# Selenium in the Body: Why It Is Needed, Where It Comes From, and Why Excess Can Be Dangerous - Canonical: https://nutrifit.health/knowledge/selenium-thyroid-antioxidant-function-tests - Language: en - Published: 2026-09-02T00:14:11.502Z - Updated: 2026-09-02T19:29:38.075Z - Author: Редакция NutriFit > Selenium: thyroid function, tests and safety — практическое руководство по источникам, дефициту, добавкам и рискам. # Selenium in the Body: Why It Is Needed, Where It Comes From, and Why Excess Can Be Dangerous Selenium is an essential trace element that the body needs in very small amounts. Unlike calcium or magnesium, daily selenium requirements are measured not in milligrams but in **micrograms**. Despite the tiny amounts involved, selenium has major biological importance. It is incorporated into selenoproteins — specialized proteins and enzymes involved in antioxidant protection, thyroid hormone metabolism, DNA synthesis, reproductive function, and immune system activity. The human body contains 25 known selenoproteins, including glutathione peroxidases, thioredoxin reductases, deiodinases, and selenoprotein P. But selenium has an important characteristic: **the gap between physiologically necessary intake and potentially excessive intake is relatively narrow**. That is why the principle “if a little is good, more must be better” does not apply to selenium. ## Why the Body Needs Selenium Selenium does not perform a single isolated function. It is integrated into several fundamental biological systems. Selenoproteins are involved in: - protecting cells against oxidative damage; - thyroid hormone metabolism; - antioxidant enzyme systems; - DNA synthesis; - reproductive function; - normal immune system activity; - regulation of certain redox processes. Some of the best-known selenium-containing enzymes are **glutathione peroxidases**. They help neutralize peroxides and limit oxidative damage to cellular components. Another important group consists of **deiodinases**, which participate in thyroid hormone metabolism. This is one reason selenium is so often discussed in the context of thyroid health. ## Selenium and the Thyroid Gland The thyroid gland contains a relatively high concentration of selenium and actively uses selenoproteins. One of their functions is related to thyroid hormone conversion. The thyroid gland produces mainly thyroxine — **T4**. Some T4 is then converted into the more biologically active hormone triiodothyronine — **T3**. Selenium-containing enzymes called deiodinases participate in this conversion. In addition, thyroid hormone synthesis involves the production of hydrogen peroxide. Selenoproteins, including glutathione peroxidases and thioredoxin reductases, help protect thyroid tissue against excessive oxidative damage. Therefore, adequate selenium status is indeed important for normal thyroid physiology. However, this does not mean that selenium supplements can treat every thyroid disorder. ## Does Selenium Treat Hypothyroidism? There is no basis for considering selenium a substitute for standard hypothyroidism treatment. If the thyroid gland does not produce enough hormones and a person has been prescribed levothyroxine, selenium cannot replace hormone replacement therapy. Selenium supplements are also not a universal way to “lower TSH.” Studies of selenium in autoimmune thyroiditis have produced mixed results. In some trials, selenium supplementation reduced concentrations of thyroid peroxidase antibodies — TPOAb. However, more clinically important outcomes, including TSH levels, thyroid structure, and quality of life, did not consistently improve. The NIH notes that current evidence is insufficient to recommend selenium supplementation as a standard treatment for autoimmune thyroiditis. This distinction is important: **a change in antibody levels on a blood test does not necessarily mean that thyroid function has improved or that the disease has resolved.** ## What About Hashimoto’s Thyroiditis? Selenium is especially heavily marketed to people with Hashimoto’s autoimmune thyroiditis. There is a biological rationale for this, because selenium is required for normal thyroid function, and some studies do show changes in thyroid antibody levels after supplementation. However, clinical trial results remain inconsistent. The American Thyroid Association also emphasizes that selenium should not be viewed as a way to prevent hypothyroidism in everyone with Hashimoto’s thyroiditis. Therefore, the approach: **“antibodies are elevated → start taking 200 mcg of selenium”** should not be applied automatically. Actual selenium status, thyroid function, diet, other medical conditions, and prescribed treatment all need to be considered. ## How Much Selenium Does a Person Need? For most adults, the recommended daily intake is only **55 mcg of selenium per day**. Requirements increase slightly during pregnancy and breastfeeding: - adults — **55 mcg per day**; - pregnancy — **60 mcg per day**; - breastfeeding — **70 mcg per day**. These amounts include selenium from all dietary sources. This means that 55 mcg is not necessarily a supplement dose. If a person already gets enough selenium from food, an additional supplement may not be needed at all. ## Why It Is Especially Important to Distinguish Between the Requirement and the Upper Limit In the United States, the Food and Nutrition Board sets the tolerable upper intake level for selenium in adults at **400 mcg per day from all sources**. But this is not the recommended intake. It is a safety threshold, not a daily target. In addition, in 2023 the European Food Safety Authority — EFSA — reviewed the evidence and established a lower tolerable upper intake level for adults: **255 mcg per day**, including during pregnancy and breastfeeding. This makes it particularly inappropriate to treat supplements containing 200–400 mcg as harmless routine prevention. The dose shown on the bottle may be several times higher than the normal physiological requirement. ## Where Selenium Is Found Selenium is widely distributed in foods. Good sources include: - fish and seafood; - meat; - poultry; - organ meats; - eggs; - dairy products; - bread and grain products; - legumes; - certain nuts. A notable feature of selenium is that its concentration in plant foods strongly depends on geography. Plants absorb selenium from the soil. Therefore, the same type of grain, legume, or nut grown in different regions can contain very different amounts of selenium. Concentration is influenced not only by total selenium in the soil but also by its chemical form, soil acidity, and other factors. For animal-derived foods, this geographic variability is usually less pronounced because animals can regulate selenium concentrations in their tissues to some extent, and livestock feed may also be supplemented with selenium. ## Brazil Nuts: A Nutritious Food With an Unusually High Selenium Content Brazil nuts deserve special attention. They are one of the most concentrated dietary sources of selenium. According to the NIH, a single Brazil nut may contain approximately **68–91 mcg of selenium**. At the same time, an average 1-ounce serving — roughly 6–8 nuts — may provide around **544 mcg**. The actual amount varies substantially depending on where the nuts were grown. For comparison: the recommended daily intake for an adult is **55 mcg**. Therefore, a single large Brazil nut may potentially provide about a full day’s requirement or even more. Eating a whole handful regularly can push total intake above the tolerable upper intake level. ## Does This Mean Brazil Nuts Are Dangerous? No. The problem is not the food itself, but the amount consumed and the extreme variability in selenium concentration. Brazil nuts are not ideal as a “natural selenium pill” because it is impossible to know exactly how much selenium is present in a specific nut. Other sources also matter. For example, a person may simultaneously get selenium from: - regular food; - Brazil nuts; - a multivitamin; - a hair and nail supplement; - a “thyroid support” supplement; - a separate selenium supplement. This unnoticed combination of several sources is what can create a risk of chronic excess. ## Selenium Deficiency Is Much Less Common Than It May Seem True severe selenium deficiency is relatively uncommon in countries where people have access to a varied diet. Risk depends not only on which foods a person eats, but also on where those foods were grown. The NIH identifies several groups at increased risk, including people living in regions with selenium-poor soil, patients undergoing long-term hemodialysis, and people living with HIV. Historically, severe selenium deficiency has been best documented in some parts of China where the selenium content of the soil is extremely low. ## Vegan Diets and Selenium A plant-based diet does not automatically mean selenium deficiency. Legumes, grains, and nuts can all contain selenium. The challenge is that selenium content in plant foods depends strongly on soil composition. Therefore, someone eating a diverse range of foods from different regions may obtain adequate selenium even on a fully plant-based diet. Risk is higher if the diet is monotonous and consists largely of locally grown plant foods from selenium-poor soil. According to the NIH, average selenium intake can be lower among people following vegetarian or vegan diets, but this does not automatically mean that they are deficient. ## Other Situations With Increased Risk Low selenium status may occur in people receiving long-term hemodialysis. During hemodialysis, some selenium is removed from the blood, while dietary restrictions and reduced appetite may further decrease intake. Historically, deficiency has also been described in patients receiving prolonged total parenteral nutrition without