Mineral ·Strong evidence ·Reviewed May 2026
Essential trace mineral crucial for thyroid function, antioxidant defense, and immune health. Narrow therapeutic window, both deficiency and excess are harmful.
The bottom line
Evidence rating strong. Most-documented uses: thyroid function, antioxidant defense, immune support. 20 sources indexed (2007–2024), with 16 interaction records on file.
Core mechanism
Incorporated as selenocysteine into 25 selenoproteins including glutathione peroxidases (GPx, neutralize hydrogen peroxide), thioredoxin reductases (regenerate antioxidants), and iodothyronine deiodinases (convert T4 to active T3 thyroid hormone).18,19
Do not exceed 400mcg/day; Brazil nuts are the richest food source (1-2 nuts = daily need)15,1
Ranked by evidence and value.
Real-world pricing across three quality tiers. Assumes L-Selenomethionine.
Assumes about 100-200 mcg/day. Selenium remains inexpensive, but unnecessary high-dose long-term use carries real toxicity risk. Updated 2026-04-02.
How much you'd eat to match a supplemental dose.
Brazil nuts vary wildly in selenium content, so they can overshoot easily.
Daily high intake from nuts plus supplements can become excessive.
Timing: With breakfast
Keep total daily selenium conservative because chronic excess can be harmful.
Dose: 100-200 mcg daily19
Timing: Any consistent time
Brazil nuts can also provide large amounts, so do not stack blindly.
Dose: 100-200 mcg daily8
Timing: With food
Stay away from chronic high-dose use.
What to test, the optimal window inside the conventional range, and how long a response takes.
Adequate selenium supplementation should raise serum selenium into range without overshooting.1,2
Use conservative dosing and recheck if supplementing long term.
When selenium deficiency is contributing to thyroid stress, TSH may move toward range with correction.1,7
Interpret with free T4, free T3, and thyroid antibodies when clinically relevant.
In people with autoimmune (Hashimoto) thyroiditis, selenium supplementation may modestly lower thyroid peroxidase (TPO) antibody titers, possibly because selenium is incorporated into antioxidant selenoproteins (such as glutathione peroxidases) that help reduce oxidative stress in thyroid tissue. The evidence here is emerging and mixed: some trials show a meaningful drop in titers while others show little or no change, and a lower antibody level has not been clearly shown to improve long-term thyroid outcomes. A reduction in titer should therefore be viewed as a surrogate marker rather than proof of clinical benefit.10,1
TPO antibody testing does not require fasting, and timing relative to your selenium dose does not affect the result, so a morning or any convenient draw is fine. Antibody titers can fluctuate substantially between draws regardless of treatment, so interpret a single value cautiously and, where possible, compare results run by the same lab using the same assay. Because this marker reflects an autoimmune thyroid condition, involve a clinician (ideally the one managing your thyroid) before starting selenium and to interpret trends alongside thyroid function. Selenium has a relatively narrow safe range and excess intake can cause toxicity, so monitor your total intake and do not stack multiple selenium-containing products without medical guidance.
Selenium supports free T3 by supplying the cofactor needed for the deiodinase enzymes that convert the storage hormone T4 into active T3 in peripheral tissues. In people who are genuinely selenium deficient, correcting that deficiency can modestly improve this conversion and nudge free T3 upward; in those who are already replete, adding more selenium is unlikely to push free T3 higher. The overall evidence here is moderate, so treat any expected change as modest rather than guaranteed.1,2
Draw thyroid labs in the morning and try to keep the timing consistent between draws, since free T3 has mild diurnal variation. Fasting is not strictly required for free T3, but a morning fasted draw makes results easier to compare and pairs well with other panels. Take selenium with food to ease tolerability, and avoid a biotin-containing supplement for roughly 48 to 72 hours before testing, as biotin can interfere with many thyroid immunoassays. Selenium has a narrow safe range and high intakes are harmful, so do not stack multiple selenium sources; treat this as a safety precaution, not a sign that more is better. Because free T3 reflects thyroid status and shifts here are usually meaningful only in deficiency, interpret results with a clinician who can review your full thyroid picture and confirm whether supplementation is appropriate for you, especially if you have a known thyroid condition or take thyroid medication.
Where this appears in the symptom-to-supplement map, ranked by relevance.
