NutriDex

The Supplement Research Compendium
🧂

Selenium

Essential antioxidant trace mineral with a narrow safe window.

Mixed evidence 🧂Mineral

✚ Updated · How we grade evidence

Evidence tier
Mixed
Research weight
Citations
11 verified / 11
Classification
Mineral
❝
What the evidence says. Conflicting results across studies; benefit uncertain.

What is Selenium?

Selenium is a mineral used for corrects selenium deficiency and supports the activity of selenoproteins (glutathione peroxidases, thioredoxin reductases) that defend cells against oxidative stress.. NutriDex grades the human evidence as Mixed. Selenium is an essential trace mineral incorporated as selenocysteine into selenoproteins such as glutathione peroxidases and thioredoxin reductases, which give it antioxidant activity and a role in thyroid hormone metabolism and immune function. Correcting a genuine deficiency clearly benefits health, and in autoimmune (Hashimoto) thyroiditis, 6 months of supplementation modestly reduces TPO antibodies and TSH, with moderate-certainty evidence but no proven change in long-term disease course. However, supplementing people who are already replete shows little benefit: the large SELECT randomized trial (35,533 men) found selenium did not prevent prostate cancer and produced a non-significant increase in type 2 diabetes risk. A Cochrane review concluded selenium does not prevent cancer and that supplementation increased diabetes risk by roughly 11% in trials. Selenium has an unusually narrow safe range — the gap between the recommended 55 mcg/day and the 400 mcg/day upper limit is small, and chronic excess causes selenosis (hair and nail loss, garlic breath, neurological effects). Overall the evidence is mixed: valuable for deficiency and possibly thyroid autoimmunity, but unhelpful or potentially harmful when status is already adequate.

Purported Benefits

✓Corrects selenium deficiency and supports the activity of selenoproteins (glutathione peroxidases, thioredoxin reductases) that defend cells against oxidative stress.
✓In Hashimoto thyroiditis, 6 months of supplementation modestly lowers thyroid peroxidase (TPO) antibodies and TSH, especially in people not on thyroid hormone replacement (moderate-certainty evidence) — though disease-modifying benefit is unproven.
✓Supports normal immune and thyroid hormone metabolism (deiodinase enzymes are selenoproteins), with the clearest gains seen when baseline status is low.
✓Does NOT prevent prostate cancer or other cancers in already-replete men, and may carry harm signals (diabetes, possibly aggressive cancer) — benefit appears limited to correcting deficiency, not exceeding requirements.

Evidence by outcome

The same supplement can be well-proven for one use and unproven for another — here is the human evidence graded outcome by outcome.

OutcomeEvidenceEffectStudies
TPO antibodies & TSH in Hashimoto thyroiditis6 months lowers TPOAb/TSH (moderate certainty) in those not on thyroid hormone; disease-modifying benefit unproven. Moderate ↑ benefit · small 3
Prostate / cancer prevention in replete menSELECT RCT (n=35,533) and Cochrane found no cancer prevention in already-replete people. Strong — no effect · negligible 2
Type 2 diabetes riskCochrane (+11%) and NPC trial (HR 1.55, dose-dependent) signal increased diabetes risk with supplementation in replete people. Moderate ⚠ risk · small 3
Correcting deficiency / selenoprotein activityClear benefit limited to correcting genuine deficiency; narrow safe range (RDA 55 vs UL 400 mcg). Strong ↑ benefit · moderate 1

Dosing & Compounds

Typical Dose
RDA 55 mcg/day for adults (60 mcg in pregnancy, 70 mcg lactation); typical supplements provide 50–200 mcg/day. Tolerable Upper Intake Level is 400 mcg/day from all sources. Most replete people in selenium-sufficient regions need no supplement.
Active Compounds
Selenomethionine (organic form, most common in supplements/yeast)Selenocysteine (the catalytic residue in selenoproteins)Sodium selenite / sodium selenate (inorganic forms)Selenium-enriched yeast

Safety & Cautions

Selenium has a narrow safe range: do not exceed the 400 mcg/day Tolerable Upper Intake Level from all sources (food + supplements), as chronic excess causes selenosis — hair loss, brittle or lost nails, garlic-odor breath, metallic taste, skin rash, GI upset, fatigue, irritability and peripheral neuropathy. Acute overdose (gram-level) can cause severe toxicity including cardiac and respiratory failure. People who are already selenium-replete (most individuals in selenium-rich regions such as much of North America) gain little and may face increased risk of type 2 diabetes and, in some analyses, more aggressive prostate cancer; supplementation is best reserved for documented deficiency or under medical guidance. Brazil nuts are exceptionally selenium-dense (often 50–90+ mcg per nut), so stacking them with supplements can push intake past safe limits. Use caution if you have diabetes or impaired glucose tolerance, and discuss with a clinician if you are pregnant, breastfeeding, on thyroid medication, or taking anticoagulants. Those with adequate dietary intake should generally avoid routine high-dose supplementation. Educational only — always check with your doctor or pharmacist before combining Selenium with any medicine.

