What Is Selenomethionine?

Important for Thyroid Function, Reproduction, and DNA Production

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.

Top view of wooden spoon with brazil nuts on it
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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 the possible benefits of selenomethionine and some of the research on selenium's health impacts. It will also help you know what to look for if you decide to try a SeMet product.

Dietary supplements are not regulated like drugs in the United States, meaning the Food and Drug Administration (FDA) does not approve them for safety and effectiveness before products are marketed. When possible, choose a supplement that has been tested by a trusted third party, such as USP, ConsumerLabs, or NSF. However, even if supplements are third-party tested, that doesn’t mean that they are necessarily safe for all or effective in general. It is important to talk to your healthcare provider about any supplements you plan to take and to check in about any potential interactions with other supplements or medications.

Supplement Facts

  • Active ingredient: Selenium
  • Alternate names: Selenium, SeMet
  • Legal status: Legal to sell over the counter (OTC)
  • Suggested dose: Adults who are not pregnant or breastfeeding should take around 55 micrograms (mcg) per day
  • Safety considerations: Taking too much selenium long-term can cause selenium poisoning

Uses of Selenomethionine

Supplement use should be individualized and vetted by a healthcare professional, such as a registered dietitian, pharmacist, or doctor. No supplement is intended to treat, cure, or prevent disease.

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.

Thyroid Disease

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 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 attack 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 damages the heart muscle, leading to heart failure and other heart problems. The disease 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.


Selenomethionine does not seem to help with cancer prevention.

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 form of 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 to support a role for selenium—in SeMet form or otherwise—for treating it.

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

What Are The Side Effects of Selenomethionine?

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.

Some people may experience mild side effects when taking selenomethionine at lower doses. These may include:

  • Stomach discomfort
  • Headache
  • Rash

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


Selenomethionine can be toxic at high doses, especially when taken long-term. Taking lower doses for long periods of time may increase your risk of developing diabetes.

Dosage: How Much Selenomethionine Should I Take?

Always speak with a healthcare provider before taking a supplement to ensure that the supplement and dosage are appropriate for your individual needs. 

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 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 Happens If I Take Too Much Selenomethionine?

Taking too much selenium for long periods of time can cause selenosis. This is the medical term for selenium poisoning. People with selenosis may experience symptoms such as:

  • Garlic breath odor
  • Metallic taste in the mouth
  • Nausea
  • Diarrhea
  • Fatigue
  • Irritability
  • Hair and nail loss
  • Skin lesions

Selenosis can also cause nerve damage. People with severe selenosis may experience:

  • Tremors
  • Respiratory distress
  • Kidney failure
  • Heart failure
  • Death


Selenium may increase the risk of bleeding or bruising if you take it with blood thinners such as Coumadin (warfarin) or heparin. It may have a similar effect if you take it with aspirin.

Selenium may increase or extend the length of time that barbiturates like Butisol (butabarbital) or Luminal (phenobarbital) affect you.

Talk to your healthcare provider if you are taking Zocor (simvastatin) with niacin to help raise your HDL ("good" cholesterol) levels. Selenium and other antioxidants may make this combination less effective. 

Selenium may also make certain chemotherapy drugs and immunosuppressive medication less effective.

How to Store Selenomethionine

Selenium should be kept in its original container at room temperature. Do not expose it to heat, moisture, sunlight, or freezing temperatures. 

Similar Supplements

Several other supplements have the same functions as selenomethionine and are possible alternatives:

  • Myo-inositol: This compound helps regulate hormones including insulin and thyroid-stimulating hormone. It is sometimes taken in combination with selenium to help treat hypothyroidism. 
  • Iodine: Your body needs iodine in order to produce thyroid hormones. However, it is possible to get too much iodine in your diet, which can also cause problems with your thyroid. Since iodine deficiency is uncommon in the United States, most healthcare providers don't recommend iodine supplementation.
  • Omega-3 fatty acids: These supplements may help reduce the risk of colon cancer, though so far this evidence only comes from animal studies. Because a diet rich in omega-3 fatty acids can help reduce inflammation, these supplements may also be used to help treat thyroid conditions.

Frequently Asked Questions

  • What is the difference between selenomethionine and selenium?

    Selenomethionine is a natural form of selenium. It is the same form of selenium found naturally in the foods you eat. Seafood, nuts, and meat are all good dietary sources selenomethionine.

  • What does selenium do in your body?

    Your body uses selenium to help build DNA and protect your cells against damage. It is important for metabolism and thyroid health. However, it is also possible to have too much selenium in your body. High doses can cause selenium poisoning. 

Sources of Selenomethionine and What To Look For

Most people can get enough selenium from the foods they eat. You may have trouble absorbing selenium, however, if you are on dialysis, if you have a GI condition such as Crohn's disease, or if you have HIV. In these cases, a selenium supplement may be helpful.

Food Sources of Selenomethionine

Foods high in selenium include:

  • Brazil nuts
  • Yellowfin tuna
  • Oysters and clams
  • Sardines
  • Shrimp
  • Salmon 
  • Halibut
  • Ham
  • Enriched breads and cereal
  • Pork, beef, Turkey, and chicken
  • Milk, yogurt, and cottage cheese

Selenomethionine Supplements

SeMet is a common form of selenium. SeMet supplements are most often available in the form of tablets or capsules.

Remember, though, that selenomethionine and other supplements are not regulated by the 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 the ingredients and amounts listed on the label are accurate.


Selenomethionine is present in many foods you eat, but it's also sold as a nutritional supplement. While SeMet may have health benefits, notably for thyroid function, researchers still have not shown clear 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 risks and benefits before you decide to add SeMet to your diet and nutrition plans.

10 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  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

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Additional Reading

By Brittany Poulson, MDA, RDN, CD, CDCES
Brittany Poulson, MDA, RDN, CDCES, is a registered dietitian and certified diabetes care and education specialist.