Selenium is a cofactor for glutathione peroxidase and deiodinase enzymes in the thyroid, and several trials report reduced TPO antibody levels with supplementation.10,1
Antibody lowering is most consistent; effects on actual thyroid function and symptoms are less clear, and total intake should stay near 200 mcg per day to avoid selenium toxicity. Adjunctive to clinician-managed care, not a replacement for thyroid hormone therapy.
Selenium is essential for the deiodinase enzymes that convert T4 to active T3 and for glutathione peroxidase that protects the thyroid from oxidative damage.1,10
May lower thyroid antibodies in Hashimoto's, and total intake should stay under about 200 mcg per day to avoid toxicity.
Selenium sulfide reduces Malassezia load and scalp cell turnover, a long-standing antidandruff approach.1,2
Use the topical shampoo, not oral selenium, which has a narrow safe range.
Selenium supports antioxidant enzymes in the testes and is involved in sperm structure and motility.2,8
Often studied with vitamin E; keep total selenium under about 200 mcg per day.
Selenium is incorporated into antioxidant selenoproteins that support immune cell function, and low status has been linked to weaker immune responses.1,2
Keep within recommended limits because selenium has a narrow safe range; supportive only, not a substitute for vaccination or medical care.
Selenium sulfide reduces Malassezia load on the skin and is a long-standing topical antifungal used for yeast-driven skin and scalp conditions.1,2
Use the topical wash, not oral selenium, which has a narrow safe range. A dermatologist can confirm the diagnosis, which often needs an oral or topical antifungal.
Selenium supports antioxidant defenses in prostate tissue, though trials have not confirmed a benefit for prostate size or cancer prevention.19,1
Keep total selenium under about 200 mcg per day; excess can be harmful.
Selenium is required for thyroid function, but excess can also cause hair loss.1,2
Use conservative doses only.
Selenium supports the antioxidant enzymes that protect follicle melanocytes from the oxidative stress thought to contribute to graying.19,1
Mechanistic rationale only; selenium has a narrow safe range, so do not exceed roughly 200 mcg per day total from all sources.
Evidence-based stacks that include it, with the exact dose and timing each one uses.
Cofactor for glutathione peroxidases; optimizes T-cell proliferation and antibody production19
Selenium is a cofactor for glutathione peroxidase and for the iodothyronine deiodinases that convert T4 to active T3, and adequacy helps buffer the thyroid against oxidative stress as iodine intake rises. Keep total daily intake at or below 200 mcg, since chronic intakes above roughly 400 mcg can cause selenosis.19,1
Selenium is a cofactor for glutathione peroxidases and for a selenoprotein in the sperm midpiece, supporting antioxidant defense and the structural integrity of the sperm tail. Benefit is most likely when selenium intake is low, and high chronic intake should be avoided.18,19
Selenium is incorporated into glutathione peroxidases and other selenoproteins that support antioxidant defense in prostate tissue. Large trials did not show that supplementation prevents prostate cancer and high intakes may carry risk, so dosing should stay near nutritional adequacy rather than megadoses.19,1
GPX1 Pro200Leu influenced the glutathione peroxidase activity response to selenium supplementation in a randomized controlled trial (PMID 22952184).
Recommendation: Use selenium cautiously and avoid chronic high doses, because response and toxicity margins can vary.
Selenium and vitamin E work synergistically as antioxidants. Selenium is part of glutathione peroxidase, while E breaks lipid peroxidation chains.
Recommendation: Take together for comprehensive antioxidant protection. They address different parts of the oxidative stress cascade.
Selenium is critical for thyroid hormone metabolism. Selenoproteins (deiodinases) convert T4 to active T3, and glutathione peroxidase protects the thyroid from peroxide damage.
Recommendation: Ensure adequate selenium when supplementing iodine. Selenium deficiency with iodine supplementation can worsen thyroid damage.
NAC provides glutathione, and selenium is the catalytic center of glutathione peroxidase enzymes.
Recommendation: Take together for optimal glutathione peroxidase function and antioxidant defense.
High-dose vitamin C may reduce selenite to elemental selenium, which is not absorbable. Does not affect selenomethionine forms.
Recommendation: If using sodium selenite form, separate from high-dose vitamin C by 2 hours. Selenomethionine is unaffected.
Both support immune function and antioxidant defense. Zinc supports SOD and thymulin; selenium supports GPx and selenoproteins.
