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Selenium
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In one pass Selenium is a trace element that both the antioxidant system and the thyroid rely on.
Educational content, not medical advice — consult a clinician.
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Chapter 1
Content depends on the soil
Selenium is a trace element that both the antioxidant system and the thyroid rely on. Whether you get enough depends largely on the soil your food grew in: the selenium in plants follows the soil, so the same grain from a different region can differ a lot. People who regularly eat seafood (see Salmon · see Tuna · see Oysters), eggs, and meat almost never run short, because animals keep their body selenium at fairly steady levels; real low-selenium risk is concentrated in particular regions and in people with very narrow diets.
A single Brazil nut can cover a day, and the content varies widely from batch to batch. A belt of low-selenium soil runs from northeast to southwest China, where Keshan disease (an endemic heart-muscle disease) was once common; after large-scale selenium supplementation, cases fell sharply.
In practice: food from many regions is a safer bet than relying on one ingredient; 1–2 Brazil nuts a day are enough, and they should not be eaten by the handful as a snack, because a small handful (6–8 nuts) already carries more selenium than the daily Tolerable Upper Intake Level.
A single Brazil nut can cover a day, and the content varies widely from batch to batch. A belt of low-selenium soil runs from northeast to southwest China, where Keshan disease (an endemic heart-muscle disease) was once common; after large-scale selenium supplementation, cases fell sharply.
In practice: food from many regions is a safer bet than relying on one ingredient; 1–2 Brazil nuts a day are enough, and they should not be eaten by the handful as a snack, because a small handful (6–8 nuts) already carries more selenium than the daily Tolerable Upper Intake Level.
Background · Keshan disease and the low-selenium belt
The most classic example of selenium deficiency as a public-health problem comes from China and is called Keshan disease.Keshan disease was first identified in 1935 around Keshan County in Heilongjiang. It is an endemic cardiomyopathy: diffuse degeneration of the heart muscle and heart failure, hitting preschool children and women of childbearing age hardest. The soil in these areas is extremely low in selenium (< 0.1 mg/kg), and along the chain from soil to crop to food to person, selenium is diluted at every step. From the 1970s to the 1990s, researchers ran several large selenium-supplementation programs (sodium selenite) in high-incidence areas, and the disease fell sharply. But selenium deficiency is not the whole answer: the US Office of Dietary Supplements (ODS) states that the exact cause of Keshan disease is still unknown; selenium deficiency on its own rarely causes illness directly, and it looks more like a condition that makes people more likely to fall ill when other stresses pile on.
The Keshan belt is a band of low-selenium soil stretching from northeast to southwest China, across Heilongjiang, Jilin, Liaoning, Inner Mongolia, Hebei, Henan, Shaanxi, Sichuan, Guizhou, Yunnan, and Tibet. The same belt also has a high incidence of Kashin-Beck disease (a joint-cartilage disease that begins in childhood); Kashin-Beck disease is concentrated in areas low in both selenium and iodine and is associated with selenium deficiency, but its cause is not settled either.
Elsewhere: average selenium intake is low in some European countries, especially among vegetarians and vegans; New Zealand was also low in the past and rose only after it began importing high-selenium wheat; in North America, even in low-selenium areas, most people's intake stays well above the recommendation because food comes from many places.
A larger implication: nutrition is not only a matter of eating more leafy greens; soil geochemistry sits upstream. The more regions your food comes from, the smaller the risk that a single region's food chain is collectively short of one trace element.
Chapter 2
Selenium built into proteins
Selenium is not an ordinary mineral plugged in somewhere: it is written directly into proteins as selenocysteine (abbreviated Sec, one-letter code U), often called the 21st amino acid.
This is an odd extension of the genetic code. The classic 20 amino acids are encoded by 64 codons; Sec uses UGA, which is normally a stop codon. Only when the messenger RNA carries a special structure called SECIS (selenocysteine insertion sequence) is UGA reread as Sec, and that step also needs a dedicated tRNA plus two proteins, EF-Sec and SBP2, working together.
