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, reviewed by
Kimberly Langdon M.D.
One study in The Journal of Clinical Endocrinology and Metabolism evaluated the effect of daily selenium supplementation on the level of thyroid antibodies in patients with Hashimoto’s thyroiditis treated with levothyroxine. Participants were split evenly into two groups - one group to supplement with selenium, one group to not. At the end of the study, the group treated with selenium saw a 40% decrease in the concentration of thyroid antibodies (versus 10% in the placebo group), and 25% of patients saw a complete normalization of their thyroid antibodies. Compared to the placebo group, patients treated with selenium in the study reported better well-being.
Selenium is a powerful antioxidant, helping to protect against oxidative stress initiated by reactive oxygen species and reactive nitrogen species in the tissues of human thyroid glands. It has even more potent antioxidant properties than the better-known vitamins E, C, A, and beta-carotene - but also more toxic in high doses! It’s important to manage oxidative stress, as this harmful process can damage cellular membranes, proteins, and DNA. If there is an imbalance between free radical formation and the cell’s capability to clear them, oxidative stress can speed up the body’s aging process and cause chronic and degenerative diseases.
Selenium deficiency impairs thyroid hormone metabolism by inhibiting the activity of iodothyronine deiodinases, a family of enzymes that help to convert thyroxine (T4) to the metabolically active triiodothyronine (T3). T4 and T3 are hormones produced by the thyroid, which, together, regulate your body’s temperature, metabolism, and heart rate. When thyroid hormone production changes, virtually every system in the body is affected.
Consuming selenium is a delicate balance. While it is necessary for good thyroid health, consuming too much selenium can cause toxicity. Selenium toxicity symptoms include garlic breath, nausea, diarrhea, skin rash, and hair and nail loss. The Tolerable Upper Intake Level (UL)—defined as the highest level of daily intake that is likely to pose no adverse health effects—for adults is 400 micrograms (mcg) per day.
Although the Recommended Dietary Allowance* (RDA) for selenium is 55 micrograms (mcg) per day, research suggests that higher intake may support optimal health, particularly in patients with Hashimoto's. Aim for ~200mcg daily from food and/or supplements, which may help reduce thyroid peroxidase antibodies and improve symptoms associated with the condition. Look for supplements containing selenomethionine, which is better absorbed than other forms like selenite and selenate.
Generally speaking, selenium an important role in regulating and normalizing the immune system, especially in autoimmune diseases. However, the relationship between selenium and immunity is quite complicated. Selenium supplementation's impact on immunity depends on what type of antigens and tissues are involved. For example, selenium may enhance cellular immune response (secrete antibodies) to a greater extent than humoral immune response (secretes cytokines).
Selenoproteins (selenium-containing proteins) may help to protect against cardiovascular disease. In 2017, researchers reviewed and analyzed 16 different randomized controlled trials to understand the effect of selenium supplementation on coronary heart disease. Results suggest that selenium supplementation reduces oxidative stress and inflammation in coronary heart disease. However, selenium supplementation may not be enough to reduce heart disease mortality or to improve cholesterol.
Brazil nuts, seafood, and organ meats are the richest dietary sources of selenium. Muscle meats, grains, and dairy products also contain selenium, but amounts depend on the selenium concentration of soil and water. As a result, selenium concentrations in plant-based foods can vary by geographic location.