Selenium
Trace mineral essential for thyroid hormone activation, antioxidant defense via glutathione peroxidase enzymes, and immune health. Research supports use in Hashimoto's thyroiditis.
What is Selenium?
Selenium is an essential trace element incorporated into selenoproteins as the amino acid selenocysteine. The human genome encodes 25 selenoproteins, including the glutathione peroxidase (GPx) family and the iodothyronine deiodinases that activate thyroid hormones.
Known Health Benefits
How It Works
Selenium is the catalytic center of glutathione peroxidase enzymes (GPx1–GPx4), which reduce hydrogen peroxide and lipid hydroperoxides, protecting cell membranes from oxidative damage. Type I, II, and III iodothyronine deiodinases require selenium to convert T4 to active T3 and to inactivate thyroid hormones. Thioredoxin reductases (TrxR) depend on selenium for redox signaling, DNA synthesis, and cell proliferation. In the immune system, selenium enhances T-cell proliferation and NK-cell activity.
What Research Says
A landmark 2002 RCT by Gärtner et al. in the Journal of Clinical Endocrinology & Metabolism showed 200 mcg/day selenomethionine reduced anti-TPO antibodies by 40% in Hashimoto's patients over 3 months. The SELECT trial (2009) found no benefit of selenium for prostate cancer prevention, ending earlier enthusiasm. A 2019 Cochrane review concluded selenium supplementation may reduce Hashimoto's antibody levels but evidence for clinical thyroid function improvement remains inconclusive.
Active Compounds
Selenomethionine, sodium selenite, selenocysteine
Forms & Bioavailability
Selenomethionine is absorbed > 90% in the intestine and can be non-specifically incorporated into proteins in place of methionine, creating a tissue reserve. Sodium selenite has lower absorption (~50%) and does not build tissue stores. Selenomethionine is the preferred supplemental form.
Dosage Guidance
| Use Case | Dosage |
|---|---|
| General antioxidant support | 55–100 mcg |
| Hashimoto's thyroiditis | 200 mcg selenomethionine |
| Male fertility | 100–200 mcg |
| Immune support | 100–200 mcg |
Always consult a healthcare provider for personalized dosing.
Natural Food Sources
- Brazil nuts (544 mcg per oz — 1–2 nuts meets daily needs)
- Yellowfin tuna (92 mcg per 3 oz)
- Halibut (47 mcg per 3 oz)
- Sardines (45 mcg per 3 oz)
- Turkey (31 mcg per 3 oz)
- Eggs (15 mcg per egg)
Potential Side Effects
Garlic breath, hair loss, nail brittleness, and neuropathy at high doses (selenosis)
Who Should Avoid It
- Non-melanoma skin cancer history — SELECT trial raised concerns
- Adequate or high selenium status — supplementation may increase type 2 diabetes risk
- Do not exceed 400 mcg/day — selenosis threshold
Pregnancy & Lactation
Selenium is important for fetal thyroid development and antioxidant defense. RDA is 60 mcg in pregnancy and 70 mcg during lactation. A 2014 Cochrane review found insufficient evidence to recommend selenium supplementation routinely in pregnancy, but deficiency should be corrected. Do not exceed 400 mcg/day.
Known Drug Interactions
May interact with chemotherapy drugs, statins, and blood thinners
Evidence Classification
Supported by cohort studies, case-control studies, or multiple observational studies with consistent findings.
Frequently Asked Questions
How many Brazil nuts do I need for selenium?
Just 1–2 Brazil nuts per day can provide 100–200 mcg of selenium, meeting daily needs. However, selenium content varies widely by soil, so supplementation may be more reliable for therapeutic use.
Can selenium help with Hashimoto's thyroiditis?
Yes — multiple RCTs show 200 mcg/day of selenomethionine reduces anti-TPO antibodies by 20–40% over 3–6 months. Whether this translates to improved clinical thyroid function is still debated.
Is it possible to get too much selenium?
Yes. Selenosis begins at chronic intakes above 400 mcg/day and manifests as garlic breath, brittle nails, hair loss, fatigue, and neuropathy. Acute selenium poisoning is a medical emergency.
Should I take selenium with iodine?
Selenium and iodine work synergistically for thyroid health. Selenium is required for deiodinase enzymes that activate thyroid hormones, and it protects the thyroid from oxidative damage caused by iodine metabolism. Supplementing iodine without adequate selenium may worsen thyroid autoimmunity.
Does selenium reduce cancer risk?
The Nutritional Prevention of Cancer (NPC) trial initially suggested a 50% reduction in prostate cancer, but the larger SELECT trial (35,000 men) found no benefit and a slight diabetes risk signal. Current evidence does not support selenium supplementation for cancer prevention.
What form of selenium is best?
Selenomethionine is the best-absorbed form and builds tissue reserves. It is the form used in most positive clinical trials. Sodium selenite is cheaper but has lower bioavailability and can be pro-oxidant at higher doses.
References
- Selenium supplementation in patients with autoimmune thyroiditis decreases thyroid peroxidase antibodies concentrations. Gärtner R, Gasnier BC, Dietrich JW, et al.. Journal of Clinical Endocrinology & Metabolism (2002)View study
- Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). Lippman SM, Klein EA, Goodman PJ, et al.. JAMA (2009)View study
- Selenium supplementation for Hashimoto's thyroiditis. van Zuuren EJ, Albusta AY. Cochrane Database of Systematic Reviews (2013)View study
- Selenium and thyroid disease: from pathophysiology to treatment. Köhrle J, Jakob F, Contempré B, Dumont JE. Thyroid (2005)View study
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This entry is for educational purposes only. It is not medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any supplement regimen, especially if you take medications or have health conditions.