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Selenium

Evidence Level
Strong
2 Clinical Trials
4 Documented Benefits
4/5 Evidence Score

Selenium is an essential trace mineral that the body uses to build antioxidant enzymes (such as glutathione peroxidase) and to activate thyroid hormones. Through these roles it supports thyroid function, a healthy immune response, antioxidant defense, and reproductive health. Most people obtain enough from foods such as Brazil nuts, seafood, eggs, and grains, though intake varies with soil content. Supplements typically provide 100 to 200 mcg, often as well-absorbed selenomethionine or selenium yeast. Because selenium has a relatively narrow safe range, total intake should stay below about 400 mcg per day to avoid toxicity.

Studied Dose 55 mcg/day (RDA); therapeutic studies: 100–200 mcg/day; upper limit 400 mcg/day
Active Compound Selenomethionine (organic, best absorbed) / Sodium Selenite (inorganic)
Deficiency information View details

Selenium deficiency in the US is uncommon because soil selenium content is generally adequate. The classic deficiency disease, Keshan disease (a cardiomyopathy), occurred in regions of China with severely selenium-deficient soils. Significant selenium deficiency in the US is mostly seen in people with severe malabsorption, on long-term parenteral nutrition, or with HIV.

Common symptoms

  • Cardiomyopathy — heart muscle weakening, heart failure (Keshan disease, severe deficiency)
  • Muscle pain, weakness, or wasting
  • Hair loss or thinning
  • Brittle nails, white nail beds
  • Fatigue
  • Compromised immunity, frequent infections
  • In children: Kashin-Beck disease (osteoarthropathy with joint deformities)
  • Possible association with hypothyroidism (selenium needed for thyroid hormone activation)
  • Macrocytic anemia (rare)

At-risk groups

  • People living in regions with severely selenium-poor soils (parts of China, New Zealand historically)
  • People with HIV, especially advanced disease
  • People on long-term total parenteral nutrition (TPN) without selenium
  • People with severe GI conditions affecting absorption (Crohn's, ulcerative colitis, short-bowel syndrome)
  • People on hemodialysis
  • People with severely restricted diets
  • Note: Brazil nuts contain extremely high selenium (~50-100+ µg per nut) — just 1-2 daily can exceed needs; supplementation is rarely necessary
When to see a doctor: Persistent fatigue with hair loss, brittle nails, or unexplained heart issues — especially in someone with malabsorption or HIV — warrants serum selenium or glutathione peroxidase testing. CAUTION: selenium has a relatively narrow safety margin. Toxicity (selenosis) at intakes >400 µg/day causes hair loss, brittle nails, garlic breath, and neurological symptoms. Don't supplement above the UL without medical guidance.
Explore related deficiencies →
Sources: NIH ODS — Selenium Health Professional Fact Sheet · Selenium Deficiency — StatPearls (NBK482260)

Benefits

Antioxidant defense

Selenoproteins including glutathione peroxidases (GPx1–4) neutralize hydrogen peroxide and lipid hydroperoxides, protecting cell membranes and DNA from oxidative damage.

Thyroid hormone metabolism

Iodothyronine deiodinases are selenoenzymes that convert inactive T4 to active T3. Selenium deficiency impairs thyroid hormone activation and may accelerate autoimmune thyroid disease.

Supported by Trial 1

Immune function

Selenium enhances proliferation of T and NK cells, promotes cytokine production, and supports the oxidative burst in macrophages. Deficiency is associated with increased susceptibility to viral infections.

Supported by Trial 2, Trial 1

Cancer risk reduction

Epidemiological studies suggest inverse relationship between selenium status and cancer incidence, particularly colorectal and prostate cancers. Mechanism involves DNA repair and apoptosis induction.

Supported by Trial 2

Mechanism of action

1

Selenoprotein synthesis

Selenium is incorporated as selenocysteine (the 21st amino acid) into over 25 selenoproteins via a unique UGA codon recoding mechanism. These proteins serve antioxidant, anti-inflammatory, and metabolic functions.