Recommendation: Take together for comprehensive immune and antioxidant support.
High-dose vitamin C may reduce selenite to elemental selenium, which is not absorbable. Does not affect selenomethionine forms.
Recommendation: If using sodium selenite form, separate from high-dose vitamin C by 2 hours. Selenomethionine is unaffected.
Both support immune function and antioxidant defense. Zinc Picolinate supports SOD and thymulin; selenium supports GPx and selenoproteins.
Recommendation: Take together for comprehensive immune and antioxidant support.
Both support immune function and antioxidant defense. Zinc Carnosine supports SOD and thymulin; selenium supports GPx and selenoproteins.
Recommendation: Take together for comprehensive immune and antioxidant support.
Both support immune function and antioxidant defense. Elderberry Zinc Lozenges supports SOD and thymulin; selenium supports GPx and selenoproteins.
Recommendation: Take together for comprehensive immune and antioxidant support.
Selenium is an essential cofactor for glutathione peroxidase, the enzyme through which glutathione neutralizes peroxides, so adequate selenium is needed for glutathione to exert its antioxidant function.
Recommendation: Ensure adequate selenium intake to support glutathione-dependent antioxidant activity; avoid exceeding the 400 mcg upper limit of selenium.
Adequate selenium status helps keep coenzyme Q10 in its active reduced form, and the pair together supports cardiac function more than either alone in people with low selenium.
Recommendation: Combine when selenium intake is low or marginal. A common evidence-based pairing is selenium 100 to 200 mcg with coenzyme Q10 100 to 200 mg daily. No timing separation is needed.
Selenium-dependent enzymes help regenerate ubiquinol after it is oxidized during antioxidant activity, so the two nutrients complement each other in supporting cardiac and antioxidant status.
Recommendation: Reasonable to combine, especially with marginal selenium intake. A typical pairing is selenium 100 to 200 mcg with ubiquinol 100 to 200 mg daily. No timing separation is needed.
Numbered references. Citations throughout the page link here.
Selenium supplementation reduced TPOAb and TSH levels and improved well-being in Hashimoto thyroiditis patients
In 4 RCTs, selenium was superior at 6 months in lowering clinical activity score (MD=-1.27, p<0.0001), improving total GO-QOL (RR=2.54, p<0.00001), and improving visual and psychological functioning scores.
Krannich F, Mücke R, Büntzel J et al.. A systematic review of Selenium as a complementary treatment in cancer patients. Complementary therapies in medicine. 2024
Filippini T, Fairweather-Tait S, Vinceti M. Selenium and immune function: a systematic review and meta-analysis of experimental human studies. The American journal of clinical nutrition. 2023
Zhou J, Zhang W, Cao Z et al.. Association of Selenium Levels with Neurodegenerative Disease: A Systemic Review and Meta-Analysis. Nutrients. 2023
In 5 RCTs, selenium supplementation significantly reduced total testosterone (SMD=-0.42, p=0.02) and cholesterol (SMD=-0.71, p=0.04) in PCOS patients compared to placebo.
Fakhrolmobasheri M, Mazaheri-Tehrani S, Kieliszek M et al.. COVID-19 and Selenium Deficiency: a Systematic Review. Biological trace element research. 2022
Lima LG, Santos AAMD, Gueiber TD et al.. Relation between Selenium and Female Fertility: A Systematic Review. Revista brasileira de ginecologia e obstetricia : revista da Federacao Brasileira das Sociedades de Ginecologia e Obstetricia. 2022
Selenium supplementation significantly dropped TPOAb titers at 6 months, and both TPOAb and TgAb titers at 12 months
Selenium at >=55 ug/day decreased cancer risk (RR 0.94); protective effect found in men and in populations with low baseline selenium
Selenium supplementation alone reduced cancer incidence (RR 0.76) in 9 RCTs, greatest benefit in populations with low baseline selenium
Wang P, Chen B, Huang Y et al.. Selenium intake and multiple health-related outcomes: an umbrella review of meta-analyses. Frontiers in nutrition. 2023
Current upper intake limits may be inadequate; selenium intake as low as 60 ug/day might increase type 2 diabetes risk
Selenium compounds exhibit antioxidant activity through ROS scavenging, GPx enzymatic catalysis, and metal-binding mechanisms
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