europe and the UK, we can summarise:

At a glance – adults (Europe/UK)

• Adequate intake (AI): ~70mcg/day (EFSA)[3]
• UK RNI: 75mcg (men), 60mcg (women)[5][6]
• EFSA UL: 255mcg/day from all sources[4]
• NHS: up to 350mcg/day from supplements is unlikely to cause harm[5]

US / WHO numbers – why they’re different

• The US Institute of Medicine and the WHO use a higher UL of 400mcg/day.[8][23]
• This comes from similar hair/nail toxicity data, but with different safety margins and assumptions.

EFSA has taken a more cautious approach, taking into account European diets (often lower in selenium), newer data and a preference for a wider safety buffer.[4][19]

For people in the UK or EU, EFSA’s 255 mcg/day UL is the key number.

Where does a 200mcg selenium supplement fit in safely?

Here’s the practical question: if you take a 200mcg selenium supplement, does that automatically mean you’re getting too much selenium?

Not necessarily – it depends on your diet and your situation.

Typical UK intakes and “total daily selenium”

In many European countries, including the UK, average intakes are often around 40–60mcg/day, sometimes lower in women and in people who eat little or no meat and fish.[6][7]

For example:

• daily diet: ≈50mcg/day
• plus a 200mcg/day supplement
• total: ≈250mcg/day

This sits just under EFSA’s UL of 255mcg/day for adults, and below the older 400mcg/day UL used by some other bodies.[4][8]

A few extra points:

• clinical trials have used 200 mcg/day selenium (often as selenium yeast or selenomethionine) for several years without obvious toxicity in people who were not already high in selenium[8]
• however, big trials like SELECT found that 200 mcg/day did not reduce major outcomes such as prostate cancer, and there were signals of increased diabetes risk in men who already had high selenium status[1][8]

So background status matters:

• if your selenium intake is low, 200mcg/day can raise blood levels into an “adequate” range[2][3]
• if you already eat a selenium-rich diet (plenty of fish, meat and regular Brazil nuts), adding 200mcg/day long term could take you closer to excess[2][8]

A 200mcg supplement, such as a high-strength selenium product (for example, a 200mcg capsule like this one from Capsure), can sit within EFSA’s UL if your dietary intake is moderate. But it should not be treated as a “the more, the better” dose for everyone.

Who should be extra cautious with 200mcg?

Some people should speak to their GP, pharmacist or specialist before taking 200mcg/day (or more), including:[5][8][9]

• children and teenagers – their safe upper limits are lower than adults’
• pregnant and breastfeeding women – EFSA’s UL is still 255mcg/day, but pregnancy care should be individualised
• people with kidney disease, on dialysis, or with complex long-term health conditions
• people who already eat very high-selenium diets, for example:
  • daily Brazil nuts (which can give more than 300mcg in a single serving)
  • high intakes of fish/meat plus fortified foods[8]
• anyone taking several supplements that all contain selenium (for example, a multivitamin, an “immune” formula and a stand-alone selenium product)

In these situations, a 200mcg supplement could push total intake closer to or above safe limits over time.

Practical tips: getting enough without getting too much selenium

Selenium doesn’t need to be a source of stress. A few simple steps can help you avoid both deficiency and excess.

Practical TL;DR

• Aim around the RNI / AI range (about 60–80mcg/day for most adults).
• Use food as your foundation, and think of supplements as a way to fill a gap, not to overwhelm the system.

1. Focus on food first

Regular, moderate intake from food is unlikely to cause too much selenium, and can help prevent deficiency. Good sources include:

• fish and seafood (e.g. tuna, sardines, prawns)
• meat and poultry
• eggs
• Brazil nuts (occasionally, in small amounts)[5][8]

One or two Brazil nuts a few times a week can easily cover the needs of many people. A big handful every day, on top of a good diet and supplements, might push intake towards or past upper limits.[8]

2. Check what’s already in your supplements

If you take a multivitamin, an “immune” or “hair/skin/nails” product, and then a separate selenium supplement, it’s surprisingly easy to approach or exceed 255mcg/day.

