Selenium Blood Test: Normal Ranges, Causes & What Your Results Mean

What Is Selenium?

Selenium is an essential trace mineral that plays a critical role in thyroid hormone metabolism, antioxidant defence, immune function, and reproductive health. The body incorporates selenium into selenoproteins — a family of 25 enzymes including glutathione peroxidases (antioxidant defence), thioredoxin reductases (cell growth), and deiodinases (thyroid hormone activation).

The UK is a region of relatively low selenium status due to lower soil selenium content compared to North America. Average UK dietary selenium intake is approximately 30–60 µg/day, below the recommended 75 µg/day for men and 60 µg/day for women. This subclinical deficiency may have implications for thyroid health, cancer risk, and immune resilience.

Why Is Selenium Tested?

• Thyroid disease — selenium is essential for converting T4 to active T3 via deiodinase enzymes; deficiency worsens hypothyroidism
• Autoimmune thyroiditis — selenium supplementation has been shown to reduce TPO antibodies in Hashimoto's thyroiditis
• Unexplained fatigue or muscle weakness — severe selenium deficiency causes Keshan disease (cardiomyopathy) and Kashin-Beck disease (osteoarthropathy)
• Reproductive health — selenium is incorporated into sperm selenoproteins; deficiency impairs male fertility
• Cancer risk assessment — epidemiological studies link low selenium status to increased cancer risk
• Parenteral nutrition monitoring — patients on long-term IV nutrition require selenium monitoring

Normal Ranges

What Do High Selenium Levels Mean?

• Over-supplementation — the most common cause; selenium toxicity (selenosis) begins above 800 µg/day
• Selenosis symptoms — garlic breath odour, hair loss, brittle nails, fatigue, nausea, nerve damage
• Occupational exposure — glass, electronics, and pigment manufacturing workers
• Brazil nut overconsumption — a single Brazil nut contains 70–90 µg of selenium; eating several daily can cause excess

What Do Low Selenium Levels Mean?

• Inadequate dietary intake — common in the UK due to low-selenium soils
• Malabsorption — coeliac disease, Crohn's disease, or short bowel syndrome
• Chronic kidney disease — dialysis patients are at particular risk
• HIV/AIDS — selenium depletion is associated with disease progression
• Pregnancy — increased demands can deplete stores
• Severe illness — critical illness and burns deplete selenium

How to Improve Your Selenium Levels

• Brazil nuts — just 2–3 per day provides the daily requirement (be careful not to overdo it)
• Seafood — tuna, sardines, prawns, and cod are excellent sources
• Meat and eggs — moderate selenium content; organic and grass-fed sources may have higher levels
• Supplementation — 100–200 µg/day selenomethionine is well absorbed; do not exceed 400 µg/day
• Pair with thyroid assessment — if you have Hashimoto's, discuss selenium supplementation with your endocrinologist

When Should You Get Tested?

• You have thyroid disease, especially Hashimoto's thyroiditis
• You follow a restricted diet (vegan, severe calorie restriction)
• You have malabsorption conditions
• You are on long-term parenteral nutrition
• You supplement with selenium and want to check you are not over-supplementing

Which Lola Health Tests Include Selenium?

Selenium is available as an add-on biomarker with any Lola Health blood test. For thyroid health, pair with TSH, FT4, FT3, and thyroid antibodies.