Selenium Test · Blood

If your selenium level is too low, your body loses some of its ability to neutralize the chemical byproducts of normal metabolism, the kind of low-grade damage that accumulates into heart disease, liver disease, and cognitive decline over decades. If your level is too high, that same mineral starts working against you, raising your risk of type 2 diabetes and kidney problems. The difference between protective and harmful can be surprisingly small.

Standard blood panels do not include selenium. You could have perfectly normal cholesterol, blood sugar, and liver enzymes while running quietly low on the one mineral that keeps your cells' internal cleanup crews functioning. Knowing your number lets you act on a risk factor that most people never measure.

What Selenium Does in Your Body

Selenium is a trace element you get entirely from food. Your body cannot make it. Once absorbed, the liver converts it into a special amino acid called selenocysteine, which gets built into roughly 25 different proteins collectively known as selenoproteins. These selenoproteins do three things that matter most for long-term health.

First, they power your antioxidant defenses. Enzymes called glutathione peroxidases (your cells' primary system for neutralizing unstable oxygen molecules that would otherwise damage DNA and cell membranes) depend on selenium to function. Second, selenium-dependent enzymes called deiodinases convert thyroid hormones from their inactive storage form into the active form your cells actually use. Third, selenoproteins regulate immune cell activity, helping your body mount and then resolve inflammatory responses appropriately.

Your blood carries selenium mainly in two forms: a transport protein called selenoprotein P (SELENOP), which delivers selenium to tissues, and an antioxidant enzyme called GPx3 (glutathione peroxidase 3). Together, these account for most of the selenium circulating in your plasma. When you test serum selenium, you are measuring the total amount available to fuel all of these systems.

The U-Shaped Risk Curve

Selenium is unusual among nutrients because its relationship with disease risk is not a simple "more is better" or "less is better" story. It follows what scientists call a U-shaped curve: risk is highest at both the low end and the high end, with a protective zone in the middle. Understanding this shape is the single most useful thing you can learn about this test.

When selenium is too low, your selenoproteins cannot reach full activity. Your antioxidant defenses weaken, thyroid hormone conversion slows, and immune regulation suffers. When selenium is too high, the functional selenoproteins are already saturated, and excess selenium accumulates as potentially toxic compounds that can generate the very damage they are supposed to prevent. This is why supplementing "just in case" without knowing your level can backfire.

Heart Disease and Heart Failure

The cardiovascular evidence is among the strongest for selenium. A meta-analysis of 16 prospective cohort studies following over 35,000 people found that higher selenium within a moderate range (roughly 55 to 145 µg/L in blood) was associated with lower rates of heart disease, with the benefit flattening out above that range. For every 10 µg/L increase in blood selenium, the risk of developing cardiovascular disease dropped by about 15%, up to a ceiling.

The PREVEND study, which tracked about 6,000 Dutch adults for a median of 8.4 years, found that among non-smokers, each 10 µg/L increase in selenium was associated with roughly 13% lower risk of death and 18% lower risk of developing heart failure. Smoking appeared to blunt this protective relationship entirely, which is one reason your smoking status matters when interpreting your result.

In elderly Swedish adults with low baseline selenium (average 67 µg/L), those in the lowest quartile had about 43% higher risk of death from any cause and 56% higher risk of cardiovascular death compared to those with higher levels. A separate nested case-control study within the PREDIMED trial found that people in the highest quarter of serum selenium had roughly half the odds of a first cardiovascular event compared to those in the lowest quarter.

If your selenium is in the low range and you have other cardiovascular risk factors, this is a number worth addressing. If you already fall within the moderate range, the data do not support pushing it higher.

Cancer Associations

Selenium's relationship with cancer depends heavily on the cancer type and on where you start. A prospective study in China followed over 2,100 adults and found that people in the highest quarter of serum selenium had about 44% lower risk of esophageal cancer and 53% lower risk of gastric cardia cancer compared to the lowest quarter. In Europe, the EPIC cohort study of nearly 2,000 people found that each 25 µg/L increase in serum selenium was associated with about 11% lower colorectal cancer risk in women.