For example:

• multivitamin: 50–70mcg
• “immune” or beauty blend: another 50–100mcg
• stand-alone selenium: 100–200mcg

Quick audit tip

1. Add up the selenium from all your supplements (check the labels).
2. Add a rough estimate from your diet – for many UK adults, 40–70mcg/day is a reasonable ballpark unless you’re vegan or eat a lot of offal, fish or Brazil nuts.[6][7]

If your daily total is regularly above 255mcg, you are in the range where too much selenium becomes more likely over the long term.

3. Match the dose to the situation

The right approach depends on the person. As a very general, non-personalised guide (always check with a health professional for your own case):

• Healthy adults with a varied diet
  • a multivitamin providing around 50–70mcg/day usually keeps you comfortably above deficiency and well below upper limits
• Higher risk of low intake
  (for example, very low meat/fish intake, certain medical diets, pregnancy in low-selenium areas – always under medical care)
  • in some situations, clinicians may use 100–200mcg/day where there is clear evidence or strong suspicion of low selenium, often with monitoring[2][3][9]
• People with high baseline intakes
  (for example, regular Brazil nuts plus plenty of fish/meat)
  • extra supplementation is not automatically helpful and may move you towards excess rather than benefit[1][2]

FAQ: long-tail questions about selenium, deficiency and safety

Is selenium deficiency really a UK problem, or just lab talk?

For the average healthy person in the UK, it’s unlikely you’ll develop dramatic deficiency diseases like Keshan disease. However, population data suggest that a significant number of adults – especially women – have intakes below the RNI and sometimes below the LRNI.[6][7]

Biochemical selenium deficiency may subtly affect thyroid and immune function and could play a role in fertility or heart health, particularly when combined with other issues like low iodine or chronic illness.[1][2][9]

How can I tell if I have selenium deficiency?

Symptoms alone (like tiredness or hair loss) are not reliable. Your doctor may consider testing if, for example:

• you are on long-term parenteral nutrition or a very restricted diet
• you have unexplained cardiomyopathy
• you are under specialist care for complex gut or endocrine conditions

In those situations, your doctor can arrange serum selenium testing through hospital labs such as South Tees or NWL Pathology.[10][11]

Self-diagnosing selenium deficiency and then taking high-dose supplements on your own is not recommended.

Is 200mcg of selenium a “high dose”?

Yes, 200mcg/day is considered a high-strength dose:

• it is clearly above the RNI / AI (around 60–75mcg)
• it is below EFSA’s 255mcg/day UL, but close enough that your diet really matters[3][4]

In research, 200mcg/day has often been used for months to years without obvious toxicity in people who weren’t already high in selenium.[1][8] Even so, taking 200mcg/day for many years without thinking about diet or status may not be ideal.

Can I get too much selenium from food alone?

In the UK, this is very unlikely, unless your diet is unusually rich in high-selenium foods, especially large amounts of Brazil nuts.[5][8]

Cases of selenosis from food alone are usually reported in regions with naturally very high soil selenium, not in the UK.[2][8]

Should I take selenium to prevent cancer or heart disease?

Current high-quality evidence does not support taking selenium supplements just for cancer or heart-disease prevention if your selenium status is already adequate:[1][8]

• the SELECT trial (200mcg/day) did not lower prostate cancer or cardiovascular events in well-nourished men[8]
• a Cochrane review found no cancer-prevention benefit of selenium supplements in high-income populations with adequate intakes[8]

Some observational studies show that both very low and very high selenium levels are linked with worse outcomes, again suggesting that staying in the middle range is more important than chasing “more”.[2]

Is it useful to test hair or nail selenium?

In research, hair and nail selenium can reflect longer-term exposure and are used in environmental studies. In UK clinical practice, serum or plasma selenium is the standard measure.[9][10][11]

Private tests sometimes offer hair mineral analysis, but these results can be hard to interpret. If you’re worried about selenium deficiency or excess, it’s better to discuss proper testing and interpretation with a health professional.

