> Research summary — not medical advice. This page synthesizes published research on a mechanism-level intervention. It is not a clinical recommendation. Consult a qualified healthcare provider before making any changes to diet, supplementation, or treatment.
Overview
Selenium supplementation provides the essential trace element required for synthesis of 25 human selenoproteins, including the glutathione peroxidase family (GPX1-4), thioredoxin reductases (TrxR1-3), and iodothyronine deiodinases (DIO1-3). The thyroid gland has the highest selenium concentration per gram of any human organ, making thyroid autoimmune conditions the primary indication.
> Clinical disclaimer: Selenium has a narrow therapeutic window. The difference between therapeutic dose (200 ug/day) and toxicity threshold (400 ug/day) is only 2x. All supplementation should include baseline and follow-up serum selenium monitoring.
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Mechanism of Action
Selenium is incorporated as selenocysteine (the "21st amino acid") into selenoproteins via a unique UGA codon recoding mechanism. Key functional classes:
- GPX1-4: Reduce hydrogen peroxide and lipid hydroperoxides, protecting cell membranes from oxidative damage
- TrxR1-3: Regenerate thioredoxin, maintaining intracellular redox balance and regulating NF-kB signaling
- DIO1-3: Catalyze T4→T3 conversion (DIO1/2) and T4/T3 inactivation (DIO3), governing thyroid hormone metabolism
In selenium deficiency, these enzymes lose function in a hierarchical manner — brain and endocrine tissues are protected last, but thyroid GPX activity drops early, increasing vulnerability to hydrogen-peroxide-mediated autoimmune attack.
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Dosage and Administration
| Parameter | Recommendation |
|---|---|
| Form | Selenomethionine (best absorbed, most studied in RCTs) |
| Therapeutic dose | 200 ug/day for autoimmune thyroid conditions |
| Maintenance dose | 55-100 ug/day (varies by baseline status) |
| Duration | Minimum 3 months for anti-TPO response; 6-12 months for full assessment |
| Upper limit | 400 ug/day (includes dietary intake) |
| Dietary sources | Brazil nuts (1-2 nuts = ~100 ug; highly variable by soil), seafood, organ meats |
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Monitoring
- Serum selenium: Baseline and at 3 months. Target range 100-130 ng/mL. Below 70 ng/mL indicates deficiency; above 150 ng/mL increases toxicity risk.
- Anti-TPO antibodies: For hashimotos thyroiditis — check at baseline, 3 months, 6 months.
- Cu/Se ratio: For cardiovascular risk assessment. Elevated ratio warrants selenium repletion before copper reduction.
- Selenosis signs: Monitor for garlic breath, brittle nails/hair, GI disturbance, peripheral neuropathy.
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Contraindications and Risks
- Toxicity above 400 ug/day: Selenosis presents as hair loss, nail brittleness, garlic-odor breath, fatigue, irritability, and peripheral neuropathy. Chronic excess may increase type 2 diabetes risk (SELECT trial signal).
- Selenium-replete populations: No benefit demonstrated in individuals with adequate baseline selenium (>106 ng/mL for prostate cancer; >100 ng/mL generally). The SELECT trial's null result likely reflects supplementation of already-replete American men.
- U-shaped dose-response: Both deficiency and excess associate with adverse outcomes. "More is not better" applies strongly to selenium.
- Drug interactions: May potentiate anticoagulant effects; may interact with statins and cisplatin.
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Connections
Concepts: selenoprotein-synthesis, nutritional immunity, oxidative stress, thyroid autoimmunity
Related interventions: vitamin d supplementation (synergistic with Se for Hashimoto's), iron management (thyroid function requires both Se and Fe)
Signatures: hashimotos thyroiditis, graves disease, cardiovascular disease
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> Educational content, not medical advice. This page describes mechanisms by which the intervention interacts with the microbiome and metal ecology. It is not a treatment recommendation. Clinical decisions about any intervention should be made with a qualified healthcare practitioner who knows your individual history.