What Is Selenomethionine?

Important for Thyroid Function, Reproduction, and DNA Production

Selenomethionine (SeMet) is one of the main natural food 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.

SeMet combines with proteins in the body to form antioxidants called selenoproteins, which help protect the cells in your body from damage by free radicals. When used in supplement form, SeMet is either converted directly to selenium or is stored in place of methionine in body proteins. 

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

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What Is Selenomethionine Used For?

SeMet has been studied for its role in 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. This mineral plays a key role in making thyroid hormone and in its metabolism. Because of this, SeMet 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 an autoimmune condition called Hashimoto’s disease. For the study, they were given either oral SeMet treatment or a placebo for four months.

At the end of the study, 31.3% of people who took SeMet had restored thyroid function, while that number was just 3.1% in the placebo group. The study shows that SeMet supplements could possibly help to restore normal thyroid levels in people with this disorder.

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

At the end of the study, the SeMet group showed a higher reduction in antibodies against thyroglobulin (Tg), a protein made by the thyroid, than the placebo group did. SeMet appeared to reduce these anti-Tg levels in children and adolescents with autoimmune thyroiditis.

Another SeMet study looked at whether the supplements helped to 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 (T1), at 36 weeks gestation (T2), and about six months after delivery (PP). 

There were no real differences between the groups at T1. However, the researchers saw a notable rise in selenium blood levels in the SeMet group at T2, and a decrease in autoantibodies after the baby's delivery in the SeMet 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.

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


The Adenomatous Colorectal Polyp (ACP) study was designed to see whether SeMet or vitamin E lowered the rate of of colon and rectal cancers, and the tumors where many of these cancers begin. The study included 2,286 men with one or more of these cancer-related adenomas in the colon.

In the men who got SeMet, 34.2% were diagnosed with adenomas at least one year later, which compared to 35.7% in the group given a placebo instead of SeMet. This was not a significant difference, and led to a conclusion that adding SeMet to the diet 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 SeMet, did not have an impact on any type of cancer risk.

Heart Disease

A 2015 review looked at selenium and heart disease, and found that when selenium intake was higher, the heart 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 forms for boosting selenium levels in the body, but less so from a metabolic standpoint. The data did not support the use of SeMet 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 selenium's lower impact as an antioxidant. 

However, the study results are mixed. There is no clear evidence for selenium’s role, in SeMet form or not, when treating Alzheimer's disease. It may have a proven prevention benefit one day. More studies are needed to see if SeMet helps older people and their mental function.


Adding selenium to the diet may offer thyroid and other health benefits, but science has yet to prove a direct link. SeMet supplements are a good way to boost selenium in the diet, but there may be side effects. Talk to a doctor about how to keep healthy levels of this mineral in your body.

Possible Side Effects

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

There are signs and symptoms of too much selenium in the body. They include:

  • garlic breath odor
  • a metallic taste in the mouth
  • hair loss
  • brittle finger and toe nails
  • nausea and diarrhea
  • fatigue and irritability
  • nerve damage

Always talk with your doctor before taking large doses of 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.

Dietary Reference Intakes for Selenium

The DRI for different groups include:

  • 1 to 3 years: 20 micrograms per day
  • 4 to 8 years: 30 micrograms per day
  • 9 to 13 years: 40 micrograms per day
  • 14+ years: 55 micrograms per day
  • Pregnancy: 60 micrograms per day
  • Lactation: 70 micrograms per day

What to Look For

SeMet is a common form of selenium. Remember, though, that SeMet and other supplements are not regulated by the 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 doctor for help in choosing the right supplement for you.

They can advise you on the best dosage, and recommend products certified by a third party to ensure quality, such as the U.S. Pharmacopeia, NSF International, or Consumer Lab. 


Selenomethionine is present in many foods we 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 in cancer, heart disease, or to prevent cognitive decline.

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

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