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Sodium Selenite

Evidence Level
Limited
4 Clinical Trials
4 Documented Benefits
2/5 Evidence Score

Sodium selenite (Na2SeO3) is an inexpensive inorganic form of selenium, about 45% selenium by weight, and one of the forms commonly used in dietary supplements. Selenium is built into about 25 selenoproteins, including glutathione peroxidases, which help protect cells from oxidative damage, and enzymes involved in thyroid hormone metabolism. In a small New Zealand study, selenite raised blood glutathione peroxidase activity, but in people who already got enough selenium it did not raise plasma selenium or this enzyme. Thyroid results are mixed: one trial in women with autoimmune thyroiditis lowered thyroid antibodies, while three other selenite trials found no change, and TSH and thyroid hormone levels did not change where reported. Compared with selenomethionine, selenite raises blood selenium less because it is not stored in body proteins. Keep total selenium intake under 400 mcg a day.

Studied Dose Adult RDAs for selenium range from 55 to 70 mcg/day depending on life stage. Thyroid trials used 200 mcg/day selenium as sodium selenite; a New Zealand trial used 100 mcg/day. Upper limit: 400 mcg/day from all sources.
Active Compound Sodium selenite (Na2SeO3), an inorganic selenium salt providing roughly 45% selenium by weight.

Benefits

Antioxidant selenoprotein support

Selenium from selenite is built into glutathione peroxidases, enzymes that help protect cells from oxidative damage. In a New Zealand study of 12 adults given 100 mcg/day of selenium as selenite or selenomethionine for 17 weeks, blood glutathione peroxidase activity rose with both forms. In people who already had enough selenium, selenite did not raise this enzyme further.

Supported by Trial 4, Trial 2

Thyroid hormone metabolism support

Selenoproteins help make and convert thyroid hormones. In 70 women with autoimmune thyroiditis on thyroid medication, 200 mcg/day of selenite for 3 months lowered thyroid peroxidase antibodies more than placebo, but three other selenite trials in adults, euthyroid women and children found no change in these antibodies. TSH and thyroid hormones did not change in the trials that reported them, so results are mixed.

Supported by Trial 2, Trial 3

Immune system support

Selenoproteins help protect the body from infection and oxidative damage, and adequate selenium intake supports normal immune function. In a 3-month trial in 36 adults with autoimmune thyroiditis, selenite did not change T cell cytokine patterns compared with placebo.

Supported by Trial 3, Trial 1

Raising selenium status

In New Zealand adults, 100 mcg/day of selenite raised blood glutathione peroxidase activity, while blood selenium levels tended to plateau rather than keep rising. In selenium-replete adults given about 200 to 600 mcg/day for 16 weeks, selenite did not raise plasma selenium, unlike selenomethionine.

Supported by Trial 4, Trial 1

Mechanism of action

1

Selenoprotein incorporation

Selenium is built into about 25 selenoproteins as the amino acid selenocysteine, including glutathione peroxidases, thioredoxin reductases and selenoprotein P, which support antioxidant defense.

2

Thyroid selenoprotein activity

Selenoproteins play key roles in thyroid hormone metabolism, including the conversion of the prohormone T4 into the active hormone T3, and help protect the thyroid from the hydrogen peroxide made during hormone production.

3

Not stored like selenomethionine

Selenomethionine can be built into body proteins in place of methionine, which gives the body a selenium store, so blood selenium keeps rising with continued use. Selenite is not stored this way, so blood selenium levels tend to plateau and are a poor guide to selenite intake.

4

Pro-oxidant effect at high concentrations

In lab studies, high concentrations of selenite generated superoxide in cells and triggered cell death through the mitochondria, a pro-oxidant effect that is one reason excess selenium is harmful.

