Selenium
An essential trace mineral, bought most often with the thyroid in mind.
- Thyroid
- Immunity
- Heart and circulation
- Mood
- Memory and thinking
A plain-English overview, not advice. Written from published reviews and trials and from what people say in public forums.
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On this page
What is selenium?
Selenium is a trace mineral the body needs in very small amounts and takes in from food. It works almost entirely by being built into a small family of proteins - about twenty-five of them in people - where it does jobs ranging from mopping up reactive molecules to making thyroid hormone active. Going short of it has been linked with poorer immune function, with thinking and memory slipping, and with a higher risk of dying. As a supplement it comes either in an organic form - selenomethionine, or selenium grown into yeast - or as an inorganic salt such as selenite or selenate, a difference the trials have found matters.
Why do people take selenium?
Thyroid
The thinking. The thyroid is the reason most people reach for selenium. The gland cannot make its hormone without it, and the enzymes that switch that hormone into its active form are selenium proteins. Where the immune system is making antibodies against the gland, the idea is that a fuller supply calms things down.
What research has found. Two reviews pooled the randomised trials in adults with an autoimmune thyroid condition. Antibodies to the thyroid came down on selenium in both - across 29 sets of results covering 2,358 people in the larger - and TSH eased a little in adults not taking thyroid hormone. Thyroxine levels and gland size did not shift. This is one of the best-studied uses, and several reviews of trials broadly agree.
Immunity
The thinking. Immune cells need selenium to grow and to do their work, and going short leaves them under more oxidative strain and slower to get going. That thinking is why selenium turns up in winter routines, and why researchers have kept asking whether more of it makes for a stronger response.
What research has found. A review of nine trials in healthy adults found no real change in antibody levels or white cell counts, and no further gain once blood selenium passed about 100 micrograms per litre. A trial in adults aged 50 to 64 with lowish selenium, given 50, 100 or 200 micrograms a day before a flu jab, saw shifts in both directions. Research here is still young: a few studies so far, mostly pointing the same way.
Heart and circulation
The thinking. Selenium’s place in heart supplements rests on it being an antioxidant: its enzymes mop up reactive molecules, and the long-standing thought is that less oxidative wear on blood vessels would mean fewer heart problems. Observational work fed that idea, with middling blood selenium looking better than either extreme.
What research has found. Two reviews pooled the randomised trials, and neither found selenium on its own made a difference to heart events or to deaths from any cause; in one, supplements averaging 100 micrograms a day, taken for anything between six months and nearly ten years, came out level with a dummy tablet. Antioxidant blends that included selenium did fare better than blends without it. Only a few studies have measured this so far, though they mostly agree.
Mood
The thinking. Mood is a quieter reason, and a newer one. Selenium enzymes keep the brain’s oxidative balance, and low selenium has been linked with thinking slipping in later life, so researchers have asked whether it tracks with low mood too - including in the months after having a baby.
What research has found. One review gathered twenty studies, most of them observational. Blood selenium was no different between people with low mood and people without; but across the trials that gave a supplement, depressive symptom scores came out lower than on placebo, by a small and closely agreed margin. Higher selenium intake also went with fewer cases of low mood after childbirth. This is early work, a small number of studies, mostly pointing the same way.
Memory and thinking
The thinking. The brain is sensitive to oxidative damage, and low selenium has been linked with thinking and memory slipping in later life. Older adults reach for antioxidants with that in mind, and researchers have followed the same thought into long trials in healthy volunteers and into studies of people whose memory has already changed.
What research has found. The largest trial gave selenium, vitamin E, both or a dummy tablet to 7,540 men aged 60 and over, and found new cases of dementia no different across the four groups. In adults with Alzheimer’s disease or mild memory problems the picture is patchier: a review found selenium alone shifted some test scores but not others. Studies here point in different directions, so this one is still being worked out; the dose, how long people took it and who took part all seem to matter.
What does selenium do in the body?
Selenium is not a fuel, and it is not stored up for a rainy day. The body threads it into a small family of proteins - about twenty-five in people - where it sits at the working end of the molecule, in the spot sulphur would otherwise occupy. Most of those proteins are enzymes that handle reactive oxygen; the best known of them keeps free radicals in check where inflammation is running.
Why selenium rather than sulphur? Chemists put it down to selenium holding its outer electrons more loosely: it reacts with reactive oxygen faster, and the molecule it forms in doing so is easily undone again, so the enzyme resets instead of staying spent. The rest of the family does other jobs - one tunes the inflammatory signals cells send each other, another helps keep the body’s own supply in balance, and another group turns thyroid hormone into its active form.
What do people say about taking selenium?
These are anecdotes people have shared, not evidence.
What people like
- The thyroid is far and away the commonest reason given, usually to help the body turn thyroxine into its active form.
- A good number were pointed towards it by a doctor or a clinic rather than choosing it for themselves off the shelf.
- The clearest reports come from people whose blood level had actually come back low, or who needed a slightly smaller thyroid dose afterwards.
