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Moderate evidenceVitamins & minerals

Selenomethionine

The organic selenium form found in food and used in most trials. It lowers thyroid antibodies in Hashimoto's, but the largest trial ever run found no cancer prevention and its long-term follow-up raised safety questions.

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What the science says

  • Thyroid antibodies in Hashimoto's Moderate evidence

    A systematic review and meta-analysis of 35 randomized trials found selenium lowered TSH in people not on thyroid hormone (SMD -0.21) and reduced thyroid peroxidase antibodies (SMD -0.96), with moderate certainty. Free T4, T3, thyroglobulin antibodies and thyroid volume did not change.

  • No cancer prevention, and possible long-term harm Moderate evidence

    SELECT randomised 34,887 men to 200 µg/day L-selenomethionine or placebo and was stopped for futility. A long-term post-intervention analysis found near-null or lower risk for most cancers but higher risk for Hodgkin lymphoma, multiple myeloma and ALS, based on few cases.

Dosage & safety

Studied doseTrials used 100–200 µg/day. The recognised tolerable upper intake is 400 µg/day from all sources, which is easy to exceed if you also eat Brazil nuts or take a multivitamin.
SafetyChronic excess causes selenosis: hair and nail brittleness or loss, garlic breath, nausea, rash and peripheral neuropathy. Earlier analyses of SELECT and other trials also linked long-term supplementation with increased type 2 diabetes risk. Do not stack multiple selenium-containing products, and do not supplement if your intake is already adequate.

How to take it

TimingTiming does not matter.
With food?With food.
Worth knowingTwo Brazil nuts can supply an entire day's selenium, so count food before you count capsules.

Interactions

With other supplements

  • multivitamins and other selenium-containing products

    Stacking is the usual route to selenosis, since the upper limit is only 400 µg/day from all sources. add up selenium across every product before taking a standalone one

Well-established interactions only — this is not a complete list. Always tell your doctor and pharmacist what you take.

Frequently asked questions

Does selenium help Hashimoto's thyroiditis?

Selenium lowers thyroid antibodies in Hashimoto's: a meta-analysis of 35 randomized trials found reduced thyroid peroxidase antibodies and slightly lower TSH in people not on thyroid hormone, with moderate certainty. Free T4, T3 and thyroid volume did not change.

Does selenomethionine prevent cancer?

No: the SELECT trial randomised 34,887 men to 200 mcg/day of selenomethionine or placebo and was stopped for futility. Long-term follow-up found higher risk of a few rare conditions such as Hodgkin lymphoma and ALS, based on few cases.

How much selenomethionine per day is safe?

Trials used 100–200 mcg/day of selenomethionine, and the upper limit is 400 mcg/day from all sources. Two Brazil nuts can supply a day's selenium, so count food and multivitamins before adding a capsule.

What are the side effects of too much selenium?

Chronic excess selenium causes selenosis: brittle or lost hair and nails, garlic breath, nausea, rash and nerve damage. Long-term selenium supplementation has also been linked with increased type 2 diabetes risk, so do not supplement if your intake is already adequate.

Scientific references

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