Clinical trials

1
Sodium selenite and thyroid peroxidase antibodies
PubMed

Blinded, placebo-controlled, randomized trial of 200 mcg/day selenium as sodium selenite for 3 months; all participants took levothyroxine to keep TSH normal. (Gärtner et al. 2002, J Clin Endocrinol Metab)

70 women (mean age 47.5) with autoimmune thyroiditis and high thyroid antibodies.

Thyroid peroxidase antibodies fell to 63.6% of baseline on selenite vs 88% on placebo (primary outcome), with larger drops in those with the highest starting levels. TSH, free T4 and free T3 did not change in either group.

2
Sodium selenite in euthyroid women with thyroid antibodies
PubMed

Randomized, double-blind, placebo-controlled trial of 200 mcg/day sodium selenite for 6 months. (Eskes et al. 2014, Clin Endocrinol)

61 women with normal thyroid function, thyroid peroxidase antibodies of 100 kU/l or more and no thyroid medication.

Blood selenium and selenoprotein P rose on selenite, but thyroid peroxidase antibodies, TSH and quality of life did not change compared with placebo.

3
Sodium selenite in unselected autoimmune thyroiditis patients
PubMed

Placebo-controlled trial of 200 mcg/day sodium selenite for 3 months alongside levothyroxine. (Karanikas et al. 2008, Thyroid)

36 consecutive patients aged 19 to 85 with autoimmune thyroiditis.

Thyroid peroxidase antibodies did not change significantly with selenite (524 vs 505 IU/mL), and T cell cytokine patterns did not differ from placebo.

4
Chemical form of selenium and plasma biomarkers
PubMed

Randomized 16-week trial with 10 groups: placebo or about 200, 400 or 600 mcg/day selenium as sodium selenite, selenium yeast or selenomethionine. (Burk et al. 2006, Cancer Epidemiol Biomarkers Prev)

Selenium-replete adults in the United States.

Selenomethionine and yeast raised plasma selenium in a dose-dependent way; selenite did not. Glutathione peroxidase activity and selenoprotein P did not respond to any form, since participants already had enough selenium. Urinary data suggested selenomethionine was better absorbed than selenite.

Side effects and drug interactions

Common Potential side effects

Selenium has a narrow safety margin; long-term intake above the 400 mcg/day upper limit can cause selenosis.
Selenosis most often causes hair loss and brittle or lost nails, and can also cause garlic breath, a metallic taste, skin rash, nausea, diarrhea, fatigue, irritability and nervous system problems.
At high concentrations selenite can act as a pro-oxidant, generating superoxide in lab studies.
The American Thyroid Association recommends against selenium supplements for pregnant women with thyroid autoimmunity.

Important Drug interactions

Other selenium supplements and fortified foods: combined intake can exceed the 400 mcg/day upper limit.
Cisplatin and some other medicines can affect selenium levels; tell your oncologist about any selenium supplement.
Thyroid medication: in trials selenite was taken alongside levothyroxine without changes in TSH or thyroid hormone levels, but tell your doctor if you use both.

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Frequently asked questions about Sodium Selenite

What is sodium selenite?

Sodium selenite is an inexpensive inorganic form of selenium used in many supplements. Unlike selenomethionine, it is not stored in body proteins, so it raises blood selenium less.

Selenite or selenomethionine, which is better?

Both supply usable selenium. Selenomethionine appears better absorbed and builds up blood levels more, while selenite is cheaper and not stored. For meeting basic selenium needs either form can work.

How much sodium selenite should I take?

Adult RDAs for selenium range from 55 to 70 mcg per day depending on life stage. Thyroid trials used 200 mcg per day. Keep total selenium under 400 mcg per day from all sources, since selenium is toxic in excess.

Is sodium selenite safe?

At usual supplement amounts selenium is generally safe, but it has a narrow safe range. Long-term intake above 400 mcg per day can cause hair and nail loss and other signs of selenosis.

What is Sodium Selenite used for?