Selenium drug interactions

Known or theoretical interactions between Selenium and common medications — educational, not exhaustive. Always check with your doctor or pharmacist before combining Selenium with any medicine.

Caution
Chemotherapy / radiation
High-dose selenium antioxidant may theoretically alter chemo/radiation efficacy or toxicity — consult oncologist.
Selenium supports glutathione-peroxidase antioxidant defenses that may modulate ROS-dependent cytotoxicity. NIH ODS — Selenium

Key Studies ★ 11 studies

Systematic review / meta-analysis 35 RCTs ✓ PubMed
A 2024 systematic review and meta-analysis found selenium supplementation lowered TSH (SMD −0.21) and TPO antibodies in Hashimoto patients not on thyroid hormone replacement, with moderate certainty and no change in fT4/fT3.
Systematic review / meta-analysis 21 trials / 1,610 subjects ✓ Full text
A 2023 meta-analysis reported selenium supplementation for 6 months (but not 3 months) significantly reduced serum TPO and thyroglobulin antibody levels in Hashimoto thyroiditis.
Meta-analysis Yu 2024 ✓ Full text
In a meta-analysis of 7 RCTs (n=2,276) in cardiac surgery patients, selenium shortened hospital stay (MD -1.33 days, 95% CI -2.51 to -0.16) and reduced CRP (SMD -0.18, 95% CI -0.34 to -0.02) and acute kidney injury (RR 0.76, 95% CI 0.59-0.98), but did not lower mortality (RR 1.07, 95% CI 0.84-1.37).
Meta-analysis Prostaglandins Other Lipid Mediat 2024 ✓ PubMed
A meta-analysis of 25 RCTs found selenium supplementation reduced VLDL (WMD -1.53, 95% CI -2.86 to -0.20) but produced no significant change in total cholesterol, LDL, HDL, or triglycerides in adults.
Meta-analysis Sun 2025 ✓ Source
A 2025 systematic review and meta-analysis of randomized controlled trials evaluated daily 200 mcg selenium for managing gestational diabetes mellitus, assessing effects on glucose and lipid metabolism.
Meta-analysis Zuhair 2024 ✓ PubMed
Meta-analysis (3 studies, 288 patients) in autoimmune thyroiditis: adding myo-inositol to selenium reduced TSH (SMD -1.15, 95% CI -1.60 to -0.69) and thyroglobulin antibodies (SMD -0.51, 95% CI -0.78 to -0.24) more than selenium alone; TPOAb, T3 and T4 changes were not significant. Suggests combination therapy may outperform selenium monotherapy.
Meta-analysis Jenkins 2020 ✓ PubMed
Systematic review/meta-analysis of 43 RCTs: selenium alone showed no association with CVD or all-cause mortality, but antioxidant mixtures containing selenium reduced CVD mortality (RR 0.77, 95% CI 0.62-0.97) and all-cause mortality (RR 0.90, 95% CI 0.82-0.98), whereas antioxidant mixtures WITHOUT selenium increased all-cause mortality (RR 1.09).
Cochrane systematic review 83 studies ✓ PubMed
A Cochrane review concluded there is no convincing evidence selenium prevents cancer, and that supplementation increased the risk of type 2 diabetes (RR 1.11, 95% CI 1.01–1.22) in randomized trials.
Government reference / guideline Reference standard ✓ Source
NIH Office of Dietary Supplements sets the adult RDA at 55 mcg/day and the Tolerable Upper Intake Level at 400 mcg/day, with chronic excess causing selenosis (hair/nail loss, garlic breath, GI and neurological effects).
Large RCT 35,533 men ✓ PubMed
In the SELECT RCT, selenium (200 mcg/day) did not prevent prostate cancer (HR 1.04, 99% CI 0.87–1.24) and showed a non-significant increase in type 2 diabetes (RR 1.07, 99% CI 0.94–1.22) in relatively replete men.
RCT Stranges (NPC trial) 2007 ✓ PubMed
Randomized trial (n=1202, mean 7.7 y) found selenium 200 mcg/d increased incidence of type 2 diabetes vs placebo (HR 1.55, 95% CI 1.03-2.33), with a dose-response: highest baseline plasma selenium tertile HR 2.70 (95% CI 1.30-5.61). Supplementation does not prevent and may cause diabetes.
11 sources cited · 11 verified Search all studies on PubMed ↗

Common questions about Selenium

What is Selenium used for?