- A few mention hair and skin looking better, and men taking it for sperm quality comes up repeatedly in fertility threads.
Common complaints
- Worrying about having taken too much is by far the commonest complaint, from tablets and from eating a lot of nuts.
- Hair loss, brittle nails, nausea and tiredness are the signs people name, usually repeated from reading rather than from their own experience.
- Quite a few say plainly that they cannot tell what the selenium did, having started several supplements in the same week.
- Those who took it for something other than the thyroid, aching joints for instance, mostly report noticing nothing at all.
How people take it
- Between 100 and 200 micrograms a day is the usual figure, and thyroid threads settle on 200 almost without exception.
- Nuts are offered as the alternative nearly every time it comes up, at one or two a day instead of a tablet.
- Plenty take it inside a multivitamin rather than on its own, and some alternate days to keep the weekly amount down.
- People on thyroid medication describe taking that first thing on an empty stomach and leaving their minerals until later in the day.
Read from these public discussions:
- Benefits of selenium? (Patient community forum, 2014)
- Have you tried Selenium? (Patient community forum, 2015)
- Anyone juggling iron, zinc, copper and selenium supplement timings? (Mumsnet, 2026)
- Hypothyroidism (Mumsnet, 2018)
- Is it possible to improve egg quality in three months? (Mumsnet, 2016)
- Poisoned myself? (Mumsnet, 2026)
What forms of selenium are there?
| Form | What’s different | Per serving, on products we list |
|---|---|---|
| Selenomethionine | Selenium sitting where sulphur normally sits in the amino acid methionine: an organic form. | not in our price tables yet |
| Other forms | Inorganic selenium salts such as selenite and selenate, or selenium grown into yeast. | 200 µg |
What else is worth knowing about selenium?
- Labels give the elemental selenium per serving, in micrograms, and a serving may be more than one capsule; the form is named separately, and organic and inorganic forms have not always behaved alike in trials.
- Selenium has an unusually narrow window between too little and too much. One review describes the link with health as U-shaped: extra selenium may suit someone whose level is low, while someone whose level is already good may be affected the other way.
- The same review notes that adding selenium on top of an already adequate intake has been linked with more type 2 diabetes.
- Side effects in the thyroid trials were mild. The newer review found them no more common on selenium than in the comparison groups; the older one found people on selenium more likely to report something. Doses were mostly 50 to 200 micrograms a day.
If you take medication, are pregnant, or have a health condition, check with a pharmacist or GP first.
The research behind this page (18 studies)
- Selenium Supplementation in Patients with Hashimoto Thyroiditis: A Systematic Review and Meta-Analysis of Randomized Clinical Trials. (2024, meta-analysis)
- Selenium Supplementation Significantly Reduces Thyroid Autoantibody Levels in Patients with Chronic Autoimmune Thyroiditis: A Systematic Review and Meta-Analysis. (2016, meta-analysis)
- Effect of selenium on thyroid autoimmunity and regulatory T cells in patients with Hashimoto's thyroiditis: A prospective randomized-controlled trial. (2021, randomised trial)
- Selenium and immune function: a systematic review and meta-analysis of experimental human studies. (2023, meta-analysis)
- Selenium supplementation has beneficial and detrimental effects on immunity to influenza vaccine in older adults. (2017, randomised trial)
- Selenium, antioxidants, cardiovascular disease, and all-cause mortality: a systematic review and meta-analysis of randomized controlled trials. (2020, meta-analysis)
- Selenium status and cardiovascular diseases: meta-analysis of prospective observational studies and randomized controlled trials. (2016, meta-analysis)
- The role of selenium in depression: a systematic review and meta-analysis of human observational and interventional studies. (2022, meta-analysis)
- Effects of Selenium Supplementation in Patients with Mild Cognitive Impairment or Alzheimer's Disease: A Systematic Review and Meta-Analysis. (2022, meta-analysis)
- Supranutritional Sodium Selenate Supplementation Delivers Selenium to the Central Nervous System: Results from a Randomized Controlled Pilot Trial in Alzheimer's Disease. (2019, randomised trial)
- Probiotic and selenium co-supplementation, and the effects on clinical, metabolic and genetic status in Alzheimer's disease: A randomized, double-blind, controlled trial. (2019, randomised trial)
- Association of Antioxidant Supplement Use and Dementia in the Prevention of Alzheimer's Disease by Vitamin E and Selenium Trial (PREADViSE). (2017, randomised trial)
- Selenium and human health. (2012, review)
- Selenium and Selenoproteins in Health. (2023, review)
- The Role of Selenium in Pathologies: An Updated Review. (2022, review)
- The role of selenium in inflammation and immunity: from molecular mechanisms to therapeutic opportunities. (2012, review)
- Selenium⁻Fascinating Microelement, Properties and Sources in Food. (2019, review)
- Selenium intake, status, and health: a complex relationship. (2020, review)
How much does selenium cost?
That depends on the form and the pack, so we work out the price of the same dose of every selenium product we can find and rank them.