Sodium Selenite is researched primarily for Antioxidant and Thyroid Health. Selenium from selenite is built into glutathione peroxidases, enzymes that help protect cells from oxidative damage. In a New Zealand study of 12 adults given 100 mcg/day of selenium as selenite or selenomethionine for 17 weeks, blood gluta…

What is the recommended dosage of Sodium Selenite?

The clinically studied dose is Adult RDAs for selenium range from 55 to 70 mcg/day depending on life stage. Thyroid trials used 200 mcg/day selenium as sodium selenite; a New Zealand trial used 100 mcg/day. Upper limit: 400 mcg/day from all sources. Always follow the product label and check with a healthcare provider for personal advice.

Is Sodium Selenite safe, and does it have side effects?

For most healthy adults, Sodium Selenite is well tolerated at studied doses. Reported effects can include: Selenium has a narrow safety margin; long-term intake above the 400 mcg/day upper limit can cause selenosis. Selenosis most often causes hair loss and brittle or lost nails, and can also cause garlic breath, a metallic taste, skin rash, nausea, diarrhea, fatigue, irritability and… It may also interact with some medications. Sodium Selenite is not right for everyone, so check with a healthcare provider first if you are pregnant or breastfeeding, have a medical condition, or take prescription medication.

Does Sodium Selenite interact with any medications?

Possible interactions include: Other selenium supplements and fortified foods: combined intake can exceed the 400 mcg/day upper limit. Cisplatin and some other medicines can affect selenium levels; tell your oncologist about any selenium supplement. If you take prescription medication, check with a pharmacist or doctor before using it.

How strong is the scientific evidence for Sodium Selenite?

NutraSmarts rates the evidence for Sodium Selenite as Limited (2 out of 5). It is backed by 4 clinical trials and 11 cited references summarized on this page. A higher rating reflects more, larger, and better-designed human studies.

References(11 citations)

Evidence ratings on NutraSmarts are based on the totality of human clinical research, with emphasis on randomized controlled trials, meta-analyses, and systematic reviews. The references below directly support claims made throughout this page.

