Home Ingredients Selenomethionine
AntioxidantThyroid Health

Selenomethionine

Evidence Level
Moderate
3 Clinical Trials
4 Documented Benefits
3/5 Evidence Score

Selenomethionine is the main form of selenium in food, where selenium takes the place of sulfur in the amino acid methionine; Brazil nuts, seafood and meat are rich sources. Supplements sell it as L-selenomethionine. Kinetic studies show it is absorbed better than sodium selenite and partly stored in body proteins, so blood selenium rises steadily with daily use. In adults with very low selenium intake it restored the antioxidant enzyme glutathione peroxidase at about half the dose of selenite. Thyroid research is mixed: a trial in pregnant women with thyroid antibodies found less thyroid dysfunction after delivery, while a 6-month trial in people with autoimmune thyroiditis found no difference from placebo. In SELECT, 200 µg/day did not prevent cancer in 35,533 men, and type 2 diabetes was slightly but not significantly more common.

Studied Dose Thyroid trials used 166 to 200 µg/day; SELECT used 200 µg/day. In low-selenium adults, 37 µg/day restored glutathione peroxidase. The adult RDA is 55 µg/day and total selenium should stay below the adult upper limit of 400 µg/day.
Active Compound L-selenomethionine (selenium bound in the amino acid methionine).

Benefits

Thyroid function and thyroid antibody levels

In a placebo-controlled trial of 151 pregnant women with thyroid peroxidase antibodies, 200 µg/day of selenomethionine in and after pregnancy was followed by less postpartum thyroid dysfunction (28.6 versus 48.6 percent). A 6-month trial of 166 µg/day in 76 people with untreated autoimmune thyroiditis found no difference from placebo. The American Thyroid Association advises against selenium in pregnant women with thyroid antibodies.

Supported by Trial 1, Trial 2

Antioxidant enzyme glutathione peroxidase in low-selenium adults

In 120 adults with very low selenium intake, selenomethionine restored full plasma glutathione peroxidase activity at 37 µg/day, versus 66 µg/day for sodium selenite, giving it nearly twice the bioavailability. In selenium-replete men, extra selenomethionine raised blood selenium but had little effect on this enzyme.

Supported by Trial 3

Raises blood selenium

In 28 healthy adults taking 200 µg/day, plasma selenium rose about 60 percent in men and roughly doubled in women by 9 months. Higher blood selenium does not always mean more selenoprotein activity: in people with cirrhosis, selenate raised glutathione peroxidase activity but selenomethionine did not, and in low-selenium adults neither form fully restored selenoprotein P.

Supported by Trial 3, Trial 1

Supplies selenium for selenoproteins

Selenium is part of 25 selenoproteins, including glutathione peroxidases, thioredoxin reductases and selenoprotein P, which take part in thyroid hormone metabolism, DNA synthesis, reproduction and protection from oxidative damage. Selenomethionine supplies selenium for these proteins once the body breaks it down.

Supported by Trial 3, Trial 1

Mechanism of action

1

Methionine replacement in body proteins

Selenomethionine can be built into body proteins in place of methionine. This creates a selenium store that is released as proteins are broken down, which is why blood selenium keeps rising with daily use.

2

Selenoprotein P synthesis

The liver converts selenium from selenomethionine to selenide, which is used to make selenocysteine for selenoproteins. The liver secretes selenoprotein P into the blood to supply selenium to other tissues.

3

Glutathione peroxidase function

Glutathione peroxidases contain selenium at their active site and break down hydrogen peroxide and lipid peroxides. Full activity depends on adequate selenium.

4

Iodothyronine deiodinases

Selenium-containing deiodinase enzymes convert the thyroid hormone T4 into active T3 and also inactivate thyroid hormones, one reason selenium matters for thyroid hormone metabolism.

Clinical trials

1
Selenomethionine in pregnant women with thyroid peroxidase antibodies
PubMed

Prospective, randomized, placebo-controlled trial: 200 µg/day of selenomethionine or placebo during pregnancy and after delivery (Negro et al. 2007, J Clin Endocrinol Metab)

151 euthyroid pregnant women positive for thyroid peroxidase antibodies (77 selenomethionine, 74 placebo), plus 81 antibody-negative women as a reference group.

