Selenium deficiency happens when your body doesn’t get enough of this essential trace element, either because your diet lacks it, your body can’t absorb it properly, or a medical condition depletes it faster than you can replace it. An estimated one billion people worldwide have insufficient selenium intake, making it far more common than most people realize. The recommended daily intake for adults is 55 mcg, a tiny amount that’s still easy to miss depending on where you live and what you eat.
Selenium-Poor Soil and Geography
The single biggest driver of selenium deficiency worldwide is geography. Selenium enters the food chain through soil: plants absorb it, animals eat those plants, and humans eat both. When soil selenium is low, everything grown or raised on that land carries less of it. Roughly 15% of the world’s land has soil selenium concentrations below 0.2 mg/kg, the threshold considered low. These areas cluster in arid and semiarid regions of China, Argentina, Chile, Australia, southern Africa, and the southwestern United States.
This isn’t just a theoretical concern. Parts of rural China have historically had such depleted soil that entire populations developed selenium deficiency diseases (more on those below). Even in wealthier countries, people who eat mostly locally grown food from low-selenium regions can fall short without realizing it. Soil acidity, sulfate levels, and the chemical form of selenium all affect how much plants actually take up, so even moderate soil levels don’t guarantee adequate food concentrations.
Diet and Plant-Based Eating
Your diet plays a major role regardless of where you live. The richest food sources of selenium are Brazil nuts, seafood, organ meats, and muscle meats. Grains and vegetables contain selenium too, but in amounts that depend heavily on the soil they were grown in. If your diet skips most animal products, your risk rises substantially.
A study comparing selenium levels across diet types found striking differences. Omnivores had a median serum selenium of about 63 mcg/L, while vegans averaged roughly 53 mcg/L. More telling: 40% of vegans and 33% of vegetarians had selenium levels below 50 mcg/L, a threshold associated with increased health risks. Only 2.5% of omnivores fell below that line. The activity of a key selenium-dependent antioxidant enzyme was also significantly lower in the plant-based groups, suggesting their bodies had less functional selenium to work with.
This doesn’t mean plant-based diets inevitably cause deficiency, but selenium is one of the nutrients that requires deliberate attention if you don’t eat meat or seafood. A single Brazil nut can contain 70 to 90 mcg of selenium, more than a full day’s requirement, making it one of the simplest dietary fixes available.
Gut Disorders That Block Absorption
Even with adequate dietary intake, your body has to absorb selenium through the small intestine. Conditions that damage or inflame the intestinal lining can interfere with this process. Celiac disease is one of the clearest examples. The intestinal damage caused by celiac disease directly impairs selenium absorption, and research has linked this to downstream thyroid problems, since the thyroid is especially sensitive to low selenium levels.
Crohn’s disease, ulcerative colitis, and other inflammatory bowel conditions can produce similar effects, particularly when the disease is active and the intestinal lining is compromised. People who’ve had portions of their small intestine surgically removed are also at higher risk, simply because there’s less absorptive surface area available.
Kidney Disease and Dialysis
People on dialysis for end-stage kidney disease consistently have blood selenium levels well below those of healthy adults. The reasons stack up: reduced appetite leads to lower food intake, the dialysis process itself physically removes selenium from the blood, and chronic inflammation increases the body’s demand for selenium-dependent antioxidant defenses. One study of 406 peritoneal dialysis patients found that 41.4% were selenium deficient, and lower levels correlated with greater weakness and more inflammation.
Long-term intravenous feeding (parenteral nutrition) poses a similar risk if the nutrient formulas don’t include adequate selenium supplementation, though awareness of this has improved in recent decades.
HIV and Chronic Infection
Chronic infections, particularly HIV, can deplete selenium levels as the immune system draws heavily on selenium-dependent enzymes to manage oxidative stress. Multiple studies have found that selenium levels drop as HIV progresses. Symptom-free HIV-positive individuals tend to have significantly higher serum selenium than those with more advanced disease, and people with lower selenium experience opportunistic infections more frequently.
The relationship appears to go both ways. Low selenium may accelerate disease progression, and worsening disease further depletes selenium. In clinical trials, HIV-positive individuals who received 200 mcg of daily selenium supplementation experienced a slower decline in immune cell counts compared to placebo groups. One trial in pregnant HIV-positive women in Kenya showed higher immune cell counts in the supplemented group as well.
The Thyroid Connection
Selenium and iodine work together to keep the thyroid functioning properly, and a deficiency in one magnifies the harm of the other. Your thyroid relies on selenium-containing enzymes to convert its inactive hormone (T4) into the active form (T3) that your cells actually use. When selenium is low, this conversion slows down.
The interaction gets dangerous in populations that are low in both nutrients. Iodine deficiency increases the production of hydrogen peroxide in thyroid cells as a byproduct of hormone synthesis, while selenium deficiency reduces the body’s ability to neutralize that hydrogen peroxide. The combination can cause severe thyroid damage. In Central Africa, the overlap of iodine deficiency, selenium deficiency, and high dietary thiocyanate exposure (from cassava) has been linked to a severe form of cretinism. Correcting selenium deficiency before addressing iodine deficiency can actually worsen hypothyroidism by speeding up hormone conversion and depleting already scarce iodine stores, a detail that matters for public health programs in affected regions.
Diseases Caused by Severe Deficiency
Two diseases are historically tied to selenium-depleted populations, both named for the regions where they were first identified.
Keshan disease is a form of heart muscle disease that causes heart failure, an enlarged heart, and abnormal heart rhythms. It primarily affects children and women of childbearing age. First documented in selenium-poor areas of China, it carried a fatality rate around 50% in affected communities. Selenium deficiency alone doesn’t always trigger it; a specific virus (Coxsackievirus) and chemical exposures often act as co-triggers in people whose selenium levels leave them vulnerable.
Kashin-Beck disease attacks bones, cartilage, and joints, causing deformities and restricted movement from joint enlargement. It’s found primarily in parts of Tibet, China, Siberia, and North Korea. Like Keshan disease, it has multiple contributing causes, but selenium deficiency is a recognized factor. Both diseases have become less common as supplementation programs and food distribution have improved selenium intake in affected regions.
How Deficiency Is Identified
Selenium status is measured through a blood test. Plasma or serum concentrations at or above 8 mcg/dL are generally considered sufficient for the body to produce its selenium-dependent proteins at full capacity. Levels below 7 mcg/dL are classified as deficient. In one study, pregnant women with blood selenium below that 7 mcg/dL threshold in early pregnancy were nearly eight times as likely to deliver preterm compared to those with normal levels.
Mild deficiency often produces no obvious symptoms, which is part of why it’s so widespread. Fatigue, weakened immunity, brain fog, and hair loss can all result from low selenium, but these overlap with many other conditions. Testing is typically prompted by a known risk factor (living in a low-selenium region, having a malabsorptive condition, following a restricted diet) rather than by symptoms alone.