Why did evolution keep such a fussy mechanism? Because selenium is more reactive than sulfur: the pKa of Sec is about 5.2 (about 8 for cysteine), so at the body's pH it is almost fully charged, and in delicate redox reactions it works much faster than cysteine and is hard to replace. The human body has about 25 selenoproteins; a small amount of selenium supports a whole family of key enzymes, which is why selenium deficiency can involve the heart, the thyroid, and immunity at the same time.
This is an odd extension of the genetic code. The classic 20 amino acids are encoded by 64 codons; Sec uses UGA, which is normally a stop codon. Only when the messenger RNA carries a special structure called SECIS (selenocysteine insertion sequence) is UGA reread as Sec, and that step also needs a dedicated tRNA plus two proteins, EF-Sec and SBP2, working together.
Why did evolution keep such a fussy mechanism? Because selenium is more reactive than sulfur: the pKa of Sec is about 5.2 (about 8 for cysteine), so at the body's pH it is almost fully charged, and in delicate redox reactions it works much faster than cysteine and is hard to replace. The human body has about 25 selenoproteins; a small amount of selenium supports a whole family of key enzymes, which is why selenium deficiency can involve the heart, the thyroid, and immunity at the same time.
Mechanism · The 25 human selenoproteins
What happens if the Sec in a selenoprotein is swapped for ordinary cysteine (Cys)? In experiments that mutate Sec to Cys in key selenoenzymes, enzyme activity drops sharply, and in some it nearly disappears. That is why evolution kept the costly SECIS, EF-Sec, and SBP2 machinery: key redox enzymes need Sec, and ordinary Cys cannot stand in.The human body has about 25 selenoproteins. The main families:
GPX (glutathione peroxidases 1–4 and 6): clear hydrogen peroxide and organic peroxidesTXNRD (thioredoxin reductases 1, 2, 3): keep the cell in a reduced stateDIO (deiodinases 1, 2, 3): turn into or inactivate thyroid hormone, the key to the thyroid-hormone switch (the chapter on why the thyroid depends on selenium)SELENOP (selenoprotein P, formerly SEPP1): transports and stores selenium in plasma, and is the main molecule that delivers selenium to the brain
The remaining dozen or so have more scattered functions. This division of labor explains why, although we need only tens of micrograms of selenium a day, small changes can affect several body systems.
Two plasma selenoproteins (GPX3 and selenoprotein P) are also used to judge selenium status, but inflammation affects them; and in people who already have enough selenium, supplements do not push them any higher. That fits the trial results elsewhere in this story: once selenium is sufficient, taking more does not buy more selenoenzymes.
Chapter 3
The thyroid depends on selenium
The thyroid has the highest selenium concentration of any organ, because it depends heavily on selenium enzymes. All the deiodinases contain selenium: some strip one iodine from low-activity to make high-activity , while others inactivate the hormone. Making thyroid hormone also produces hydrogen peroxide, which the selenium-containing glutathione peroxidases have to clear so the thyroid cells are not damaged by their own work.
So iodine is the raw material, and selenium is the hormone switch plus antioxidant protection. When iodine is short, selenium cannot make up for the missing raw material; when selenium is short, the thyroid copes less well with oxidative stress. In Hashimoto's thyroiditis (a disease in which the immune system attacks the thyroid), most selenium trials saw antibodies fall, but no clear improvement in the course of the disease, and selenium cannot replace thyroid hormone.
So iodine is the raw material, and selenium is the hormone switch plus antioxidant protection. When iodine is short, selenium cannot make up for the missing raw material; when selenium is short, the thyroid copes less well with oxidative stress. In Hashimoto's thyroiditis (a disease in which the immune system attacks the thyroid), most selenium trials saw antibodies fall, but no clear improvement in the course of the disease, and selenium cannot replace thyroid hormone.
Mechanism · Deiodinases and antioxidant cover
Two classes of selenoproteins work side by side in the thyroid.The deiodinases (DIO 1, 2, 3) all contain selenium: DIO 1 and 2 strip one iodine from low-activity to make high-activity ; DIO 3 does the reverse, turning T4 into inactive reverse T3 and T3 into T2 to switch the hormone off; together they tune hormone levels. Glutathione peroxidases (GPX) and thioredoxin reductases clear the by-products: thyroid peroxidase () uses hydrogen peroxide to make hormone, and these enzymes clear the excess so the thyroid cells are protected.