Postpartum thyroid dysfunction occurred in 28.6 percent of the selenomethionine group versus 48.6 percent on placebo, and permanent hypothyroidism in 11.7 versus 20.3 percent. Trials of selenium in pregnancy have been mixed overall, and the American Thyroid Association recommends against selenium supplements for pregnant women with thyroid autoimmunity.

2
L-selenomethionine in untreated euthyroid autoimmune thyroiditis (no difference from placebo)
PubMed

Blinded, placebo-controlled, randomized prospective trial: 166 µg/day of L-selenomethionine or placebo for 6 months (Esposito et al. 2017, J Endocrinol Invest)

76 euthyroid adults with Hashimoto thyroiditis not taking thyroid medication (38 per group).

TSH, free T4, free T3, thyroid peroxidase antibodies, thyroid echogenicity and CXCL10 did not differ between groups at 3 or 6 months. Within the selenium group free T4 fell and free T3 rose from baseline. The authors concluded that short-term supplementation had a limited impact on the course of the condition.

3
Selenomethionine versus selenite in low-selenium adults
PubMed

Randomized supplementation trial: placebo or up to 66 µg/day of selenium as selenomethionine or sodium selenite for 20 weeks (Xia et al. 2005, Am J Clin Nutr)

120 adults in a low-selenium area of China with average intake of about 10 µg/day.

Full plasma glutathione peroxidase expression needed 37 µg/day as selenomethionine versus 66 µg/day as selenite, so selenomethionine had nearly twice the bioavailability. Neither form fully expressed selenoprotein P at the doses tested.

Side effects and drug interactions

Common Potential side effects

Generally well tolerated at usual supplement doses.
Early signs of too much selenium are a garlic odor on the breath and a metallic taste in the mouth.
Chronically high intake causes selenosis: hair loss and brittle or lost nails, plus skin rash, nausea, diarrhea, fatigue, irritability and nervous system problems. The adult upper limit for total selenium is 400 µg/day.
Misformulated products can cause acute poisoning: in 2008, 201 people had severe reactions to a liquid supplement that contained 200 times the labeled amount of selenium.
Type 2 diabetes: in SELECT, which gave 200 µg/day of L-selenomethionine to men for a median 5.5 years, type 2 diabetes was slightly more common with selenium but the difference was not statistically significant (relative risk 1.07, 99% CI 0.94 to 1.22). SELECT also recorded more alopecia and mild dermatitis with selenium. People with diabetes or prediabetes should talk to a doctor before taking 200 µg/day.

Important Drug interactions

Cisplatin: this chemotherapy drug can lower selenium levels in hair and serum, and whether that matters clinically is not known. People on chemotherapy should ask their oncologist before taking selenium.
Other selenium sources: multivitamins, selenium yeast and Brazil nuts all add to total intake, which should stay below 400 µg/day.
Thyroid medication: selenium takes part in thyroid hormone metabolism, so people treated for a thyroid condition should tell their doctor before starting it.

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Frequently asked questions about Selenomethionine

What is selenomethionine?

Selenomethionine is the main form of selenium in food, where selenium takes the place of sulfur in the amino acid methionine. It is well absorbed, and the body stores part of it in proteins.

Is selenomethionine the best form of selenium?

It is well absorbed, and the body stores part of it in proteins, which raises blood selenium steadily. Higher blood selenium does not always mean more selenoprotein activity, though: in one trial in people with cirrhosis, selenate raised an antioxidant selenoprotein and selenomethionine did not.

How much selenomethionine should I take?

The selenium RDA is 55 mcg per day; supplements often provide 100 to 200 mcg. Keep total selenium under 400 mcg per day from all sources, since selenium has a narrow safe range and excess is toxic.

Is selenomethionine safe?

At normal amounts it is safe and well tolerated. Because it is stored in the body, very high long-term intake can build up and cause selenosis (hair loss, brittle nails, garlic breath), so stay within recommended limits.

What is Selenomethionine used for?

Selenomethionine is researched primarily for Antioxidant and Thyroid Health. In a placebo-controlled trial of 151 pregnant women with thyroid peroxidase antibodies, 200 µg/day of selenomethionine in and after pregnancy was followed by less postpartum thyroid dysfunction (28.6 versus 48.6 percent).

