Selenium
Essential trace mineral. Drives glutathione peroxidase, supports thyroid hormone conversion, and regulates immune tone. A few Brazil nuts a week often does the job.
Strong for reducing TPO antibody levels in Hashimoto's (multiple RCTs). Moderate for cancer prevention (the early enthusiasm has been tempered; SELECT trial null for prostate). Useful in immune support and male fertility (sperm quality). Effects on TPO antibodies: 3–6 months.
Selenium is built into ~25 selenoproteins, the most important being glutathione peroxidase (a major antioxidant enzyme) and the deiodinases that convert thyroid hormone T4 into the active T3. Adequate selenium also dampens autoimmune thyroid antibodies (TPO) and modestly supports immune resilience. The catch: the range between deficient and toxic is unusually narrow. Brazil nuts can vary 10-fold in selenium content depending on the soil they grew in, which makes 'a few Brazil nuts a day' surprisingly variable.
- Form
- Selenomethionine — the form most readily incorporated into selenoproteins. Best general choice. Selenium yeast — mixed selenoamino-acids, well-studied. Sodium selenate / selenite — inorganic forms, less well-absorbed. Brazil nuts are a natural food source but content is highly variable — supplementation is more reliable.
- Dose
- 100–200 mcg/day. Upper limit ~400 mcg/day from all sources combined — easy to overshoot if eating Brazil nuts daily AND taking a multivitamin with selenium.
- Timing
- Anytime, with or without food.
- Schedule
- Daily.
- Side effects
- Selenosis at high doses (>400 mcg/day chronically): garlic breath, hair loss, brittle nails, GI upset, mood changes, neurological issues. The toxic ceiling is closer than people realise.
- Interactions
- Anticoagulants (mild additive at high doses). Statins (some evidence selenium improves their efficacy). Lithium (additive thyroid effects).
- Contraindications
- Pre-existing selenosis. Skin cancer history (paradoxical signal from SELECT — non-melanoma skin cancer increased in supplemented group). Caution if eating significant Brazil nuts daily — don't double-dose.
- Lab markers
- Serum selenium or whole-blood selenium. Glutathione peroxidase activity (functional marker). TPO antibodies in those with thyroid autoimmunity.
- Food sources
- Brazil nuts (50–100 mcg per nut — 1–2 per day is often enough). Seafood (tuna, sardines, oysters, shrimp). Organ meats (liver, kidney). Eggs (especially yolk). Mushrooms. Sunflower seeds. Soil selenium content varies regionally — plant sources from selenium-rich areas (US plains, parts of Brazil) are far higher than from selenium-poor areas (parts of the UK, China, Europe).
Pairs with
Iodine (selenium-protects the thyroid from iodine-induced oxidative stress; iodine + selenium together is the standard thyroid stack). Vitamin E (joint antioxidant role — glutathione peroxidase + vitamin E recycle each other). Zinc.
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