Selenium is most often taken for Corrects selenium deficiency and supports the activity of selenoproteins (glutathione peroxidases, thioredoxin reductases) that defend cells against oxidative stress., In Hashimoto thyroiditis, 6 months of supplementation modestly lowers thyroid peroxidase (TPO) antibodies and TSH, especially in people not on thyroid hormone replacement (moderate-certainty evidence) — though disease-modifying benefit is unproven., Supports normal immune and thyroid hormone metabolism (deiodinase enzymes are selenoproteins), with the clearest gains seen when baseline status is low., Does NOT prevent prostate cancer or other cancers in already-replete men, and may carry harm signals (diabetes, possibly aggressive cancer) — benefit appears limited to correcting deficiency, not exceeding requirements.. Essential antioxidant trace mineral with a narrow safe window.

Does Selenium work — what does the evidence say?

Mixed evidence. Conflicting results across studies; benefit uncertain. Selenium is an essential trace mineral incorporated as selenocysteine into selenoproteins such as glutathione peroxidases and thioredoxin reductases, which give it antioxidant activity and a role in thyroid hormone metabolism and immune function. Correcting a genuine deficiency clearly benefits health, and in autoimmune (Hashimoto) thyroiditis, 6 months of supplementation modestly reduces TPO antibodies and TSH, with moderate-certainty evidence but no proven change in long-term disease course. However, supplementing people who are already replete shows little benefit: the large SELECT randomized trial (35,533 men) found selenium did not prevent prostate cancer and produced a non-significant increase in type 2 diabetes risk. A Cochrane review concluded selenium does not prevent cancer and that supplementation increased diabetes risk by roughly 11% in trials. Selenium has an unusually narrow safe range — the gap between the recommended 55 mcg/day and the 400 mcg/day upper limit is small, and chronic excess causes selenosis (hair and nail loss, garlic breath, neurological effects). Overall the evidence is mixed: valuable for deficiency and possibly thyroid autoimmunity, but unhelpful or potentially harmful when status is already adequate.

What is the typical dose of Selenium?

RDA 55 mcg/day for adults (60 mcg in pregnancy, 70 mcg lactation); typical supplements provide 50–200 mcg/day. Tolerable Upper Intake Level is 400 mcg/day from all sources. Most replete people in selenium-sufficient regions need no supplement.

Is Selenium safe? Any cautions or side effects?

Selenium has a narrow safe range: do not exceed the 400 mcg/day Tolerable Upper Intake Level from all sources (food + supplements), as chronic excess causes selenosis — hair loss, brittle or lost nails, garlic-odor breath, metallic taste, skin rash, GI upset, fatigue, irritability and peripheral neuropathy. Acute overdose (gram-level) can cause severe toxicity including cardiac and respiratory failure. People who are already selenium-replete (most individuals in selenium-rich regions such as much of North America) gain little and may face increased risk of type 2 diabetes and, in some analyses, more aggressive prostate cancer; supplementation is best reserved for documented deficiency or under medical guidance. Brazil nuts are exceptionally selenium-dense (often 50–90+ mcg per nut), so stacking them with supplements can push intake past safe limits. Use caution if you have diabetes or impaired glucose tolerance, and discuss with a clinician if you are pregnant, breastfeeding, on thyroid medication, or taking anticoagulants. Those with adequate dietary intake should generally avoid routine high-dose supplementation.

How many studies support Selenium?

NutriDex cites 11 sources for Selenium, graded "Mixed".

Does Selenium interact with any medications?

Yes — known or theoretical interactions include: Chemotherapy / radiation (caution). This is educational and not exhaustive; always check with your doctor or pharmacist before combining Selenium with any medicine.

Cite this page
APA

Peh, D. (2026). Selenium: Benefits, Dosage, Side Effects & Evidence. NutriDex — The Supplement Research Compendium. Retrieved 26 Jun 2026, from https://nutridex.info/s/selenium

BibTeX
@misc{nutridex_selenium,
  author       = {Peh, Daryl},
  title        = {Selenium: Benefits, Dosage, Side Effects \& Evidence},
  year         = {2026},
  howpublished = {NutriDex --- The Supplement Research Compendium},
  url          = {https://nutridex.info/s/selenium},
  note         = {Reviewed by Dr Daryl Peh, MBBS Singapore, MMed FM. Accessed 2026-06-26}
}

For medical claims, citing the underlying primary studies linked above is preferred. NutriDex is an educational reference, not medical advice.

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