Does Selenomethionine interact with any medications?

Possible interactions include: Cisplatin: this chemotherapy drug can lower selenium levels in hair and serum, and whether that matters clinically is not known. People on chemotherapy should ask their oncologist before taking selenium. If you take prescription medication, check with a pharmacist or doctor before using it.

How strong is the scientific evidence for Selenomethionine?

NutraSmarts rates the evidence for Selenomethionine as Moderate (3 out of 5). It is backed by 3 study summaries 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. Patterson BH, Combs GF Jr, Taylor PR, Patterson KY, Moler JE, Wastney ME. Selenium Kinetics in Humans Change Following 2 Years of Supplementation With Selenomethionine. Front Endocrinol (Lausanne). 2021;12:621687. doi: 10.3389/fendo.2021.621687.PubMedUsed to support: 31 adults were studied with oral selenomethionine and selenite tracers before and after 2 years of 200 µg/day of selenomethionine. Fractional absorption did not change, the amount absorbed rose, and urinary excretion did not account for all of it, so selenomethionine was incorporated into body pools. This describes selenium handling, not health outcomes.
  2. Marshall JR, Burk RF, Payne Ondracek R, Hill KE, Perloff M, Davis W, Pili R, George S, Bergan R. Selenomethionine and methyl selenocysteine: multiple-dose pharmacokinetics in selenium-replete men. Oncotarget. 2017;8(16):26312-26322. doi: 10.18632/oncotarget.15460.PubMedUsed to support: 29 selenium-replete men were randomized to selenomethionine or methylselenocysteine for 84 days. Selenomethionine raised blood selenium more than methylselenocysteine, but neither form had more than a minimal effect on selenoprotein P or glutathione peroxidase, and no toxicity was seen.
  3. Burk RF, Hill KE, Motley AK, Byrne DW, Norsworthy BK. Selenium deficiency occurs in some patients with moderate-to-severe cirrhosis and can be corrected by administration of selenate but not selenomethionine: a randomized controlled trial. Am J Clin Nutr. 2015;102(5):1126-33. doi: 10.3945/ajcn.115.110932.PubMedUsed to support: Patients with cirrhosis received placebo, selenate (200 or 400 µg/day) or selenomethionine (200 µg/day) for 4 weeks. In moderate cirrhosis, both selenate doses raised glutathione peroxidase activity more than placebo, but selenomethionine did not, suggesting impaired selenomethionine metabolism in severe liver disease.
  4. Kokarnig S, Tsirigotaki A, Wiesenhofer T, Lackner V, Francesconi KA, Pergantis SA, Kuehnelt D. Concurrent quantitative HPLC-mass spectrometry profiling of small selenium species in human serum and urine after ingestion of selenium supplements. J Trace Elem Med Biol. 2015;29:83-90. doi: 10.1016/j.jtemb.2014.06.012.PubMedUsed to support: Volunteers took selenate, selenite, selenomethionine, methylselenocysteine or selenized yeast, and selenium species were measured in serum for 5 hours and urine for 24 hours. Selenosugar 1 was the main urinary metabolite for most forms. This characterizes metabolism, not health outcomes.
  5. 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: A blinded, placebo-controlled trial in 70 women with autoimmune thyroiditis on levothyroxine. Note the form: patients took 200 µg/day of sodium selenite, not selenomethionine. Thyroid peroxidase antibodies fell to 63.6 percent of baseline versus 88 percent with placebo, and ultrasound echogenicity normalized in some patients.
  6. Xia Y, Hill KE, Byrne DW, Xu J, Burk RF. Effectiveness of selenium supplements in a low-selenium area of China. Am J Clin Nutr. 2005;81(4):829-34. doi: 10.1093/ajcn/81.4.829.PubMedUsed to support: 120 adults in a low-selenium area of China (intake about 10 µg/day) took placebo or up to 66 µg/day of selenium as selenomethionine or selenite for 20 weeks. Full glutathione peroxidase expression needed 37 µg/day as selenomethionine versus 66 µg/day as selenite, nearly twice the bioavailability; selenoprotein P was not fully expressed by either form. The participants were deficient, unlike most supplement users.
