Selenium
A trace mineral your thyroid can't work without. It also powers your body's master antioxidant (glutathione) and supports immune defence.
Selenium is an essential trace mineral that your body needs in tiny amounts but can't make on its own. It's critical for thyroid hormone production, glutathione recycling (your body's main antioxidant), and a well-functioning immune system. Indian soils tend to be low in selenium, making deficiency common.
Good for you if: You have Hashimoto's or thyroid antibodies, live in a low-selenium soil region (most of India), want to support your glutathione system, or are looking to optimise immune function.
Dive deeper into the researchCommon side effects
- Nausea or metallic taste (usually at higher doses)
- Garlic-like breath odour if you exceed safe levels
- Hair and nail brittleness with chronic excess (>400 mcg/day)
What does selenium do?
Selenium is a building block for about 25 special proteins in your body called selenoproteins. The most important ones do three things:
- Run your thyroid — Your thyroid makes an inactive hormone (T4). Selenium-dependent enzymes convert it into the active form (T3) that actually controls your metabolism.
- Recycle glutathione — Glutathione is your body's master antioxidant. Selenium is needed by the enzyme (glutathione peroxidase) that recharges it after it neutralises free radicals.
- Support immune cells — Selenium helps your immune cells respond faster and more accurately to infections, and reduces excessive inflammation.
What can you expect?
- Improved thyroid function — lower TPO antibodies (21–40% in studies), better T4-to-T3 conversion
- Stronger antioxidant defence — higher glutathione peroxidase activity
- Better immune response — faster viral clearance, reduced duration of infections
- Potential mood lift — selenium deficiency is linked to anxiety and low mood
You won't "feel" selenium the way you feel caffeine. The benefits show up in lab work over 3–6 months, and as fewer sick days over time.
How to take it
100–200 mcg selenomethionine daily with food — take it with breakfast or lunch. No need to time it around other supplements.
If you eat 2–3 Brazil nuts daily (each has ~70–90 mcg selenium), you may not need a supplement. Brazil nuts are the richest food source by far.
Selenium has a narrow safety window. The upper limit is 400 mcg/day from all sources (food + supplements). More is definitely not better — chronic excess causes hair loss, nail problems, and nerve damage.
Which form to buy?
Selenomethionine is the most commonly recommended form. It's an organic form of selenium that your body absorbs well and stores efficiently in tissues. It's the form used in most major clinical trials.
Selenium yeast contains multiple organic selenium forms and is also well-studied. Good choice if you can find it.
Sodium selenite is cheaper but less bioavailable. It works, but selenomethionine is preferred when available.
Want to see if selenium is actually working for you?
eterni tracks your thyroid markers and selenium levels before and after — so you're not just guessing.
Get early accessFrequently Asked Questions
How much selenium should I take per day?
55–100 mcg per day from supplements is a safe and effective range. Most people don't need more than 200 mcg from all sources combined (food + supplements). If you eat 2–3 Brazil nuts a day, you may not need a supplement at all — one Brazil nut can contain 70–90 mcg.
Is selenium important for thyroid health?
Yes — your thyroid contains more selenium per gram than any other organ in your body. Selenium is essential for converting the inactive thyroid hormone T4 into the active T3 form. Studies show selenium supplementation reduces thyroid antibodies (TPO-Ab) in people with Hashimoto's thyroiditis by 21–40%.
Can you take too much selenium?
Yes — selenium has a narrow safety window. The upper limit is 400 mcg/day from all sources. Chronic intake above 800 mcg can cause selenosis: hair loss, brittle nails, garlic breath, fatigue, and nerve damage. This is why dosing correctly matters more with selenium than most supplements.
Which form of selenium is best?
Selenomethionine is well-absorbed and builds up selenium stores in your body. Sodium selenite is an inorganic form that's cheaper but less bioavailable. For thyroid-specific support, some studies use selenium yeast (which contains multiple organic forms). All three work — selenomethionine is the most popular choice.
How it works in your body
Selenium is incorporated into proteins as the amino acid selenocysteine (the "21st amino acid"). These selenoproteins include glutathione peroxidases (GPx1–GPx6), thioredoxin reductases, and iodothyronine deiodinases (types 1, 2, and 3 — the enzymes that convert T4 to T3).
The thyroid gland has the highest selenium concentration per gram of any organ. When selenium is deficient, T4-to-T3 conversion drops, leading to higher T4 and lower T3 — even when the thyroid itself is producing adequate hormone. This pattern is common in subclinical hypothyroidism.
Selenium also regulates the immune system via its role in selenoprotein P (SELENOP) and GPx. Deficiency shifts the immune response toward excessive inflammation while weakening adaptive immunity.
What the studies show
- Hashimoto's thyroiditis: A meta-analysis of 16 RCTs found 200 mcg/day selenium reduced TPO antibodies by 21–40% over 3–12 months, with greater effects in those who were selenium-deficient at baseline
- Immune function: Selenium supplementation (100–200 mcg/day) improved viral clearance and reduced infection severity in multiple RCTs, including HIV and influenza models
- Cancer: The SELECT trial (35,000+ men) found no benefit for prostate cancer prevention with selenium — but participants had adequate selenium levels at baseline. Benefits appear mainly in deficient populations.
- Indian soil data: Studies show that Indo-Gangetic plains and many peninsular regions have low-selenium soils, contributing to widespread subclinical deficiency in vegetarians
Side effects & safety
Selenium is safe within the recommended range but has a narrower therapeutic window than most supplements:
- At 55–200 mcg/day — well-tolerated with minimal side effects. Occasional nausea if taken on an empty stomach.
- At 200–400 mcg/day — generally safe but approaching the upper limit. Monitor with blood levels.
- Above 400 mcg/day (chronic) — risk of selenosis: hair loss, nail brittleness/white spots, garlic breath, fatigue, irritability, mild nerve damage
- Above 800 mcg/day — toxic. Can cause serious GI distress, kidney/liver damage, and neurological symptoms.
Who should be careful: People already eating selenium-rich diets (many Brazil nuts daily), those with kidney disease (reduced excretion), and anyone on anticoagulants (selenium may mildly enhance effects).
Which labs to check
- Serum selenium — optimal range 100–150 ng/mL. Below 85 ng/mL suggests deficiency.
- Glutathione peroxidase (GPx) — functional marker of selenium status
- TPO antibodies — if supplementing for thyroid support, track these every 3–6 months
- Thyroid panel (TSH, Free T3, Free T4) — look for improved T3:T4 ratio
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