2

Glutathione peroxidase activation

Selenium is the catalytic center of GPx enzymes that reduce hydrogen peroxide and organic hydroperoxides to water and alcohols using glutathione as the electron donor, directly protecting against oxidative cell damage.

3

Thioredoxin reductase activity

Selenium-containing thioredoxin reductases maintain thioredoxin in its reduced state, enabling DNA synthesis, peroxiredoxin recycling, and transcription factor regulation.

Clinical trials

1
Selenium for Hashimoto's Thyroiditis — Clinical Trial

Clinical trial of 200 µg/day selenomethionine vs placebo in 70 patients with autoimmune thyroiditis (Hashimoto's) for 3 months. Outcomes: anti-TPO antibodies, thyroid ultrasound. (Gärtner et al. 2002, J Clin Endocrinol Metab)

70 Hashimoto's patients.

Selenium reduced anti-TPO antibody titers and improved thyroid ultrasound echogenicity vs placebo. Note: subsequent CATALYST trial (2019) and other replications have shown mixed effects on clinical outcomes (overt hypothyroidism prevention not clearly established). Modest evidence; ATA does not strongly endorse selenium for routine Hashimoto's management.

2
SELECT Trial — Selenium and Vitamin E for Prostate Cancer Prevention

Large clinical trial (SELECT — Selenium and Vitamin E Cancer Prevention Trial) of selenium (200 µg/day selenomethionine) and/or vitamin E in 35,533 men. (JAMA — or related)

35,533 men. Long-term follow-up.

Primary endpoint negative: neither selenium, vitamin E, nor combination reduced prostate cancer incidence vs placebo. Secondary findings: vitamin E alone increased prostate cancer risk (~17%); selenium showed potential harm signal in subgroups (T2DM risk). Critical context: this large rigorous trial reversed earlier enthusiasm for selenium chemoprevention. Selenium supplementation in selenium-replete populations may harm rather than help.

Side effects and drug interactions

Common Potential side effects

Selenosis at doses above 400 mcg/day: garlic breath, hair loss, nail brittleness
Nausea and GI discomfort with inorganic forms (selenite)
Fatigue, irritability, and peripheral neuropathy with severe chronic excess

Important Drug interactions

Chemotherapy agents (cisplatin) — selenium may reduce nephrotoxicity but could reduce efficacy
Statins — selenium may modestly affect statin efficacy via CoQ10 pathway
Anticoagulants — high-dose selenium may affect clotting; monitor

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Frequently asked questions about Selenium

How much selenium should I take?

The RDA is 55 mcg per day for adults, and most supplements provide 100 to 200 mcg. Selenium has a relatively narrow safe range, so avoid exceeding 400 mcg per day from all sources, since too much can be toxic.

What is selenium good for?

Selenium is a trace mineral essential for thyroid hormone metabolism and for antioxidant enzymes. It supports thyroid, immune, and antioxidant function. Many people get enough from food, especially Brazil nuts.

Can I get enough selenium from Brazil nuts?

Yes, Brazil nuts are exceptionally high in selenium; just one or two can supply a full day's requirement. Because content varies and overdoing it is possible, a few nuts a few times a week is plenty, and you may not need a separate supplement.

Can you take too much selenium?

Yes. Selenium toxicity (selenosis) can occur above about 400 mcg per day and may cause hair loss, brittle nails, garlic breath, and nerve issues. Stay within the recommended range and avoid stacking multiple high-selenium sources.

What is Selenium?

Selenium is an essential trace mineral that the body uses to build antioxidant enzymes (such as glutathione peroxidase) and to activate thyroid hormones. Through these roles it supports thyroid function, a healthy immune response, antioxidant defense, and reproductive health.

What is Selenium used for?

