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Selenium (Selenomethionine)

Thyroid · antioxidant · immune

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Written to our editorial standards · reviewed against published research· Updated 2 Jun 2026
Last reviewed: May 30, 2026
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Selenium: Thyroid · antioxidant · immune. Critical for thyroid hormone conversion and glutathione peroxidase, one of the body's main antioxidant enzymes. Standard dose 200 mcg, taken in the morning, about $10/month. Evidence rating: strong evidence.

What is Selenium?

Selenium is a trace mineral essential for the activity of selenoproteins, including glutathione peroxidase, one of the body's primary antioxidant enzymes, and the deiodinase enzymes that convert thyroid hormone T4 into its active form T3. Brazil nuts are the richest food source, but soil depletion makes consistent intake variable. Adequate selenium supports thyroid function, immune resilience, and protection against oxidative stress. The selenomethionine form has the highest bioavailability. Daily intake of 100-200 mcg covers most adults; the upper limit is 400 mcg/day. Excessive intake from high-dose supplements should be avoided.

WHY IT MATTERS

Critical for thyroid hormone conversion and glutathione peroxidase, one of the body's main antioxidant enzymes.

Natural food sources

Where possible, get Selenium from whole foods first. Common dietary sources include:

Brazil nutsTunaEggsSardines

Where to buy

NOW Foods Selenium 200mcg Yeast Free
RECOMMENDED
NOW Foods, Selenium
200 mcg · Morning · ~$10/month

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Best for your goal

Interactions

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Common questions

What is Selenium?
Selenium is a trace mineral essential for the activity of selenoproteins, including glutathione peroxidase, one of the body's primary antioxidant enzymes, and the deiodinase enzymes that convert thyroid hormone T4 into its active form T3. Brazil nuts are the richest food source, but soil depletion makes consistent intake variable. Adequate selenium supports thyroid function, immune resilience, and protection against oxidative stress. The selenomethionine form has the highest bioavailability. Daily intake of 100-200 mcg covers most adults; the upper limit is 400 mcg/day. Excessive intake from high-dose supplements should be avoided.
What is Selenium used for?
Thyroid · antioxidant · immune. Critical for thyroid hormone conversion and glutathione peroxidase, one of the body's main antioxidant enzymes.
What is the standard dose of Selenium?
200 mcg, typically taken in the morning. Approximate cost is $10 per month.
How strong is the evidence for Selenium?
Strong evidence for its primary uses. See the full study list on the research page.

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