  7. Negro R, Greco G, Mangieri T, Pezzarossa A, Dazzi D, Hassan H. The influence of selenium supplementation on postpartum thyroid status in pregnant women with thyroid peroxidase autoantibodies. J Clin Endocrinol Metab. 2007;92(4):1263-8. doi: 10.1210/jc.2006-1821.PubMedUsed to support: In 151 euthyroid pregnant women with thyroid peroxidase antibodies, 200 µg/day of selenomethionine during pregnancy and after delivery was followed by less postpartum thyroid dysfunction (28.6 versus 48.6 percent) and less permanent hypothyroidism (11.7 versus 20.3 percent) than placebo.
  8. Esposito D, Rotondi M, Accardo G, Vallone G, Conzo G, Docimo G, Selvaggi F, Cappelli C, Chiovato L, Giugliano D, Pasquali D. Influence of short-term selenium supplementation on the natural course of Hashimoto's thyroiditis: clinical results of a blinded placebo-controlled randomized prospective trial. J Endocrinol Invest. 2017;40(1):83-89. doi: 10.1007/s40618-016-0535-4.PubMedUsed to support: In 76 untreated euthyroid patients with Hashimoto thyroiditis, 166 µg/day of L-selenomethionine for 6 months did not change TSH, thyroid hormones, thyroid peroxidase antibodies, echogenicity or CXCL10 compared with placebo.
  9. Combs GF Jr, Midthune DN, Patterson KY, Canfield WK, Hill AD, Levander OA, Taylor PR, Moler JE, Patterson BH. Effects of selenomethionine supplementation on selenium status and thyroid hormone concentrations in healthy adults. Am J Clin Nutr. 2009;89(6):1808-14. doi: 10.3945/ajcn.2008.27356.PubMedUsed to support: 28 healthy adults took 200 µg/day of selenomethionine for 28 months. By 9 months plasma selenium rose from 1.78 to 2.85 µmol/L in men and from 1.64 to 3.32 µmol/L in women. T4 and TSH were unchanged and T3 rose 5 percent per year in men only. No placebo group.
  10. Lippman SM, Klein EA, Goodman PJ, Lucia MS, Thompson IM, Ford LG, Parnes HL, Minasian LM, Gaziano JM, Hartline JA, Parsons JK, Bearden JD 3rd, Crawford ED, Goodman GE, Claudio J, Winquist E, Cook ED, Karp DD, Walther P, Lieber MM, Kristal AR, Darke AK, Arnold KB, Ganz PA, Santella RM, Albanes D, Taylor PR, Probstfield JL, Jagpal TJ, Crowley JJ, Meyskens FL Jr, Baker LH, Coltman CA Jr. Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2009;301(1):39-51. doi: 10.1001/jama.2008.864.PubMedUsed to support: In 35,533 relatively healthy men, 200 µg/day of selenium from L-selenomethionine did not reduce prostate or other prespecified cancers. Type 2 diabetes was slightly but not significantly more common with selenium (relative risk 1.07, 99% CI 0.94 to 1.22), and alopecia and grade 1 to 2 dermatitis were more frequent.
  11. Office of Dietary Supplements, National Institutes of Health. Selenium: Fact Sheet for Health Professionals (updated September 4, 2025). NIH Office of Dietary Supplements. 2025;Not PubMed-indexed..SourceUsed to support: Official NIH fact sheet. States that in foods selenium is present mainly as selenomethionine; that selenium is part of 25 selenoproteins (including glutathione peroxidases, thioredoxin reductases and selenoprotein P) that take part in thyroid hormone metabolism, DNA synthesis, reproduction and protection from oxidative damage; that the adult RDA is 55 µg/day and the adult upper limit 400 µg/day; that early signs of excess are garlic breath and a metallic taste, and chronic excess (selenosis) causes hair loss and brittle or lost nails, rash, nausea, diarrhea, fatigue, irritability and nervous system problems; that most people in the United States get enough selenium; that trials of selenium in pregnant women have been mixed and the American Thyroid Association recommends against selenium supplements for pregnant women with thyroid autoimmunity; that in 2008, 201 people had severe reactions to a liquid supplement containing 200 times the labeled amount of selenium; and that cisplatin can lower selenium levels in hair and serum, with unknown clinical significance. Not PubMed-indexed.