Supplement analysis backed by scientific evidence · PT

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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.

Evidence last reviewed: August 2026

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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.

Scientific references

Where to buy

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