Selenium — Research Profile

Yes. Brazil nuts are the richest food source of selenium, containing 68-91mcg per nut on average. Just 1-2 Brazil nuts daily provides adequate selenium for most adults. A 2008 Thomson et al. study confirmed that 2 Brazil nuts daily was as effective as 100mcg selenomethionine supplement for raising selenium status.

Selenium is essential for thyroid hormone metabolism. For Hashimoto's thyroiditis specifically, multiple studies show 200mcg daily reduces TPO antibodies. However, evidence that this translates to improved thyroid function or reduced medication needs is less consistent. Selenium is most beneficial for thyroid patients who are selenium-deficient.

The upper limit is 400mcg/day. Chronic intake above this causes selenosis, characterized by garlic breath, hair loss, nail changes, fatigue, and gastrointestinal problems. Acute toxicity is possible above 1,000mcg. If eating Brazil nuts, limit to 2-3 per day as selenium content varies widely.

No single form is best for every purpose. Selenomethionine and selenium-enriched yeast are organic forms that incorporate into body proteins, and the NIH fact sheet notes selenomethionine was more effective than sodium selenite at lowering thyroid peroxidase antibodies in autoimmune thyroiditis, possibly because of higher absorption. Inorganic sodium selenite and selenate are also well absorbed and typically cheaper; the body takes up about 90% of selenium from all four of these forms.

Selenium's clearest benefit is correcting deficiency, which restores antioxidant selenoenzyme activity and normal thyroid hormone metabolism. Randomized trials and meta-analyses show supplementation lowers thyroid peroxidase antibodies in Hashimoto's thyroiditis and may assist Graves' disease management. For cancer prevention, however, a Cochrane review and the SELECT trial found no benefit from supplements in people who are already selenium-replete.

The adult RDA is 55 mcg per day, rising to 60 mcg in pregnancy and 70 mcg during lactation. Thyroid-antibody trials have commonly used 200 mcg per day of selenomethionine, a therapeutic dose that should be kept well below the adult Tolerable Upper Intake Level of 400 mcg per day. Most people in selenium-adequate regions already meet the RDA through diet.

Selenium does not need to be taken at a specific time of day; it is absorbed efficiently (up to about 90%) and works by sustaining selenoprotein activity over weeks rather than through an acute daily effect. Taking it with a meal can reduce the chance of nausea. There is no evidence that morning versus evening dosing changes outcomes.

At intakes near the RDA, selenium rarely causes side effects. Excessive intake causes selenosis, whose signs include a garlic-like breath odor, a metallic taste, hair loss, brittle or lost nails, skin rash, nausea, diarrhea, fatigue, irritability, and nervous system abnormalities. These reflect chronic intake above the 400 mcg per day upper limit rather than ordinary supplemental doses.

Documented drug interactions are limited. The chemotherapy drug cisplatin can reduce selenium concentrations in hair and serum, though the clinical significance is unclear. Because selenium supports thyroid hormone metabolism, people taking medication for Hashimoto's or Graves' disease should have thyroid function monitored when they add it.

Most people in selenium-adequate regions meet their needs through food and do not require a supplement. Those more likely to benefit include people whose diets rely on crops grown in low-selenium soils, individuals on long-term hemodialysis, people living with HIV, and patients with autoimmune thyroid disease, where supplementation has reduced thyroid antibodies in trials. A couple of Brazil nuts provide a substantial dose without a pill.

Timing depends on the goal. Correcting a deficiency raises blood selenium and restores the antioxidant selenoenzyme glutathione peroxidase within weeks. In autoimmune thyroid disease, trials that measured drops in thyroid peroxidase antibodies with selenomethionine generally ran 3 to 6 months before meaningful changes appeared, and if you are already selenium-replete, supplementing produces no noticeable symptomatic effect.

At intakes near the adult RDA of 55 mcg per day and below the Tolerable Upper Intake Level of 400 mcg per day from all sources, long-term selenium is considered safe. The main risk of sustained high intake is selenosis; the SELECT trial gave 200 mcg per day for about 5.5 years without reducing prostate cancer, which raised questions about routine high-dose supplementation in people who are already replete. Keeping total intake at or below 400 mcg per day is the key limit.

Yes. The adult Tolerable Upper Intake Level is 400 mcg per day from food plus supplements, and chronically exceeding it can cause selenosis. Early indicators are a garlic odor on the breath and a metallic taste, followed by hair loss and nail brittleness or loss, skin rash, nausea, diarrhea, fatigue, irritability, and nervous system abnormalities.

Selenium is often included in thyroid formulas alongside iodine because both minerals are needed for normal thyroid hormone metabolism. It is also commonly paired with vitamin E as an antioxidant combination, although the SELECT trial found that pairing produced no cancer-prevention benefit. Add up selenium from any multivitamin, Brazil nuts, and standalone products so the daily total stays under the 400 mcg upper limit.

Check the elemental selenium dose in micrograms; most products supply 50 to 200 mcg. The body absorbs up to about 90% of selenium whether it comes as selenomethionine, selenium-enriched yeast, sodium selenite, or sodium selenate, so all of these forms are well absorbed, with the organic forms (selenomethionine and yeast) being more readily retained in tissue. Because the margin between the RDA and the 400 mcg upper limit is narrow, avoid stacking a high-dose standalone product on top of a multivitamin.