adequate selenium. Modern parenteral nutrition formulations generally include trace elements, but this setting still requires medical monitoring. Risk may also increase in severe conditions associated with poor nutrition or malabsorption. In such cases, however, clinicians usually evaluate overall nutritional status rather than selenium alone. ## What Does Selenium Deficiency Look Like? This is a difficult question because moderate low intake does not produce a reliable set of symptoms that can be used for self-diagnosis. Symptoms such as: - fatigue; - weakness; - reduced performance; - muscle discomfort are nonspecific and occur in a wide range of conditions. In patients with severe deficiency during prolonged parenteral nutrition, muscle pain and tenderness have been described. In areas with extremely low selenium intake, certain specific diseases have historically been reported. **Keshan disease** is an endemic cardiomyopathy that was historically observed mainly in selenium-deficient regions of China. **Kashin–Beck disease** is a chronic disorder affecting bones and joints that has occurred in parts of China and Siberia. However, selenium deficiency alone is not always the sole cause of these diseases. ## Is Hair Loss a Sign of Selenium Deficiency? This is a common but problematic claim. Hair loss can occur in many different conditions, including: - iron deficiency; - thyroid disease; - inadequate calorie or protein intake; - certain illnesses; - stress; - medication effects. Hair loss alone cannot be used to diagnose selenium deficiency. In fact, **significant hair loss and nail changes are classic signs of chronic selenium excess — selenosis**. Therefore, taking large doses of selenium on your own “for hair loss” is especially questionable: if selenium intake is already sufficient, supplementation could theoretically worsen the problem. ## Selenium and Iodine Are Interconnected Selenium and iodine are involved in different stages of thyroid hormone physiology. Severe selenium deficiency may worsen the effects of iodine deficiency and impair thyroid hormone metabolism. However, this does not mean that every abnormal TSH result should lead to simultaneous selenium and iodine supplementation. Both deficiency and excess of nutrients can matter, and thyroid diseases have many possible causes. Supplements therefore do not replace proper laboratory evaluation of thyroid function. ## How Selenium Status Is Assessed The most common laboratory approach is measuring selenium concentration in **plasma or serum**. According to the NIH, a concentration of approximately **8 mcg/dL — 80 mcg/L — or higher** in healthy individuals is generally considered sufficient for selenoprotein synthesis. However, this number should not be treated as a universal cutoff for self-diagnosis. Reference ranges depend on the laboratory and testing method. In addition, selenium concentrations in plasma and serum mainly reflect relatively recent selenium intake. ## Are There More Accurate Tests? Other measurements also exist. Whole-blood selenium concentrations may better reflect longer-term selenium status because they include selenium contained in red blood cells. Selenium levels in hair and nails are mainly used to assess longer-term exposure over months or years. There are also functional markers, including: - selenoprotein P; - glutathione peroxidase 3. However, these tests are more complex to interpret. For example, selenoprotein concentrations can change during inflammation and oxidative stress, so they do not always reflect selenium stores alone. For an otherwise healthy person without risk factors, broad “selenium screening” is generally not the starting point when investigating nonspecific symptoms. ## TSH and Free T4 Are Not Selenium Tests This distinction is important. **TSH and free T4 assess thyroid function.** They do not directly show how much selenium is present in the body. A person can have normal TSH and T4 values with different levels of selenium intake. Conversely, an abnormal TSH should not automatically be explained by selenium deficiency. When thyroid disease is suspected, thyroid function and the overall clinical situation should be evaluated first rather than starting micronutrient supplements empirically. ## Which Forms of Selenium Are Used in Supplements? Common forms used in dietary supplements include: - selenomethionine; - selenium-enriched yeast; - sodium selenite; - sodium selenate. According to the NIH, the body can absorb a large proportion of selenium from these forms — up to approximately 90%. Selenomethionine is an organic form of selenium and is frequently used in supplements. However, seeing the word “selenomethionine” on a label does not mean that a person needs that supplement. The main question is not only which form is used, but **whether additional selenium is needed at all and what the total intake is**. ## Why It Is Easy