Selenium (Se) — guide

What is selenium

Selenium (Se) is an essential trace element required for thyroid function, antioxidant protection, and immune health. In the body, selenium is incorporated into selenoproteins — enzymes that protect cells from oxidative stress and participate in the conversion of thyroid hormone T4 into the active form T3.

Serum selenium reflects dietary intake and body stores. It is measured in micrograms per litre (µg/L).

Reference range

The normal serum selenium concentration is 70–150 µg/L. Many sources consider 100–120 µg/L optimal — selenoprotein activity is highest within this range.

Selenium levels can vary depending on the selenium content of the soil in a region. In European countries, where soil is poor in selenium, population levels are often lower than in North America.

Why selenium may be low

The most common causes of low selenium:

• Insufficient dietary intake — especially a monotonous diet or a vegan diet without supplementation
• Malabsorption — Crohn's disease, coeliac disease, short bowel syndrome
• Kidney disease — haemodialysis can reduce selenium levels
• Autoimmune thyroid disease — in Hashimoto's thyroiditis there may be a link between low selenium and higher anti-TPO antibodies
• Long-term parenteral nutrition without selenium supplementation
• Pregnancy and breastfeeding — increased requirements

Why selenium may be high

Elevated selenium (selenosis) is rare:

• Excessive supplement use — the most common cause in industrialised countries
• Occupational exposure — electronics and glass manufacturing
• Living in a region with very high soil selenium content

Symptoms of selenosis may include hair loss, brittle nails, a garlic odour on the breath, digestive disturbances, and peripheral neuropathy.

What to do next

If selenium is low:

• Review your diet — selenium-rich foods: Brazil nuts (1–2 per day can meet the daily requirement), seafood, meat, eggs, sunflower seeds
• Discuss with your doctor whether supplementation is needed, especially if you have an autoimmune thyroid condition
• Check thyroid function (TSH, free T4, anti-TPO)

If selenium is high — review whether you are taking too many supplements and reduce the dose.

Monitoring over time

It is reasonable to recheck selenium every 6–12 months if you are taking supplements or have malabsorption. With a stable diet and a normal level, frequent retesting is not necessary.

If selenium was low and you started supplementation or changed your diet, repeat the test after 2–3 months to assess the response.

Questions for your doctor

1. Could my selenium level be related to an autoimmune thyroid process? 2. Should I take selenium supplements, or are dietary changes sufficient? 3. Is it worth checking other trace elements (zinc, copper) if selenium is low? 4. How often should I repeat the selenium test?