Selenium (oral salts and chelates: selenomethionine, sodium selenite, selenium yeast)

Mineral supplements. Trace elements

ATC code: A12CE-SELENIUM (Selenium (oral salts and chelates, generic group))

Brand names – supplements

Thorne Selenomethionine, Pure Encapsulations Selenium, Life Extension Super Selenium Complex

Supplements are not tested in clinical trials and are not registered as medications.

Mechanism of action

Selenium is a component of selenoproteins (more than 25 identified in humans), including glutathione peroxidases (antioxidant defence), iodothyronine deiodinases (thyroid hormone conversion from T4 to T3), and thioredoxin reductases (redox regulation). An essential trace element. Soil and food selenium content varies markedly by geography: eastern Siberia, north-western regions of Russia, and north-eastern China are low-selenium regions.

Indications

Selenium deficiency

First line

Selenium preparations are first-line for confirmed selenium deficiency. Treatment dose: 100–200 µg elemental selenium daily for 1–3 months with plasma selenium monitoring. In residents of regions with low soil selenium (eastern Siberia, north-western Russia), supplementation at the RDA is reasonable on a varied diet but not mandatory. In patients on long-term parenteral nutrition, daily supplementation of 60–100 µg is standard.

Sources

Cochrane: Selenium supplementation for cancer prevention (2018)

Graves orbitopathy (thyroid eye disease)

Adjunct

Selenomethionine 100 µg twice daily for 6 months in patients with mild-to-moderate active thyroid eye disease showed improved quality of life and slowed disease progression in the NEJM 2011 RCT (159 patients). ETA 2021 included selenium in mild Graves orbitopathy recommendations as a level A evidence agent. The effect is more pronounced in Europe (a low-selenium soil region); in the US and Japan the effect was smaller.

Use is justified only in the active phase of mild disease. In severe or sight-threatening disease: glucocorticoids, teprotumumab, orbital decompression.

Sources

Hashimoto thyroiditis (chronic autoimmune thyroiditis)

Individual decision

The 2013 Cochrane review showed a moderate reduction in TPO antibody titres with selenomethionine 200 µg daily in autoimmune thyroiditis. The clinical significance of antibody titre reduction is debated – long-term effects on progression to hypothyroidism, levothyroxine dose, or symptoms have not been confirmed in large RCTs. Russian endocrinology guidelines 2021 do not include selenium as first-line in Hashimoto thyroiditis. The decision is individualised.

Sources

Cochrane: Selenium supplementation for Hashimoto thyroiditis (2013)

Anti-aging and longevity (marketed indication)

Not recommended

Selenium for slowing ageing or extending lifespan in non-deficient individuals is not mentioned in international guidelines. The antioxidant ageing hypothesis underlying this marketing argument has not received compelling clinical support. Long-term high-dose selenium is dangerous due to selenosis.

Sources

Cochrane: Selenium supplementation for cancer prevention (2018)

Cancer prevention

Not recommended

The large SELECT RCT (35,533 men, 7 years of follow-up, JAMA 2009) showed no protective effect of selenium 200 µg and vitamin E 400 IU daily on prostate or other cancers. In a subset of patients, prostate cancer risk increased in the selenium arm. Cochrane 2018 confirmed no effect of selenium on cancer prevention or all-cause mortality in non-selenium-deficient people. Selenium for cancer prevention is not supported by international guidelines.

Sources

Practical notes

Timing and administration

Take regardless of meals. Selenomethionine and selenium yeast are absorbed better than inorganic forms (sodium selenite). Combination with vitamin C slightly enhances absorption.

Dose titration

Adult RDA: 55 µg (60 µg for women). Prophylactic supplement dose: 50–100 µg daily. Treatment dose for deficiency: 100–200 µg daily. For mild Graves orbitopathy: 100 µg twice daily for 6 months. Upper tolerable intake level (UL): 400 µg daily. Doses above 800 µg daily long-term cause selenosis.

Monitoring

With long-term high doses (above 200 µg daily), monitor plasma selenium. Signs of selenosis: garlic odour of breath and skin, brittle hair and nails, dermatitis, peripheral neuropathy, and in severe cases hepatic cirrhosis.

Food and drinks

Dietary sources: Brazil nuts (1–2 nuts cover the RDA), marine fish (tuna, sardines), meat, eggs, whole grains, mushrooms. Food selenium content directly depends on regional soil selenium. Residents of selenium-deficient regions are advised to include seafood or products from selenium-rich regions.

Common myths

Myth: “selenium prevents cancer”. Fact: the SELECT RCT (over 35,000 men) did not confirm a protective effect; in a subgroup selenium increased prostate cancer risk. Cochrane 2018: no effect on cancer incidence.

Myth: “selenium is good for everyone's thyroid”. Fact: the effect on AIT antibody titre reduction is modest, on clinical outcomes – not proven. In patients without confirmed selenium deficiency and without AIT, supplementation is unjustified.

Myth: “more selenium, less ageing”. Fact: in non-deficient people, the effect is not proven. Selenium in excess is toxic – doses above 400 µg daily long-term cause selenosis. The safety window between the RDA (55 µg) and toxic doses is narrow.

Safety

Contraindications

Hypersensitivity to selenium
Selenosis (any signs of overdose)
Pregnancy and breastfeeding at doses above the RDA

Serious adverse effects

Selenosis: brittle hair and nails, dermatitis, peripheral neuropathy with long-term doses above 800 µg daily
Hepatic cirrhosis and cardiomyopathy in severe chronic intoxication

Common adverse effects

Garlic breath at doses above 400 µg daily
Nausea, epigastric discomfort
Metallic taste

Uncommon adverse effects

Skin rash, pruritus
Fatigue, irritability

PregnancyFDA C

Safe at physiological doses (60 µg daily). Doses above 200 µg daily in pregnancy are not recommended. In pregnant women with confirmed deficiency, supplementation is monitored by plasma levels.

Breastfeeding

Actively secreted into breast milk. Maternal supplementation at physiological doses is safe and increases milk selenium in deficient mothers.