to Accidentally Get Too Much Selenium Selenium is included in many combined supplement products. It may be present at the same time in: - a standard multivitamin; - a thyroid support formula; - a hair and nail supplement; - a mineral complex; - a separate selenium supplement. The NIH notes that many multivitamins provide around 55 mcg, combination products often contain 50–200 mcg, and selenium-only supplements may contain 100–400 mcg per dose. As a result, two or three supplements taken simultaneously can produce a very different total dose from what a person assumes when looking at each bottle separately. ## What Is Selenosis? **Selenosis** is a condition caused by chronic excessive selenium intake. Typical signs include: - hair loss; - brittle, deformed, or lost nails; - garlic-like breath odor; - metallic taste; - nausea; - diarrhea; - skin rash; - fatigue; - irritability; - neurological disturbances. Garlic breath and a metallic taste may be early signs of excessive selenium exposure. If such symptoms appear, it is important to evaluate not just one selenium supplement but all possible sources combined. ## Acute Selenium Overdose Very large amounts of selenium can cause severe poisoning. Possible manifestations include: - severe gastrointestinal symptoms; - neurological disturbances; - tremor; - profound weakness; - kidney injury; - cardiac abnormalities; - breathing problems. In particularly severe cases, kidney failure, heart failure, and other life-threatening complications have been reported. If a person has knowingly taken a very large amount of selenium and develops severe vomiting, confusion, breathing difficulty, profound weakness, tremor, or other serious symptoms, urgent medical evaluation is required. ## Selenium During Pregnancy Selenium requirements increase slightly during pregnancy to **60 mcg per day**. But this does not mean that a high-dose selenium supplement is necessary. The requirement refers to total intake from food and supplements. Particular caution is warranted when using selenium independently for autoimmune thyroid disorders during pregnancy. The NIH notes that evidence for this approach is inconsistent, and the American Thyroid Association does not recommend routine selenium supplementation for pregnant women who are positive for thyroid peroxidase antibodies. Therefore, supplement use during pregnancy should take into account the entire diet and the composition of any prenatal vitamin and mineral supplement. ## Selenium and the Immune System Selenoproteins do participate in both innate and adaptive immune function. That is a real biological function of selenium. But it does not follow that taking selenium above physiological requirements makes the immune system “stronger.” If selenium status is already adequate, increasing the dose further does not guarantee additional protection against infections. Selenium is therefore better viewed as an **essential nutrient**, not as a universal immune booster. ## Selenium Is Not a Cancer Prevention Treatment Selenium has also been extensively studied in relation to cancer prevention. Observational studies have sometimes found associations between lower selenium status and higher risk of certain cancers. However, such associations do not prove that selenium supplementation prevents cancer. Large randomized trials have not shown that selenium supplements reduce the risk of prostate cancer or other cancers in people who already have adequate selenium intake. Therefore, high-dose selenium should not be taken “for cancer prevention.” ## Selenium and Medications Selenium supplements may be relevant in the context of certain drug therapies. For example, the NIH notes an interaction involving **cisplatin**, a chemotherapy drug. Cisplatin can reduce selenium concentrations in the body, but there is not enough evidence to recommend self-supplementation with selenium to reduce treatment-related side effects. During cancer treatment, any antioxidant or mineral supplement should be discussed with the treating medical team. The same general caution is appropriate for people taking many medications, those with chronic kidney disease, and those with complex endocrine disorders. ## What to Do if Selenium Deficiency or Excess Is Suspected **1. Evaluate your usual diet.**Consider how often you eat fish, seafood, eggs, meat, dairy products, grains, legumes, and nuts. **2. Check all supplements at the same time.**Add up selenium from multivitamins, hair supplements, thyroid formulas, and standalone selenium products. **3. Do not use Brazil nuts as a precisely dosed selenium tablet.**The selenium content of individual nuts varies substantially. **4. Do not diagnose deficiency based on fatigue or hair loss.**These symptoms are nonspecific, and hair loss is also a classic sign of excessive selenium intake. **5. Look for more common causes of symptoms first.