100–200 µg/day as selenomethionine, for 3-6 months; take care not to exceed 400 µg/day sustained.
29 trials: moderate improvement in signs of Hashimoto's thyroiditis (anti-TPO in some studies). TSH does not change consistently.
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Selenium is an essential trace mineral whose biological function is exerted almost exclusively through its incorporation into a select group of proteins (selenoproteins), which include the glutathione peroxidases (first-line antioxidant defense), the thioredoxin reductases (regeneration of endogenous antioxidants), and the thyroid deiodinases (conversion of inactive T4 to active T3); this functional concentration in critical systems explains why even mild deficiencies can translate into detectable dysfunction of the immune system, thyroid function, or general antioxidant defense.
The selenium content of foods depends enormously on the soil where the plant was grown or the animal grazed: Brazil nuts are the most concentrated dietary source (a single nut can provide more than 100 percent of the recommended daily intake), followed by shellfish, organ meats, and meats; in Europe the soils are relatively poor in selenium compared with those in the Americas, which results in lower average intakes and a higher prevalence of suboptimal status, especially in countries such as the United Kingdom or Finland (where the government went so far as to add selenium to fertilizers to improve the population's status).
In autoimmune thyroiditis (Hashimoto's), several randomized trials have shown that supplementation with 100-200 micrograms of selenium per day (preferably as selenomethionine) reduces antithyroid antibodies (anti-TPO) after 3-6 months of treatment, with modest but replicable effect sizes; the clinical translation of that antibody reduction into symptom improvement or long-term preservation of thyroid function is more debated.
In Graves' disease (autoimmune hyperthyroidism), supplementation with 200 micrograms/day for 6 months has shown improvements in quality of life and in some ocular markers in patients with mild Graves' ophthalmopathy, a specific indication supported by a well-designed European trial published in NEJM in 2011.
In viral infections, several studies have shown that supplementation corrects immune defects in people with low selenium status; however, the large cancer prevention trials (especially SELECT in prostate cancer) did not show a benefit of supplementation in well-nourished populations, and even suggested possible risks in people with high baseline status, an important lesson about the U-shaped curve of many micronutrients.
In male fertility, several small trials have shown improvements in sperm motility and morphology with supplementation at 100-200 micrograms/day for 3 months, especially when combined with vitamin E or other antioxidants, an indication consistent with the role of selenoproteins in sperm maturation and in testicular antioxidant protection.
The usual nutritional doses are 55-70 micrograms per day (recommended intake in adults); the therapeutic doses in thyroiditis or fertility are 100-200 micrograms per day of selenomethionine or selenocysteine (better bioavailability than inorganic sodium selenite); the established upper safety limit is 400 micrograms per day, and it should not be exceeded chronically because of the risk of toxicity (selenosis).
It is one of the minerals with the narrowest therapeutic margin: deficiency and excess cause different but equally serious problems; the signs of chronic toxicity include garlic-smelling breath, brittle nails and hair, gastrointestinal disturbances, and, in severe cases, peripheral neuropathy; people with active thyroiditis should monitor thyroid function during supplementation because transient changes can occasionally occur; in pregnancy and breastfeeding, requirements increase slightly and moderate supplementation is safe.
Information, not medical advice. Scientific evidence reflects the quality of the studies.