So iodine plus selenium is a division of labor: iodine is the raw material (the names T4 and T3 carry the 4 and 3 iodine atoms), and selenium is the hormone switch plus the thyroid's antioxidant protection. When iodine is short, adding selenium blindly does not solve the raw-material problem; when selenium is short, the thyroid is less able to handle oxidative stress. In population studies, low selenium status is associated with a higher risk of thyroid disease, but mainly in women, and it is an observed association.
Hashimoto's thyroiditis and selenium: the Toulis 2010 pooled 4 , all in people with Hashimoto's who were already taking levothyroxine, most using 200 µg of selenium a day. After 3 months of selenium, anti-TPO antibodies were clearly lower than in controls, and more people reported better well-being or mood; the levothyroxine dose and the thyroid's appearance on ultrasound were either unchanged or not reported. The authors concluded that an improvement in thyroid function and structure has to be shown before routine selenium can be recommended. The Winther 2020 review reaches the same judgment: selenium mostly lowers antibodies but shows no improvement in the course of the disease, and guideline recommendations extend only to Graves' ophthalmopathy (the eye disease that can accompany an overactive thyroid).
To be clear, selenium cannot replace levothyroxine: once Hashimoto's has progressed to hypothyroidism, hormone replacement is still required. The slogan selenium saves the thyroid cannot be taken out of context; iodine, selenium, autoimmune status, and hormone levels have to be read together.
Evidence · A null trial and advice in pregnancy
Not every trial saw antibodies fall. Karanikas 2008 ran a controlled trial in 36 consecutive people with Hashimoto's who were all taking levothyroxine and had not been selected by disease activity: 18 added sodium selenite 200 µg/day for 3 months and 18 took placebo; in the selenium group anti- went from 524 to 505 /mL, not a significant change, and the cytokine profile of circulating T cells did not change either; the paper's title is its result: No immunological benefit. Positive and null trials side by side, plus the lack of , are why the overall evidence counts only as a signal, not a settled answer.Pregnancy needs a separate word: the American Thyroid Association (ATA) guideline of 2017 made a weak recommendation against selenium supplements for pregnant women who are TPO-antibody positive, based on moderate-quality evidence.
If you have confirmed Hashimoto's with very high anti-TPO and live in a low-selenium region, discuss with your endocrinologist whether to try selenium for a period, at what dose, and for how long; while taking it, keep the safety window in mind (the Tolerable Upper Intake Level is 400 µg/day, and it is a ceiling, not a target).
A few limits worth remembering: selenium cannot replace levothyroxine, and once Hashimoto's reaches hypothyroidism, hormone replacement remains the main treatment; there is no evidence that selenium prevents Hashimoto's, though avoiding long-term extreme low selenium is a reasonable goal; whether taking selenium plus iodine together worsens immune damage in Hashimoto's is still theoretical, with weak evidence. The full management of Hashimoto's is covered in the Hashimoto's Thyroiditis story.
karanikas-2008-se-ait
Chapter 4
Immunity & redox
Selenium's place in immunity is keeping oxidative damage where it belongs. Immune cells deliberately make hydrogen peroxide and superoxide to kill microbes, and the same weapons can hurt the immune cells themselves and nearby tissue; the selenium enzymes glutathione peroxidase (GPX) and thioredoxin reductase (TXNRD) reduce the excess peroxides so the damage stays around the pathogen.
Low selenium blunts this regulation, but that does not make high-dose selenium an immune upgrade. The largest trial, SELECT, gave 35,533 men 200 µg of selenium a day and saw no prevention of prostate cancer or other cancers. The effect of selenium is U-shaped: correcting a deficiency matters, while adding more on top of enough may do harm.
Low selenium blunts this regulation, but that does not make high-dose selenium an immune upgrade. The largest trial, SELECT, gave 35,533 men 200 µg of selenium a day and saw no prevention of prostate cancer or other cancers. The effect of selenium is U-shaped: correcting a deficiency matters, while adding more on top of enough may do harm.