Selenium is researched primarily for Antioxidant, Immune Support, and Metabolic Health. Selenoproteins including glutathione peroxidases (GPx1–4) neutralize hydrogen peroxide and lipid hydroperoxides, protecting cell membranes and DNA from oxidative damage.

What are the signs of Selenium deficiency?

Selenium deficiency in the US is uncommon because soil selenium content is generally adequate. The classic deficiency disease, Keshan disease (a cardiomyopathy), occurred in regions of China with severely selenium-deficient soils.

What is the recommended dosage of Selenium?

The clinically studied dose is 55 mcg/day (RDA); therapeutic studies: 100–200 mcg/day; upper limit 400 mcg/day Always follow the product label and check with a healthcare provider for personal advice.

Is Selenium safe, and does it have side effects?

For most healthy adults, Selenium is well tolerated at studied doses. Reported effects can include: Selenosis at doses above 400 mcg/day: garlic breath, hair loss, nail brittleness Nausea and GI discomfort with inorganic forms (selenite) It may also interact with some medications. Selenium is not right for everyone, so check with a healthcare provider first if you are pregnant or breastfeeding, have a medical condition, or take prescription medication.

Does Selenium interact with any medications?

Possible interactions include: Chemotherapy agents (cisplatin) — selenium may reduce nephrotoxicity but could reduce efficacy Statins — selenium may modestly affect statin efficacy via CoQ10 pathway If you take prescription medication, check with a pharmacist or doctor before using it.

How strong is the scientific evidence for Selenium?

NutraSmarts rates the evidence for Selenium as Strong (4 out of 5). It is backed by 2 clinical trials and 4 cited references summarized on this page. A higher rating reflects more, larger, and better-designed human studies.

References(4 citations)

Evidence ratings on NutraSmarts are based on the totality of human clinical research, with emphasis on randomized controlled trials, meta-analyses, and systematic reviews. The references below directly support claims made throughout this page.

  1. Toulis KA, Anastasilakis AD, Tzellos TG, Goulis DG, Kouvelas D. Selenium supplementation in the treatment of Hashimoto's thyroiditis: a systematic review and a meta-analysis. Thyroid. 2010;20(10):1163-73. doi: 10.1089/thy.2009.0351.PubMedUsed to support: Supports the thyroid claim: meta-analysis of RCTs found selenium (mainly selenomethionine) lowers anti-TPO antibody titers in Hashimoto's thyroiditis, though the authors caution the clinical significance and effect on thyroid function/quality of life remain uncertain.
  2. Klein EA, Thompson IM Jr, Tangen CM, Crowley JJ, Lucia MS, Goodman PJ, et al. Vitamin E and the risk of prostate cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2011;306(14):1549-56. doi: 10.1001/jama.2011.1437.PubMedUsed to support: Cancer prevention not supported: in this large RCT of selenium-replete men, selenium (alone or with vitamin E) did not prevent prostate cancer, and vitamin E significantly increased prostate cancer risk. Argues against selenium supplementation for cancer prevention in well-nourished people.
  3. Stranges S, Marshall JR, Natarajan R, Donahue RP, Trevisan M, Combs GF, et al. Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Ann Intern Med. 2007;147(4):217-23. doi: 10.7326/0003-4819-147-4-200708210-00175.PubMedUsed to support: Safety/harm signal: secondary analysis of an RCT found long-term selenium (200 mcg/day) was associated with a statistically significant increased incidence of type 2 diabetes, especially in those with higher baseline selenium. Supports caution about excess selenium intake.
  4. Rayman MP. Selenium and human health. Lancet. 2012;379(9822):1256-68. doi: 10.1016/S0140-6736(11)61452-9.PubMedUsed to support: Authoritative selenoprotein/deficiency review: explains selenium's role in selenoproteins and the U-shaped risk curve, supporting correction of deficiency while warning that supplementation in selenium-replete populations may cause harm (including diabetes risk) rather than benefit.