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.
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
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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.
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 14 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 16 Sept 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 2026-09-16}
}
For medical claims, citing the underlying primary studies linked above is preferred. NutriDex is an educational reference, not medical advice.