What Is Selenomethionine?

Important for Thyroid Function, Reproduction, and DNA Production

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Selenomethionine (SeMet) is one of the main natural forms of selenium. This nutrient is found in many foods, including nuts, whole grains, meat, eggs, and dairy.

As a form of selenium, SeMet is important for normal thyroid gland function. It plays a role in reproduction, DNA production, and protecting the body from infection. It also has been studied for heart and other health benefits.

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Selenomethionine combines with proteins in the body to form antioxidants called selenoproteins. These compounds help protect against free radicals, the unstable molecules that can cause damage to cells in your body.

This article looks at possible benefits of selenomethionine and some of the research on selenium's health impacts. It also will help you know what to look for if you decide to try a SeMet product.

What Is Selenomethionine Used For?

Much of the research on SeMet has centered around thyroid disease, cancer, and heart disease prevention. It also has been studied for its role in mental health and how it may prevent cognitive decline.


Selenium levels in the human body are highest in the thyroid gland. The mineral plays a key role in making thyroid hormone and in its metabolism. Because of this, selenomethionine has been studied for its effects on diseases of the thyroid.

One study looked at 192 people who had hypothyroidism, or mildly low and symptom-free thyroid levels because of Hashimoto’s disease. For the study, participants were given either 83 micrograms (mcg) of oral SeMet or a placebo (sham treatment) every day for four months.

At the end of the study, 31.3% of people who took SeMet had restored thyroid function. That number was just 3.1% in the placebo group.

Another study looked at how children and teens with autoimmune thyroiditis would respond to SeMet. Seventy-one people were given 200 mcg of selenomethionine or a placebo every day for six months.

At the end of the study, the SeMet group showed a higher reduction in immune proteins (antibodies) that go after thyroglobulin (Tg), a protein made by the thyroid, than the placebo group.

Another selenomethionine study looked at whether the supplements helped protect against thyroid autoimmunity during and after pregnancy. In it, 45 women who had thyroiditis in pregnancy were given either SeMet or a placebo. They were evaluated at around 10 weeks gestation, at 36 weeks gestation, and about six months after delivery. 

There were no real differences between the groups at the first check-in. However, the researchers saw a notable rise in selenium blood levels in the SeMet group at the second evaluation. There was also a decrease in autoantibodies, an antibody type often made by the body after an infection, following the baby's delivery in the selenomethionine group.

Based on these three studies, it seems that SeMet may help with some thyroid conditions in certain groups of people, including adults, pregnant women, children, and adolescents.

What Is Keshan Disease?

Keshan disease is the only known disorder linked directly to low levels of selenium. It is often found in places with low soil levels of selenium, meaning the mineral levels in food grown there may be lower too. Keshan disease was discovered in 1935 in China, where it continues to cause fatal heart problems in children and younger women today.


The Adenomatous Colorectal Polyp (ACP) study was designed to see whether SeMet or vitamin E lowered the rate of colon and rectal cancers, as well as adenomas—tumors, or abnormal growths, that can lead to these cancers.

The study, based on a subset of people in a wider research program, included 2,286 men with one or more of these cancer-related adenomas who were treated for seven to 12 years.

The colorectal study subset was divided into four groups: those who got only selenium, those who got only vitamin E, those who got both, and those who got neither and had only placebos.

In the men who got selenomethionine, 34.2% were diagnosed with adenomas at least one year later, compared to 35.7% in the group given a placebo. This was not a significant difference and led to a conclusion that taking SeMet had no effect on colon cancer prevention for these people.

A 2018 review of previous studies also found that taking selenium, including in the form of selenomethionine, did not have an impact on any type of cancer risk.

Heart Disease

A 2015 review found that when selenium intake was higher, heart disease risk was lower. Yet the researchers were unable to tell if it was selenium alone that actually prevented heart disease or if there were other factors at work.

They also reported that SeMet is one of the best organic ways to boost selenium levels in the body. However, that selenium is not the most efficient in terms of how well it is processed and used by the body. The data did not support the use of selenomethionine for preventing heart disease, especially in healthy people who get enough selenium from their diet.

More clinical trials are needed to better understand the SeMet-heart risk connection.

Cognitive Function

Selenium levels in the body often decline with age. Because of this, low selenium may be linked to age-related cognitive declines, possibly because of the subsequent reduction in its impact as an antioxidant. However, study results are mixed.

Alzheimer's disease is often diagnosed in older people and leads to progressive cognitive decline. Yet there is no clear evidence for selenium’s role—in SeMet form or otherwise—when treating it.

Selenium may have a proven cognitive decline prevention benefit one day, but more studies are needed.


SeMet supplements are a good way to boost selenium in the diet. Adding selenium to the diet may offer thyroid, cancer prevention, heart health, and other benefits. The research results remain mixed, however, and there is not enough evidence to support recommended uses just yet.