Mechanism · How selenium keeps immune fire contained
Immune cells (neutrophils and macrophages) deliberately create oxidative stress to kill microbes: hydrogen peroxide, superoxide, hypochlorous acid. But the same weapons can hurt the immune cells themselves and the healthy tissue around them. Two classes of selenium enzymes, glutathione peroxidases and thioredoxin reductases, reduce the excess peroxides so the damage stays confined around the pathogen; when selenium is low, this regulation blunts and inflammation more easily gets out of hand.In population studies, low selenium status is associated with weaker immune function and higher mortality (reviewed by Rayman 2012); these are observed associations. But low selenium blunts immunity does not mean high-dose selenium upgrades immunity. The weightiest counter-lesson comes from the SELECT trial:
Lippman 2009 (JAMA): 35,533 men aged 50 or 55 and older were randomized to selenium (200 µg a day as selenomethionine), vitamin E, both, or placebo; after a median follow-up of 5.46 years, neither selenium nor vitamin E lowered prostate cancer, and no other cancer endpoint differed. There was slightly more type 2 diabetes in the selenium group, but the difference was not statistically significant ( 1.07, P = 0.16)Klein 2011 (JAMA): with longer follow-up, the vitamin-E-only group (400 a day) had about a 17% higher prostate cancer risk ( 1.17, P = 0.008), this time statistically significant; the selenium group still showed no benefit
Why did it fail? These men already had enough selenium. The Rayman 2012 review puts it plainly: supplementation can help only when intake is inadequate, and adding more for people who already have enough may raise their risk of type 2 diabetes.
This history leaves two conclusions. First, the assumption that a plausible mechanism means it works is unreliable and has to be tested in randomized trials. Second, correcting a deficiency and adding more on top of enough are two different points on the U-shaped curve. Ordinary men do not need selenium to prevent prostate cancer, nor to boost immunity.
Evidence · Before and after SELECT
SELECT was originally designed on the strength of the earlier NPC trial (Clark 1996): NPC set out to test whether selenium prevents the return of skin cancer, and it did not, but in later analyses the men taking selenium had less prostate cancer, which gave rise to the hypothesis that selenium supplements lower cancer risk. SELECT was far larger and more tightly controlled, and it found no drop in cancer risk from selenium; the vitamin-E-only group actually had more prostate cancer, while selenium and vitamin E together did not differ significantly from placebo.NPC's own follow-up analysis flashed a yellow light too: Stranges 2007 found that in the 200 µg-a-day selenium group the for type 2 diabetes was 1.55 (95% 1.03–2.33), concentrated in people whose baseline selenium was already high.
The Vinceti 2018 Cochrane systematic review pooled 10 randomized trials: well-designed trials showed that selenium supplements do not reduce cancer risk (high certainty of evidence); some trials reported signals of more high-grade prostate cancer and more type 2 diabetes; and the NPC-derived hypothesis that selenium supplements lower cancer risk in people with low baseline selenium was not confirmed.
Large negative results like these are real scientific progress: they prune away a whole set of repeatedly marketed stories that selenium lowers cancer risk and vitamin E slows aging.
For daily life: on this basis the SELECT researchers recommended that men over 55 avoid selenium supplements above recommended intakes. Ordinary men do not need selenium to prevent prostate cancer; total selenium intake should not exceed the Tolerable Upper Intake Level of 400 µg/day over the long term, and for people who already have enough selenium, even 200 µg may not be a net benefit.
stranges-2007-selenium-diabetesclark-1996-npc-selenium
Chapter 5
Safe and toxic doses are close
Selenium's safety window is narrower than most nutrients': the adult recommended intake is 55 µg/day and the Tolerable Upper Intake Level is 400 µg/day, only about sevenfold apart, so long-term excess really happens. The most common signs of chronic selenium poisoning are hair loss and nails that become brittle or fall off; early signs are garlic-smelling breath and a metallic taste in the mouth; skin rash, nausea and diarrhea, fatigue, irritability, and nervous-system problems can also occur.
One Brazil nut can swing a day's selenium, and the content varies sharply with the soil of the tree. One or two a day usually covers the need; eaten by the handful as a snack, they carry a real risk of poisoning. People who eat seafood, eggs, meat, and grains as usual mostly do not need extra selenium.
