Selenium is a trace mineral with two main jobs: running one of your three primary antioxidant enzymes (glutathione peroxidase, covered on Our Cellular Biology) and enabling the enzymes that convert T4 into active T3 (covered on Our Hormonal System). It's not a supplement everyone needs to take. Most people in developed countries get enough from food.

Who this is actually relevant for: people with Hashimoto's or other autoimmune thyroid conditions, people who feel hypothyroid despite "normal" standard thyroid panels (usually a T4-to-T3 conversion problem), men trying to conceive or restoring fertility after a cycle, and people living in regions with genuinely low-selenium soil (parts of the UK, Europe, China, New Zealand). If none of that applies and you eat a varied diet with some seafood, eggs, or meat, you're probably already covered and additional supplementation won't do much. Selenium is one of the few supplements where more is not better, it has a real upper limit and pushing past it creates its own problems.

Before supplementing, test free T3 and ideally serum selenium. Selenium works by removing a bottleneck in T3 production when conversion is impaired. It doesn't push T3 above normal in most people who are already converting fine, but if you don't know where your free T3 sits, you're guessing.

deep dive

Dosage

• RDA: 55 mcg/day for adults. This is the minimum to prevent overt deficiency, not the functional optimal
• Functional target: 100-200 mcg/day total intake (food plus supplementation combined). This range supports optimal glutathione peroxidase activity and thyroid hormone conversion without entering the toxicity zone
• Therapeutic doses:
  • Hashimoto's thyroiditis: 200 mcg/day, the dose used in the meta-analysis trials. Effects on antibody reduction show over 3-6 months
  • Male fertility: 200 mcg/day for 3-6 months before evaluating sperm parameter changes
  • Subclinical hypothyroidism with low free T3: 100-200 mcg/day
• Upper limit: 400 mcg/day is the tolerable upper limit. Above this, chronic selenium toxicity (selenosis) becomes a risk. Do not exceed 400 mcg daily from all sources combined
• Best forms: selenomethionine and selenocysteine are organic forms, well-absorbed, and incorporated directly into selenoproteins. A comparison study found selenomethionine raised plasma selenium more effectively than inorganic sodium selenite or selenate. Selenomethionine is the standard form in most quality supplements
• Selenium-enriched yeast is another well-studied form, providing selenium predominantly as selenomethionine. Used in most of the major clinical trials including SELECT and NPC
• Avoid: sodium selenate and sodium selenite (inorganic forms, less well absorbed, cheaper)
• Brazil nuts: 1-2 Brazil nuts daily from Brazilian or other selenium-rich soil provides 70-200 mcg. The catch is enormous variability, a single Brazil nut can contain anywhere from 50 to 400 mcg depending on the soil it was grown in. If you're using Brazil nuts as your primary selenium source, you're running an unpredictable dose. Stick to 1-2 per day maximum and don't rely on them exclusively if precision matters
• Food sources: Brazil nuts (68-400 mcg per nut, highly variable), tuna (90 mcg per 100g), sardines (50 mcg per 100g), beef (30 mcg per 100g), chicken (25 mcg per 100g), eggs (15 mcg per egg), mushrooms (10-15 mcg per 100g)
• No dosing adjustment needed for women beyond the RDA slightly lower (55 mcg is the same for both sexes). Pregnancy increases RDA to 60 mcg, breastfeeding to 70 mcg
• Timing: selenium is fat-soluble and absorbed with meals. Take with food for best absorption. Time of day doesn't matter

Here's what you can expect