  1. Gärtner R, Gasnier BC, Dietrich JW, Krebs B, Angstwurm MW. Selenium supplementation in patients with autoimmune thyroiditis decreases thyroid peroxidase antibodies concentrations. J Clin Endocrinol Metab. 2002;87(4):1687-91. doi: 10.1210/jcem.87.4.8421.PubMedUsed to support: Randomized, blinded, placebo-controlled trial in 70 women with autoimmune thyroiditis on levothyroxine: 200 mcg/day selenium as sodium selenite for 3 months lowered thyroid peroxidase antibodies to 63.6% of baseline vs 88% on placebo; TSH and free thyroid hormones did not change.
  2. Burk RF, Norsworthy BK, Hill KE, Motley AK, Byrne DW. Effects of chemical form of selenium on plasma biomarkers in a high-dose human supplementation trial. Cancer Epidemiol Biomarkers Prev. 2006;15(4):804-10. doi: 10.1158/1055-9965.EPI-05-0950.PubMedUsed to support: Randomized 16-week trial in selenium-replete adults: selenomethionine and selenium yeast raised plasma selenium dose-dependently but selenite did not; glutathione peroxidase and selenoprotein P did not respond to any form, and selenomethionine appeared better absorbed than selenite.
  3. Davis TZ, Tiwary AK, Stegelmeier BL, Pfister JA, Panter KE, Hall JO. Comparative oral dose toxicokinetics of sodium selenite and selenomethionine. J Appl Toxicol. 2017;37(2):231-238. doi: 10.1002/jat.3350.PubMedUsed to support: Animal (lamb) study: equimolar oral selenomethionine was absorbed faster and gave higher blood selenium and greater bioavailability than sodium selenite, with a similar retention time.
  4. Eskes SA, Endert E, Fliers E, Birnie E, Hollenbach B, Schomburg L, Köhrle J, Wiersinga WM. Selenite supplementation in euthyroid subjects with thyroid peroxidase antibodies. Clin Endocrinol (Oxf). 2014;80(3):444-51. doi: 10.1111/cen.12284.PubMedUsed to support: Randomized, double-blind trial in 61 euthyroid women with thyroid peroxidase antibodies: 200 mcg/day sodium selenite for 6 months raised selenium status but did not change antibodies, TSH or quality of life versus placebo.
  5. Karanikas G, Schuetz M, Kontur S, Duan H, Kommata S, Schoen R, Antoni A, Kletter K, Dudczak R, Willheim M. No immunological benefit of selenium in consecutive patients with autoimmune thyroiditis. Thyroid. 2008;18(1):7-12. doi: 10.1089/thy.2007.0127.PubMedUsed to support: In 36 patients with autoimmune thyroiditis on levothyroxine, 200 mcg/day sodium selenite for 3 months did not change thyroid peroxidase antibodies or T cell cytokine patterns compared with placebo.
  6. Bonfig W, Gärtner R, Schmidt H. Selenium supplementation does not decrease thyroid peroxidase antibody concentration in children and adolescents with autoimmune thyroiditis. ScientificWorldJournal. 2010;10:990-6. doi: 10.1100/tsw.2010.91.PubMedUsed to support: In 49 children and teens with autoimmune thyroiditis on levothyroxine, adding 100 or 200 mcg/day sodium selenite for 12 months did not lower thyroid peroxidase antibodies.
  7. van Zuuren EJ, Albusta AY, Fedorowicz Z, Carter B, Pijl H. Selenium supplementation for Hashimoto's thyroiditis. Cochrane Database Syst Rev. 2013;2013(6):CD010223. doi: 10.1002/14651858.CD010223.pub2.PubMedUsed to support: Cochrane review of 4 trials (463 people) at unclear to high risk of bias: selenomethionine lowered thyroid antibodies in some trials, sodium selenite did not in one trial, and the evidence was judged too incomplete for confident decisions.
  8. Thomson CD, Robinson MF, Campbell DR, Rea HM. Effect of prolonged supplementation with daily supplements of selenomethionine and sodium selenite on glutathione peroxidase activity in blood of New Zealand residents. Am J Clin Nutr. 1982;36(1):24-31. doi: 10.1093/ajcn/36.1.24.PubMedUsed to support: In 12 New Zealand residents given 100 mcg/day selenium as selenite or selenomethionine, blood glutathione peroxidase activity rose in all; blood selenium rose more with selenomethionine and tended to plateau with selenite.
  9. Xiang N, Zhao R, Zhong W. Sodium selenite induces apoptosis by generation of superoxide via the mitochondrial-dependent pathway in human prostate cancer cells. Cancer Chemother Pharmacol. 2009;63(2):351-62. doi: 10.1007/s00280-008-0745-3.PubMedUsed to support: Cell study: sodium selenite generated superoxide in mitochondria and triggered cell death, showing its pro-oxidant action at high concentrations.
  10. NIH Office of Dietary Supplements. Selenium: Fact Sheet for Health Professionals. National Institutes of Health. 2025;Online fact sheet, updated September 4, 2025..SourceUsed to support: Not PubMed-indexed; official NIH fact sheet. Selenium is part of 25 selenoproteins with roles in thyroid hormone metabolism and protection from oxidative damage and infection; supplements include sodium selenite; adult RDAs of 55 to 70 mcg and an adult upper limit of 400 mcg; lists selenosis signs; notes mixed thyroid antibody trial results and the American Thyroid Association advice against selenium in pregnant women with thyroid autoimmunity.
  11. Schrauzer GN. The nutritional significance, metabolism and toxicology of selenomethionine. Adv Food Nutr Res. 2003;47:73-112. doi: 10.1016/s1043-4526(03)47002-2.PubMedUsed to support: Review: selenomethionine can be built into body proteins in place of methionine, giving a reversible selenium store, a property not shared by other natural selenium amino acids; notes that sodium selenite and selenate are not the normal food forms of selenium.