Selenium

Selenium

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

How it works. Selenium gets incorporated into about 25 different proteins in the human body, called selenoproteins, where it sits at the active site of the enzyme as a specific amino acid called selenocysteine. The two most important families:
Glutathione peroxidase (GPx). One of the three primary antioxidant enzymes covered on Our Cellular Biology. GPx neutralises hydrogen peroxide and lipid peroxides (oxidised fats in cell membranes) by converting them to water and stable alcohols. Without selenium, GPx cannot function, and the antioxidant defence system loses one of its three main pillars. The consequences of this are systemic: more oxidative damage to cell membranes, DNA, and proteins across every tissue.
Iodothyronine deiodinases. The enzymes that convert T4 into T3. The thyroid produces mostly T4 (inactive) with a small amount of T3 (active), and peripheral tissues (primarily liver, kidney, and muscle) convert T4 into T3 as needed. Deiodinase enzymes require selenium to function. Low selenium means impaired conversion, which shows up as normal TSH, normal T4, but low free T3, the most commonly missed pattern of suboptimal thyroid function. Symptoms: fatigue, cold intolerance, sluggish metabolism, brain fog, poor recovery from training, despite "normal" standard thyroid panels.
Thioredoxin reductase. A third family of selenoenzymes involved in antioxidant defence and DNA synthesis. Supports cell proliferation and redox balance.
Thyroid and Hashimoto's. Selenium's most robust clinical application is in autoimmune thyroid disease. A meta-analysis of nine trials found selenium supplementation (typically 200 mcg daily) significantly reduced thyroid peroxidase antibodies (anti-TPO) in patients with Hashimoto's thyroiditis. Anti-TPO is the antibody that drives the autoimmune attack on the thyroid gland, and reducing it slows the progression of the disease. Selenium doesn't cure Hashimoto's but it meaningfully reduces the antibody load in most patients. This is particularly relevant because Hashimoto's is 5-8 times more common in women than men and often develops between ages 30-50, making this one of the few interventions women specifically benefit from more than men.
Sperm and male fertility. Selenium is concentrated in the testes and is essential for sperm motility. The selenoprotein GPx4 is a structural component of the sperm midpiece, where it supports mitochondrial function and flagellar integrity. A trial of men with idiopathic infertility found selenium supplementation (200 mcg daily) for 6 months significantly improved sperm motility and conception rates compared to placebo. This is particularly relevant for men trying to conceive or recovering from cycle-induced fertility suppression.
Immune function. Selenium supports T-cell proliferation, natural killer cell activity, and antibody production. A review found selenium deficiency impairs immune response to viral infections and may accelerate viral mutation rates (a mechanism studied in detail with HIV and influenza). Supplementation in deficient populations consistently improves immune markers. In selenium-replete individuals, additional supplementation doesn't provide further benefit.
Cancer and mortality. The cancer prevention claim for selenium comes from the Nutritional Prevention of Cancer (NPC) trial in the 1990s, which found significant reductions in prostate, lung, and colorectal cancer with 200 mcg daily supplementation. The follow-up SELECT trial, larger and more rigorous, found no benefit and a small increase in type 2 diabetes risk in men who already had adequate selenium status. The current interpretation: selenium supplementation protects against cancer specifically in people who are deficient at baseline. In selenium-replete populations, more is not better and may be mildly harmful. Don't take selenium for cancer prevention if you're already getting enough.
Women specifically. Beyond the Hashimoto's application (where women benefit disproportionately), selenium supports thyroid function through the same T4-to-T3 conversion mechanism relevant to everyone. Women are also more likely to have subclinical thyroid dysfunction, and selenium is one of the higher-leverage interventions for the conversion-impaired pattern. During pregnancy, selenium requirements rise modestly to support fetal thyroid development and placental antioxidant function.
More T3 is not universally better. The Selenium → T3 framing on this page is accurate for people whose T3 is genuinely low because conversion is impaired. It's misleading if you read it as "more T3 is always better." The relationship between T3 and health is U-shaped. Low T3 causes fatigue, cold intolerance, and slow metabolism. Supraphysiological T3 causes muscle wasting, bone loss (T3 stimulates osteoclast activity, and chronic elevation meaningfully raises fracture risk), anxiety, insomnia, tachycardia, and increased atrial fibrillation risk even when only subclinically elevated. Selenium is a conversion rate-limiter remover, not a T3 booster. If your free T3 is already in the 3.0-4.0 pg/mL range, supplementation won't push it higher because the deiodinase enzymes self-regulate based on demand. But if you self-diagnose "low T3" from symptoms alone and stack selenium with iodine and possibly T3 medication, you can push yourself into the overshoot zone where symptoms (anxiety, insomnia, racing heart) get misread as continued deficiency and drive further intervention. Test first, then supplement.
The iodine caveat for Hashimoto's. Selenium supplementation in someone who is simultaneously iodine-deficient can paradoxically worsen thyroid damage. The selenium-dependent enzymes inside the thyroid (particularly GPx) become more active while the gland is struggling to produce hormone with inadequate iodine, and hydrogen peroxide byproducts accumulate. Check iodine status alongside selenium if you have Hashimoto's, don't just start selenium blind.