For breast cancer, a Swedish cohort of over 1,000 women found that those with the highest pre-diagnostic selenium levels had about 37% lower overall mortality and 40% lower breast cancer-specific mortality compared to those with the lowest levels. A meta-analysis of population-based prospective studies found a modest overall protective effect for dietary selenium intake at or above recommended levels.

The caveat: large chemoprevention trials using selenium supplements in already-replete populations have not shown clear cancer protection. The benefit appears concentrated in people who start with low or borderline selenium status.

Liver Disease

A systematic review and meta-analysis of 50 studies (covering nearly 10,000 cases and 13,000 controls) found that higher selenium was associated with about 41% lower risk of advanced chronic liver diseases, including hepatitis, cirrhosis, and liver cancer. Patients with these conditions consistently showed much lower selenium than healthy controls. The liver is the organ that makes most selenoproteins, so when the liver is damaged, selenium status drops as a consequence, but the low selenium may also accelerate further damage by weakening antioxidant protection.

Type 2 Diabetes: The Other Side of the Curve

Here is where the U-shaped curve becomes especially relevant. A systematic review and dose-response meta-analysis found that risk of type 2 diabetes increased in a roughly linear fashion as blood selenium rose. At about 140 µg/L, the risk was approximately 3.6 times higher than at levels below 45 µg/L. A randomized trial of 200 µg/day selenium supplementation in people who were not selenium-deficient found an increase in diabetes risk.

This does not mean selenium causes diabetes in everyone. It means that in populations that already have adequate selenium (most of the United States, for example), pushing levels higher with supplements appears to worsen metabolic risk. This is one of the strongest arguments for testing before supplementing.

Thyroid Health

The thyroid gland has the highest selenium concentration of any endocrine organ. Selenium-dependent deiodinase enzymes convert the inactive thyroid hormone T4 into the active form T3, and selenoproteins also protect thyroid tissue from damage during hormone production. A six-year prospective study of over 1,200 people in rural China found that those with low selenium (below 80 µg/L) had roughly 3.7 times the risk of developing Hashimoto's thyroiditis, an autoimmune condition that attacks the thyroid, compared to those with adequate levels.

Cognitive Function

A meta-analysis of studies on selenium and neurodegenerative disease found that people with Alzheimer's disease had significantly lower selenium levels than healthy individuals. In a study of over 850 older adults, higher serum selenium was associated with better cognitive function. These associations do not prove that raising selenium prevents dementia, but they suggest that selenium status is part of the picture when assessing brain health in older adults.

Kidney Function: A Caution

Mendelian randomization, a method that uses genetic variants as natural experiments to test cause-and-effect, found that genetically higher selenium levels were linked to lower eGFR (estimated glomerular filtration rate, the standard measure of kidney function) and higher blood urea nitrogen in people of European ancestry. This suggests that chronically high selenium may genuinely impair kidney function, adding another reason not to supplement beyond what you need.

Reference Ranges

These ranges come from multiple clinical and population-based sources measuring serum or plasma selenium. They are orientation ranges, not universal targets. Your lab may report slightly different numbers depending on the method used. The U.S. population tends to run higher than European populations because of differences in soil selenium content and food supply.

Compare your results within the same lab over time for the most meaningful trend. A single reading provides a snapshot, but your trajectory is what matters for decision-making.

When Results Can Be Misleading

Several common situations can shift your measured selenium without reflecting a true change in your body's selenium status. Acute illness and inflammation are the most important. Selenium behaves partly as a negative acute-phase reactant, meaning your level drops temporarily whenever your body is fighting an infection or dealing with significant inflammation. A blood draw during or shortly after a cold, surgery, or any acute inflammatory event may show a falsely low number.

• Inflammation and acute illness: Higher CRP (a marker of inflammation) is independently associated with lower serum selenium. Wait at least two to three weeks after recovery from an acute illness before testing.
• Smoking: Current smokers consistently show lower selenium levels. Smoking also changes the relationship between selenium and health outcomes, so your smoking status should always be factored into interpretation.
• BMI: Higher body mass index is independently associated with lower selenium, possibly due to dilution in a larger body volume or altered metabolism.
• Iron deficiency and anemia: Low iron status is linked to lower selenium. If you are iron-deficient, your selenium result may underestimate your true selenium availability once iron is corrected.