One Brazil nut can swing a day's selenium, and the content varies sharply with the soil of the tree. One or two a day usually covers the need; eaten by the handful as a snack, they carry a real risk of poisoning. People who eat seafood, eggs, meat, and grains as usual mostly do not need extra selenium.
Safety · What selenium poisoning looks like
Selenium's safety window is narrower than most nutrients': the recommended intake is 55 µg and the Tolerable Upper Intake Level is 400 µg, only about 7 times apart.The signs of chronic selenium poisoning (selenosis), as summarized by the US ODS:
Most common: hair loss, and nails that become brittle or fall offEarly signs: garlic-smelling breath (the body turns extra selenium into volatile methylated compounds that leave through the lungs) and a metallic taste in the mouthOthers: skin rash, nausea, diarrhea, fatigue, irritability, and nervous-system problems (numb hands and feet, for example)
Brazil nuts are the most drug-like food among nuts: each nut holds about 68–91 µg, and one ounce (about 28 g, 6–8 nuts) averages 544 µg, already above the Tolerable Upper Intake Level; other analyses have measured widely varying values. So 1–2 nuts a day are very likely enough and low-risk, 3–5 may approach or exceed the ceiling, and eating them by the handful as a snack means chronic excess.
Add the diabetes signal from the selenium trials (the chapter on immunity), and the a little extra never hurts mindset is especially wrong for selenium.
A few practical points:
People who eat seafood, eggs, meat, and grains as usual mostly do not need extra seleniumThe body absorbs most of the selenium from common supplement forms (selenomethionine, selenium yeast, sodium selenite, sodium selenate), up to about 90%; for ordinary people, the amount matters more than the formSingle-ingredient selenium supplements commonly hold 100–400 µg per tablet, so one tablet can approach the ceiling; add up food and supplements before you take oneGenuine reasons to supplement are limited: for example, confirmed Hashimoto's in someone living in a low-selenium region, decided together with an endocrinologistDo not eat Brazil nuts by the handful as an everyday snack; among nuts they sit in a dose-risk class of their own
Numbers · How much selenium is in a Brazil nut
The Brazil nut (Bertholletia excelsa) is the food with the highest natural selenium content, and one of the few where a single nut can swing a day's selenium intake.According to US Department of Agriculture data, one ounce (6–8 nuts) averages 544 µg of selenium, or about 68–91 µg per nut; but the US ODS also cautions that other analyses have measured widely varying values, depending on the soil the tree grew in, and nuts in the same bag can differ. Against the recommended intake of 55 µg/day and the Tolerable Upper Intake Level of 400 µg/day, 1–2 nuts a day will very likely cover the recommendation at low risk, while 3–5 may approach or exceed the ceiling. In ODS's words: eaten regularly, Brazil nuts could cause selenium toxicity.
A few practical suggestions:
Do not eat Brazil nuts by the handful as an everyday snack2 nuts a day are often used as a substitute for a selenium supplement, but know that batches differSeafood (tuna, shrimp, sardines), meat, eggs, and grains are steadier sources of selenium with less batch-to-batch variationMost people in everyday life do not need selenium supplements
A special case: in Hashimoto's, selenium lowers antibodies (the Toulis 2010 , trials mostly at 200 µg a day for 3 months) but shows no improvement in the course of the disease; this is a limited effect in diagnosed patients, not a reason for ordinary people to take selenium to prevent thyroid disease.