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

If you're deficient or at the low end of functional sufficient, you may notice improved energy, warmer extremities, and less brain fog within 4-8 weeks as thyroid conversion improves. These effects are strongest in people who had the classic conversion-impaired pattern (normal TSH, normal T4, low free T3).
If you have Hashimoto's, antibody (anti-TPO) reduction typically shows up on bloodwork over 3-6 months. You may or may not feel a subjective difference, the main outcome is slowing progression of the disease and reducing inflammatory damage to the thyroid gland over years.
If you're supplementing for fertility, sperm parameter improvements take 3-6 months because sperm production is a 72-day cycle. Don't evaluate the effect before then.
If you're already selenium-replete, you won't notice much of anything. Taking more selenium than you need doesn't improve function and can slightly elevate diabetes risk over time. This is a supplement where more is not better.

Side effects & risks

  • Selenosis (chronic selenium toxicity) develops at intakes above 400 mcg daily sustained over weeks to months. Symptoms: garlic breath, hair loss, brittle nails, skin rashes, GI upset, fatigue, and neurological symptoms in severe cases. The brittle nails and hair loss are the characteristic signs. If you develop these on selenium supplementation, stop immediately and get serum selenium checked
  • Acute toxicity from single large doses (several milligrams) can cause severe GI symptoms and is rare but serious. Stick to the functional dosing range
  • Type 2 diabetes risk. The SELECT trial found a small but statistically significant increase in type 2 diabetes incidence in men who were selenium-replete at baseline and took 200 mcg daily. The effect was small (roughly 1.5% absolute increase over 5 years) but suggests supplementation isn't benign in people who are already getting enough. If you have normal selenium status, don't supplement at the upper end for no reason
  • Brazil nut overconsumption is the most common cause of accidental selenium toxicity. Eating a handful of Brazil nuts daily for months can push you into selenosis territory. Stick to 1-2 per day
  • Drug interactions: selenium may modestly reduce the efficacy of statins (one mechanism is through interaction with coenzyme Q10 production). Clinically significant interactions are rare, but flag selenium supplementation to whoever is managing your medications
  • Inorganic forms (selenite, selenate) are more likely to cause GI upset than organic forms (selenomethionine). Use selenomethionine if you've had problems with cheaper selenium supplements
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Blood markers

Free T3 (optimal 3.0-4.0 pg/mL) should be tested before starting selenium. This is what tells you whether conversion is actually your problem. If free T3 is already in the optimal range, selenium supplementation isn't doing anything for thyroid function and you're only exposing yourself to the downside (diabetes signal at chronic 200 mcg dosing). If free T3 is low with normal TSH and free T4, selenium is one of the causes to rule out. Recheck at 3 months after starting
Serum selenium (optimal range 120-150 ng/mL). Check at baseline before supplementing. Below 100 ng/mL indicates insufficient status. Above 160 ng/mL means you don't need to supplement further. This pairs with free T3 for a complete picture before starting
Iodine status (urinary iodine or serum iodine) if you have Hashimoto's. Selenium supplementation in the context of iodine deficiency can worsen thyroid damage. Check this before starting selenium for Hashimoto's
Anti-TPO antibodies (optimal below 35 IU/mL, or as low as possible). Relevant only if supplementing selenium for Hashimoto's. Check at baseline and every 6 months to track antibody reduction
Semen analysis for men supplementing for fertility restoration (particularly post-cycle). Sperm count, motility, and morphology at baseline and at 3-6 months. Bloodwork doesn't track this, you need an actual semen analysis
Fasting glucose and HbA1c if you're planning to supplement 200 mcg daily long-term, worth checking periodically given the SELECT trial signal on diabetes risk
Who actually needs what: anyone considering selenium should test free T3 and serum selenium first. Hashimoto's patients add iodine status and anti-TPO. Men supplementing for fertility track semen parameters, not bloodwork. For general health at functional doses (100 mcg or less from diet or a multivitamin), no ongoing bloodwork is required
Avalible everywhere