Selenium
Also known as: Se
What Does Selenium Measure?
Selenium is an essential trace mineral measured in the blood, typically as serum or plasma selenium levels. A blood selenium test reflects your recent dietary intake and body stores of this micronutrient. Selenium is incorporated into special proteins called selenoproteins, which perform a wide range of critical biological functions throughout the body.
Why Does Selenium Matter?
Selenium plays a vital role in thyroid hormone metabolism, DNA synthesis, and protection against oxidative damage and infection. It is a key component of glutathione peroxidase, one of the body's most important antioxidant enzymes. Selenium status has been linked to thyroid function, immune health, fertility, cardiovascular health, and cancer risk. Both deficiency and excess can cause serious health problems, making it important to maintain optimal levels through diet and, when necessary, supplementation under medical supervision.
Normal Ranges
Males
70–150 mcg/L (micrograms per liter) or 0.89–1.90 micromol/L
Females
70–150 mcg/L (micrograms per liter) or 0.89–1.90 micromol/L
Children
Varies by age; typically 58–234 mcg/L in infants, approaching adult ranges by adolescence
Causes of High Levels
- Excessive selenium supplementation (selenosis from high-dose supplements)
- Occupational exposure to selenium in industrial or agricultural settings
- High dietary intake of selenium-rich foods such as Brazil nuts consumed in large quantities
- Selenium-containing medications or intravenous nutrition formulas with excess selenium
- Living in geographic regions with selenium-rich soil leading to elevated food content
- Certain herbal supplements or traditional medicines containing undisclosed selenium
Causes of Low Levels
- Inadequate dietary intake, especially in selenium-poor geographic regions
- Malabsorption syndromes such as Crohn's disease, celiac disease, or short bowel syndrome
- Total parenteral nutrition (TPN) without adequate selenium supplementation
How to Improve Your Selenium
Diet
- Eat 1–2 Brazil nuts per day, which can provide 70–90 mcg of selenium each (do not exceed 3–4 nuts daily)
- Include selenium-rich seafood such as tuna, halibut, sardines, and shrimp regularly in your diet
- Consume organ meats like beef liver or kidney, which are excellent natural selenium sources
- Add whole grains such as whole wheat bread, oatmeal, and brown rice, which contain moderate selenium
- Include eggs and dairy products as accessible everyday sources of selenium
Supplements
- Selenomethionine 100–200 mcg/day is the most bioavailable organic form, preferred for deficiency correction
- Sodium selenite 50–100 mcg/day as an inorganic option, though less well-absorbed than organic forms
- Selenium yeast 200 mcg/day, a well-studied form used in clinical trials for thyroid and antioxidant support
- Do not exceed 400 mcg/day total selenium from all sources to avoid toxicity (selenosis)
- Combine with Vitamin E, as these work synergistically to enhance antioxidant protection
Related Biomarkers
Frequently Asked Questions
What are the symptoms of selenium deficiency?
Selenium deficiency can cause fatigue, muscle weakness, hair loss, brittle nails, thyroid dysfunction, weakened immunity, and in severe cases, heart problems (Keshan disease) or joint pain (Kashin-Beck disease). Many people with mild deficiency have no obvious symptoms until levels are measured via a blood test.
Can you get too much selenium, and what are the signs of selenium toxicity?
Yes, selenium toxicity (selenosis) is possible, especially from excessive supplementation or eating large amounts of Brazil nuts daily. Signs include a garlic-like odor on the breath, hair loss, nail brittleness or loss, nausea, diarrhea, fatigue, irritability, and in severe cases, nerve damage. The safe upper limit for adults is 400 mcg per day from all sources combined.
How does selenium affect thyroid health?
Selenium is essential for converting the inactive thyroid hormone T4 into the active form T3, a process carried out by selenium-containing enzymes called deiodinases. It also protects the thyroid gland from oxidative damage caused during hormone production. Low selenium is associated with hypothyroidism, Hashimoto's thyroiditis, and Graves' disease. Supplementing selenium has been shown in studies to reduce thyroid antibodies in autoimmune thyroid conditions.
Does selenium help with cancer prevention?
Research has shown mixed results. Observational studies have linked adequate selenium status with reduced risk of certain cancers, particularly prostate, colorectal, and lung cancers. However, large randomized trials like SELECT (Selenium and Vitamin E Cancer Prevention Trial) did not confirm that supplementation reduces prostate cancer risk and actually raised some concerns. The current consensus is that maintaining sufficient selenium through diet is beneficial, but high-dose supplementation for cancer prevention is not recommended without medical guidance.