Selenium Blood Test: Levels, Deficiency & Toxicity

What Is Selenium and Why Do We Test It?

Selenium is an essential trace mineral that plays a critical role in antioxidant defence, thyroid hormone metabolism, and immune function. It is incorporated into selenoproteins, including glutathione peroxidases, which protect cells from oxidative damage.

In my clinic, I routinely check selenium levels when a patient presents with unexplained fatigue, recurrent infections, or symptoms of thyroid dysfunction that don't match standard thyroid blood work. A simple blood test can reveal whether selenium status is contributing to the picture.

When Your Doctor Might Order a Selenium Test

Unexplained muscle weakness or pain

Low selenium can manifest as muscle aches, weakness, or even cardiomyopathy (Keshan disease) in severe depletion. I've seen patients with persistent myalgia who improved dramatically after correcting a mild deficiency.

Thyroid abnormalities

Selenium is essential for converting T4 to active T3. Patients with Hashimoto's thyroiditis or those on levothyroxine sometimes benefit from selenium supplementation after we check their baseline level.

Nutritional concerns

People on parenteral nutrition, those with malabsorption (Crohn's, celiac, gastric bypass), or strict vegetarians in low-selenium soil regions are at risk. I always screen these groups.

Reference Ranges for Selenium in Blood

The following are typical reference ranges for serum or plasma selenium. Values may vary slightly by laboratory method (e.g., atomic absorption spectrometry vs. ICP-MS). Always interpret with your lab's specific reference interval.

Is High Selenium Dangerous?

Yes, chronic excessive selenium intake can lead to selenosis. Symptoms include garlic breath, metallic taste, brittle hair and nails, gastrointestinal upset, and in severe cases, neurological damage or liver cirrhosis. In my practice, I rarely see toxicity except in patients using unregulated supplements with megadoses.

The tolerable upper intake level for adults is 400 µg/day from all sources. Blood selenium above 150–200 µg/L may indicate excess. Acute poisoning (e.g., from industrial accidents) is very rare but can be life-threatening.

Selenium Deficiency – More Common Than You Think

Low selenium is associated with Keshan disease (a cardiomyopathy, primarily in young women and children in selenium-deficient regions), Kashin-Beck disease (an osteoarthropathy), and an increased risk of infections. Milder deficiency can cause fatigue, cognitive decline, and hair thinning.

I often see low levels in patients with inflammatory bowel disease, HIV, or those living in areas with selenium-poor soil (e.g., parts of China, Europe, and New Zealand). Supplementation should be guided by blood levels to avoid toxicity.

Selenium During Pregnancy

Optimal selenium status during pregnancy supports fetal thyroid development and reduces the risk of miscarriage and preeclampsia. The RDA increases to 60 µg/day. Blood levels tend to fall in later pregnancy due to plasma volume expansion; values near the lower end of the reference range are common but should not drop below 55 µg/L.

If you're pregnant or planning pregnancy, I recommend checking selenium as part of your first-trimester nutrient panel, especially if you have a restrictive diet or malabsorption.

How to Prepare for the Selenium Blood Test

No special preparation is needed. A simple blood draw from a vein is performed. Fasting is not required. However, if you take selenium supplements, let your doctor know so we can interpret results accurately.

Some medications (e.g., platinum-based chemotherapy, certain antacids) can interfere with selenium metabolism. Provide a complete medication list.

LOINC Standard Code

The internationally recognised LOINC code for selenium in serum or plasma is 5686-1 (Selenium [Mass/volume] in Serum or Plasma). This ensures consistency across laboratories worldwide.