Bottom line

• Selenium has a narrow safe range: both deficiency and excess are undesirable. The goal is to stay in the middle.
• In the UK, mildly low intakes are fairly common, especially in women, but severe deficiency is rare.[6][7]
• The UK RNI is around 60–75mcg/day, EFSA’s adequate intake is 70mcg/day, and EFSA’s upper limit for adults is 255mcg/day.[3][4][5]
• The NHS suggests that up to 350mcg/day from supplements is unlikely to cause harm in adults, but this is not a target and “more” is not “better”.[5]
• A 200mcg/day supplement is a high-strength option. It can still sit below EFSA’s UL if your dietary intake is moderate, but it is not a casual, risk-free dose for everyone – especially if you also take other selenium-containing products.[3][4][8]
• If you’re worried that you may have selenium deficiency or that you’re getting too much selenium, the safest option is to talk to your GP, pharmacist or a relevant specialist rather than guessing and adjusting doses on your own.

Product Related to This Article from Capsure

Selenium 200mcg 120 Capsules

£6.05

Shop Now

References

1. Rayman MP. Selenium and human health. Lancet. 2012;379(9822):1256-68. Available from: https://pubmed.ncbi.nlm.nih.gov/22381456
2. Rayman MP. Selenium intake, status, and health: a complex relationship. Hormones (Athens). 2020;19(1):9-14. Available from: https://pubmed.ncbi.nlm.nih.gov/31388899
3. EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Scientific opinion on dietary reference values for selenium. EFSA J. 2014;12(10):3846. Available from: https://www.efsa.europa.eu/en/efsajournal/pub/3846
4. Turck D, Bohn T, Castenmiller J, de Henauw S, Hirsch-Ernst KI, et al. Scientific opinion on the tolerable upper intake level for selenium. EFSA J. 2023;21(1):e07704. Available from: https://www.efsa.europa.eu/en/efsajournal/pub/7704
5. National Health Service. Vitamins and minerals – others: selenium. In: Vitamins and minerals – others. [Internet]. London: NHS; 2020 [cited 2025 Nov 27]. Available from: https://www.nhs.uk/conditions/vitamins-and-minerals/others/
6. Stoffaneller R, Morse NL. A review of dietary selenium intake and selenium status in Europe and the Middle East. Nutrients. 2015;7(3):1494-537. Available from: https://www.mdpi.com/2072-6643/7/3/1494
7. Scientific Advisory Committee on Nutrition. SACN statement on selenium and health – 2013 [Internet]. London: Public Health England; 2013 [cited 2025 Nov 28]. Available from:
   https://assets.publishing.service.gov.uk/media/5a7ea6b440f0b62305b824ce/SACN_Selenium_and_Health_2013.pdf
8. Office of Dietary Supplements. Selenium – Health professional fact sheet. [Internet]. Bethesda (MD): National Institutes of Health; 2022 [updated 2024; cited 2025 Nov 27]. Available from: https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/
9. Shreenath A, Hashmi MF, Dooley J. Selenium deficiency. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan– [cited 2025 Nov 27]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482260/
10. South Tees Hospitals NHS Foundation Trust. Selenium. [Internet]. Middlesbrough: South Tees Hospitals NHS Foundation Trust; 2022 [cited 2025 Nov 27]. Available from: https://www.southtees.nhs.uk/services/pathology/tests/selenium/
11. North West London Pathology. Selenium (Se). [Internet]. London: North West London Pathology; 2025 [cited 2025 Nov 27]. Available from: https://www.nwlpathology.nhs.uk/test/selenium/
12. MacFarquhar JK, Broussard DL, Melstrom P, Hutchinson R, Wolkin A, et al. Acute selenium toxicity associated with a dietary supplement. Arch Intern Med. 2010;170(3):256-61. Available from: https://pubmed.ncbi.nlm.nih.gov/20142570
13. Morris JS, Crane SB. Selenium toxicity from a misformulated dietary supplement, adverse health effects, and the temporal response in biomarkers. Nutrients. 2013;5(4):1024-57. Available from: https://www.mdpi.com/2072-6643/5/4/1024
14. Rayman MP. Selenium intake, status and health: a complex relationship. Hormones (Athens). 2020;19(1):9-14. (Additional discussion of U-shaped associations). Available from:
    https://pmc.ncbi.nlm.nih.gov/articles/PMC7033057/
15. European Food Safety Authority. Overview on tolerable upper intake levels as derived by the Scientific Committee on Food and the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) [Internet]. Parma: EFSA; 2025 [cited 2025 Nov 28]. Available from:
    https://www.efsa.europa.eu/sites/default/files/2024-05/ul-summary-report.pdf