Possible Side Effects

The upper dietary intake level for selenium is 400 mcg per day for adults.

Generally, side effects will only occur when there is too much selenium in the body. This is rare when taking supplements as directed, but it can happen if the doses you take are too high.

Signs and symptoms of too much selenium in the body may include:

  • Garlic breath odor
  • Metallic taste in the mouth
  • Hair loss
  • Brittle finger and toe nails
  • Nausea
  • Diarrhea
  • Fatigue
  • Irritability
  • Nerve damage

Always talk with your healthcare provider before taking any dietary supplement, including SeMet. Be sure to follow the directions on the product label.

Dosage and Preparation

There are no recommended SeMet dosages. The supplements are most often available in the form of tablets or capsules, with usual dosage amounts of 200 micrograms.

The National Academies of Sciences sets the Dietary Reference Intakes (DRIs) for vitamins and minerals, including selenium. The DRIs for selenium are based on age, as well as life stage, including people who are pregnant or breastfeeding. The DRIs include your intake from all sources, such as food, drinks, and the supplements.

 Age/Stage  DRI for Selenium
 1 to 3 years  20 mcg/day
 4 to 8 years  30 mcg/day
 9 to 13 years  40 mcg/day
 14+ years  55 mcg/day
 Pregnancy  60 mcg/day
 Breastfeeding  70 mcg/day

What to Look For

SeMet is a common form of selenium. Remember, though, that selenomethionine and other supplements are not regulated by the U.S. Food and Drug Administration (FDA) or any other government agency. The quality of the products and how effective they are varies greatly because of this.

Simply reading the label may not provide enough information to make a well-informed buying decision about SeMet. It’s best to ask your healthcare provider for help in choosing the right supplement for you. They can also advise you on the best dosage for you.

As a general rule, look for products certified by a third party, such as U.S. Pharmacopeia, NSF International, or Consumer Lab. This ensures quality and that what's listed on the label is actually what's in the bottle.


Selenomethionine is present in many foods you eat, but it's also sold as a nutritional supplement. While SeMet may have health benefits, notably in thyroid care, researchers still have not shown a clear link to benefits for cancer, heart disease, or cognitive decline.

The supplements are generally safe to take. Still, be sure to ask your healthcare provider about your own health issues before you decide to add SeMet to your diet and nutrition plans.

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  1. Pirola I, Gandossi E, Agosti B, Delbarba A, Cappelli C. Selenium supplementation could restore euthyroidism in subclinical hypothyroid patients with autoimmune thyroiditisEndokrynol Pol. 2016;67(6):567-571. doi:10.5603/EP.2016.0064

  2. Kyrgios I, Giza S, Kotanidou EP, et al. l-selenomethionine supplementation in children and adolescents with autoimmune thyroiditis: A randomized double-blind placebo-controlled clinical trialJ Clin Pharm Ther. 2019;44(1):102-108. doi:10.1111/jcpt.12765

  3. Mantovani G, Isidori AM, Moretti C, et al. Selenium supplementation in the management of thyroid autoimmunity during pregnancy: results of the "SERENA study", a randomized, double-blind, placebo-controlled trialEndocrine. 2019;66(3):542-550. doi:10.1007/s12020-019-01958-1

  4. Shi Y, Yang W, Tang X, Yan Q, Cai X, Wu F. Keshan disease: a potentially fatal endemic cardiomyopathy in remote mountains of chinaFront Pediatr. 2021;9:576916. doi: 10.3389/fped.2021.576916

  5. Lance P, Alberts DS, Thompson PA, et al. Colorectal adenomas in participants of the SELECT randomized trial of selenium and vitamin E for prostate cancer preventionCancer Prev Res (Phila). 2017;10(1):45-54. doi:10.1158/1940-6207.CAPR-16-0104

  6. Vinceti M, Filippini T, Del Giovane C, et al. Selenium for preventing cancerCochrane Database Syst Rev. 2018;1(1):CD005195. doi:10.1002/14651858.CD005195.pub4

  7. Benstoem C, Goetzenich A, Kraemer S, et al. Selenium and its supplementation in cardiovascular disease--what do we know?Nutrients. 2015;7(5):3094-3118. doi:10.3390/nu7053094

  8. Robberecht H, De Bruyne T, Davioud-Charvet E, Mackrill J, Hermans N. Selenium status in elderly people: Longevity and age-related diseasesCurr Pharm Des. 2019;25(15):1694-1706. doi:10.2174/1381612825666190701144709

  9. Loef M, Schrauzer GN, Walach H. Selenium and Alzheimer's disease: A systematic reviewJ Alzheimers Dis. 2011;26(1):81-104. doi:10.3233/JAD-2011-110414

  10. Institute of Medicine. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, DC: The National Academies Press. 2000. doi:10.17226/9810.

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