References · 11
- National Institutes of Health, Office of Dietary Supplements. (2021). Selenium — Fact Sheet for Health Professionals. Table 2: Brazil nuts, 1 ounce (6-8 nuts), 544 mcg selenium (989% DV); the text says Brazil nuts contain 68-91 mcg per nut and could cause selenium toxicity if consumed regularly, and that values from other analyses vary widely. The version read carries 'Updated: April 15, 2024' (Wayback Machine snapshot of 31 December 2024), newer than the 2021 date above (fact sheet, read 2026-09-24). The September 4, 2025 update (Wayback snapshot 19 September 2026) adds: the body absorbs up to about 90% of selenium from selenomethionine, selenium-enriched yeast, selenite and selenate; selenium-only supplements typically contain 100 to 400 mcg; Keshan disease, an endemic cardiomyopathy first identified in 1935 in low-selenium parts of China, fell dramatically after selenium intervention trials in the 1970s-1990s; in 2017 the American Thyroid Association issued a weak recommendation against selenium supplements for TPOAb-positive pregnant women (fact sheet). ods.od.nih.gov/factsheets/Selenium-HealthProfessional
- Rayman, M. P. (2012). Selenium and human health. The Lancet, 379(9822), 1256–1268. 10.1016/S0140-6736(11)61452-9
- Stoffaneller, R., & Morse, N. L. (2015). A review of dietary selenium intake and selenium status in Europe and the Middle East. Nutrients, 7(3), 1494-1537. 10.3390/nu7031494
- Combs, G. F. (2015). Biomarkers of selenium status. Nutrients, 7(4), 2209-2236. 10.3390/nu7042209
- Steinbrenner, H., Al-Quraishy, S., Dkhil, M. A., Wunderlich, F., & Sies, H. (2015). Dietary selenium in adjuvant therapy of viral and bacterial infections. Advances in Nutrition, 6(1), 73-82. 10.3945/an.114.007575
- National Institutes of Health, Office of Dietary Supplements. (2022). Iodine — Fact Sheet for Health Professionals. Fact sheet (updated November 5, 2024; Wayback snapshot 16 September 2026): commercial seaweeds range from 16 to 2,984 mcg iodine per gram; dried nori, 2 tablespoons flaked (5 g), 116 mcg; US iodized salt is labelled 45 mcg iodine per gram (measured 47.5-50.7); two RCTs giving 150 or 200 mcg/day iodine from early pregnancy to delivery found no effect on child cognitive, language or motor scores at 1.5-2 years, and one found no benefit at 5-6 years either (fact sheet). ods.od.nih.gov/factsheets/Iodine-HealthProfessional
- Winther, K. H., Rayman, M. P., Bonnema, S. J., & Hegedüs, L. (2020). Selenium in thyroid disorders — essential knowledge for clinicians. Nature Reviews Endocrinology, 16(3), 165-176. 10.1038/s41574-019-0311-6
- Toulis, K. A., Anastasilakis, A. D., Tzellos, T. G., Goulis, D. G., & Kouvelas, D. (2010). Selenium supplementation in the treatment of Hashimoto's thyroiditis: a systematic review and a meta-analysis. Thyroid, 20(10), 1163-1173. Pooled 4 RCTs in Hashimoto's (mostly on levothyroxine): selenium for 3 months lowered anti-TPO titers (weighted mean difference -271, no percentage) and more often improved reported well-being/mood; levothyroxine needs and thyroid ultrasound were unaltered or under-reported (abstract, PMID 20883174). 10.1089/thy.2009.0351
- Vinceti, M., Filippini, T., Del Giovane, C., et al. (2018). Selenium for preventing cancer. Cochrane Database of Systematic Reviews, 1, CD005195. 10.1002/14651858.CD005195.pub4
- Lippman, S. M., et al. (2009). Effect of selenium and vitamin E on risk of prostate cancer (SELECT): a randomized controlled trial. JAMA, 301(1), 39–51. 35,533 men (African American 50 or older, others 55 or older; PSA 4 ng/mL or less) randomised to selenium 200 micrograms/day (L-selenomethionine), vitamin E 400 IU/day, both, or placebo. Median follow-up 5.46 years at the 23 October 2008 data cut. Prostate cancer HR (99% CI): vitamin E 1.13 (0.95-1.35), selenium 1.04, combination 1.05 - none significant; no other prespecified cancer end point differed. Type 2 diabetes with selenium RR 1.07 (0.94-1.22), P = .16, not significant (abstract, PMID 19066370). 10.1001/jama.2008.864
- Klein, E. A., Thompson, I. M., Tangen, C. M., Crowley, J. J., Lucia, M. S., Goodman, P. J., et al. (2011). Vitamin E and the risk of prostate cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA, 306(14), 1549-1556. Among 35,533 men, vitamin E 400 IU/day significantly increased prostate cancer risk (about 17% relative increase; 76 vs 65 cases per 1,000). 10.1001/jama.2011.1437