**Depending on the clinical situation, a clinician may evaluate a complete blood count, ferritin, vitamin B12, TSH, free T4, and other markers. **6. Test selenium when there is a real indication.**Testing may be useful when there are genuine risk factors, severe nutritional disorders, certain diseases, or suspected toxicity. **7. Do not replace thyroid treatment with selenium.**Prescribed levothyroxine or other therapy should not be discontinued because a micronutrient supplement has been started. **8. Do not aim for the upper intake limit.**55 mcg is the normal physiological requirement for most adults, whereas 255–400 mcg represents different regulatory upper safety limits, not desirable daily intake. **9. If overdose is suspected, stop additional sources and seek medical evaluation.**This is especially important if severe gastrointestinal, neurological, or cardiovascular symptoms are present. ## What Counts as a Good Outcome? The goal is not to achieve the highest possible selenium concentration in the blood or to take selenium “just in case.” A good outcome means: - adequate but not excessive selenium intake; - a varied diet; - no unnecessary duplication between supplements; - normal thyroid system function; - proper diagnosis of thyroid disease independently of selenium status; - absence of signs of selenosis; - use of supplements only when there is a reasonable indication. The main principle is simple: **selenium is essential, but its benefits depend on adequacy, not on maximizing the dose.** If the diet already provides the physiological requirement, additional selenium may offer no extra benefit. At high doses, an essential trace element can instead become a source of toxicity. *This material is for educational purposes only and does not replace medical consultation, diagnosis, or individually prescribed treatment.* ## Sources 1. **National Institutes of Health, Office of Dietary Supplements (NIH ODS). Selenium — Fact Sheet for Health Professionals.** A primary reference on the physiological role of selenium and selenoproteins, recommended intake, dietary sources, selenium status assessment, risk groups, supplement forms, thyroid disease, toxicity, and medication interactions. 2. **National Institutes of Health, Office of Dietary Supplements (NIH ODS). Selenium — Fact Sheet for Consumers.** Practical information on daily selenium requirements, food sources, Brazil nuts, deficiency, supplements, signs of excessive intake, and tolerable upper intake levels. 3. **National Institutes of Health, Office of Dietary Supplements (NIH ODS). Dietary Supplements for Immune Function and Infectious Diseases — Selenium.** Information on the role of selenium and selenoproteins in innate and adaptive immune function and the limited evidence supporting additional selenium supplementation for prevention of infectious diseases. 4. **Merck Manual Professional Edition. Selenium Deficiency.** Clinical information on the rarity of severe selenium deficiency, Keshan disease, Kashin–Beck disease, deficiency during prolonged parenteral nutrition, the relationship with iodine deficiency, and diagnostic approaches. 5. **Merck Manual Professional Edition. Selenium Toxicity.** Clinical guidance on manifestations of chronic and acute selenium excess, including gastrointestinal symptoms, hair loss, nail changes, garlic-like breath odor, neurological abnormalities, and approaches to diagnosing selenosis. 6. **American Thyroid Association. Does Selenium Supplementation Prevent Hypothyroidism in Hashimoto’s Thyroiditis?** An overview of current evidence regarding selenium use in Hashimoto’s autoimmune thyroiditis and the limited evidence that supplementation can prevent progression to hypothyroidism. 7. **American Association of Clinical Endocrinologists and American Thyroid Association. Clinical Practice Guidelines for Hypothyroidism in Adults.** Clinical guidelines on hypothyroidism emphasizing the lack of sufficient evidence for routine selenium use to prevent or treat hypothyroidism. 8. **National Institutes of Health, Office of Dietary Supplements (NIH ODS). Dietary Supplements and Life Stages: Pregnancy — Fact Sheet for Health Professionals.** Reference information on recommended selenium intake during pregnancy and other micronutrients required during this period. 9. **European Food Safety Authority (EFSA). Scientific Opinion on the Tolerable Upper Intake Level for Selenium.** The scientific basis for the European tolerable upper intake level of **255 mcg per day for adults**, established with consideration of chronic excess risks, including alopecia. 10. **American Society for Parenteral and Enteral Nutrition (ASPEN). Appropriate Dosing for Parenteral Nutrition.** Recommendations on the inclusion of selenium and other trace elements in parenteral nutrition regimens for adult patients. ## Sources - [NIH ODS — Selenium Fact Sheet](https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/)