Our pick · Selenium

NOW Supplements Selenium 100mcg

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Our pick

NOW Supplements Selenium 100mcg

L-selenomethionine form (best absorbed), GMP-certified facility, ConsumerLab-tested brand

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Dosage

Typical Dose

100-200 mcg/day is the common supplemental range. 200 mcg/day is used in most clinical trials.

RDA

RDA: 55 mcg/day for adults.

Upper Limit

400 mcg/day. Chronic intake above this level risks selenosis (hair loss, nail brittleness, GI issues, neurological symptoms).

Forms

Selenomethionine is well-absorbed and incorporated into body proteins. Sodium selenite is an inorganic form with adequate absorption. Selenium yeast provides mixed organic forms.

Safety

General

Safe at doses within the RDA to 200 mcg/day. The margin between therapeutic and toxic doses is narrower than for most minerals.

Cautions

Selenosis occurs above 400 mcg/day chronically — symptoms include garlic breath, hair loss, nail changes, fatigue, and nausea.
The SELECT trial found 200 mcg/day selenium did not prevent prostate cancer and may have slightly increased type 2 diabetes risk in selenium-replete men.

Interactions

May interact with cisplatin and other chemotherapy agents.
High-dose vitamin C may reduce selenium absorption if taken simultaneously.
Statins may have additive antioxidant effects.

Selenium

An essential trace mineral critical for thyroid function and antioxidant defense — beneficial for those with low selenium status, but excess carries risk.

Immune Metabolic & Weight

BEvidence

By editorialUpdated 2026-05-251 min read

What it's actually good for

Selenium is an essential trace mineral that humans need in small amounts for proper thyroid function, antioxidant defense, and immune regulation. It is incorporated into selenoproteins including glutathione peroxidase (antioxidant protection) and deiodinases (thyroid hormone activation). The strongest supplementation case is for people with low selenium status or thyroid autoimmunity — multiple trials show reduced thyroid antibodies with 200 mcg/day. However, selenium has a relatively narrow safety window, and supplementation in already-replete populations has not shown benefit and may carry risk. Soil selenium content varies dramatically by region (low in parts of Europe and China, adequate in most of North America), making geographic context relevant.

Claim-by-claim

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Essential for thyroid hormone metabolism and may support thyroid autoimmunity

Selenium is required for deiodinase enzymes that convert T4 to active T3. Multiple RCTs show selenium supplementation (200 mcg/day) reduces TPO antibodies in Hashimoto's thyroiditis, though clinical thyroid function improvement is less consistent.

Supports antioxidant defense via glutathione peroxidase and selenoproteins

Selenium is a cofactor for glutathione peroxidase and thioredoxin reductase — key antioxidant enzymes. Adequate selenium status is associated with reduced oxidative stress markers. Supplementation benefits are clearest in populations with low baseline selenium.

Sources

1 cited

[01]GOVTSelenium — Fact Sheet for Health ProfessionalsNIH Office of Dietary Supplements. 2024

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Medical disclaimer. The information on this site is provided for educational purposes only and is not intended as medical advice. It does not constitute a diagnosis, treatment plan, or recommendation for any specific health condition. Always consult a qualified healthcare professional before making changes to your supplement regimen, diet, or lifestyle — especially if you are pregnant, nursing, taking medications, or managing a medical condition.

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