Not getting enough selenium disrupts several critical body systems, from thyroid hormone production to heart muscle integrity to your ability to fight off infections. Adults need just 55 micrograms a day, an amount easily met through diet in most parts of the world. But when intake falls short, whether from poor soil conditions, digestive disorders, or restricted diets, the consequences range from subtle (fatigue, brain fog, low mood) to severe (heart failure, joint deformity, impaired fertility).
How Selenium Works in Your Body
Selenium gets built into 25 different proteins in the human body, collectively called selenoproteins. These proteins serve as the backbone of your antioxidant defense system, neutralizing the reactive molecules that damage cells. They also regulate thyroid hormone activation, support immune signaling, and protect sperm cells from oxidative damage. When selenium runs low, these proteins can’t be made in sufficient quantities, and the systems that depend on them start to falter.
Your Thyroid Slows Down
One of the earliest and most common effects of low selenium is disrupted thyroid function. Your thyroid gland produces a hormone called T4, but T4 is relatively inactive. It needs to be converted into T3, the biologically active form, in tissues throughout the body. That conversion is carried out by selenium-dependent enzymes. Without enough selenium, T4 builds up while T3 drops, essentially leaving your body with a thyroid hormone it can’t fully use.
The result feels a lot like hypothyroidism: fatigue, weight gain, sensitivity to cold, sluggish metabolism. The thyroid gland itself contains more selenium per gram of tissue than any other organ, which makes it especially vulnerable to shortfalls. Even a modest deficiency can measurably shift the ratio of T4 to T3 in blood tests, though severe deficiency is needed to significantly impair T3 production.
Your Immune System Weakens
Selenium deficiency makes infections more likely and more severe. Low levels reduce the activity of key antioxidant enzymes in immune cells, leaving them less effective at clearing pathogens. But the relationship goes beyond a weakened defense. Research has shown that selenium deficiency can actually make viruses themselves more dangerous.
In a striking series of experiments, researchers infected selenium-deficient mice with a normally harmless strain of Coxsackie virus (the same family of viruses linked to Keshan disease). In the selenium-deficient animals, the virus mutated at seven specific points in its genome, transforming from a benign strain into one that caused heart inflammation. Six of those seven mutations matched changes seen in the virulent form of the virus found in selenium-depleted regions. In other words, a nutrient deficiency in the host provided the conditions for the virus to evolve into something more harmful.
This pattern extends to other infections. Selenium deficiency has been associated with worse outcomes in influenza A, higher viral loads across multiple infections, and a greater overall susceptibility to both viral and bacterial illness.
Heart Muscle Damage: Keshan Disease
The most dramatic cardiac consequence of selenium deficiency is Keshan disease, named after a county in northeast China where it was first identified. The condition causes death and scarring of heart muscle tissue, leading to heart failure, dangerous heart rhythm problems, and in acute cases, cardiogenic shock. Early symptoms include general malaise, dizziness, shortness of breath, nausea, and chest discomfort.
Keshan disease occurs almost exclusively in regions where the soil is severely depleted of selenium, particularly a belt stretching from northeast to southwest China. Viral infections, especially Coxsackie virus, appear to act as a co-trigger alongside the deficiency. Selenium supplementation programs in affected regions have dramatically reduced the incidence of the disease, confirming the mineral’s central role.
Joint and Bone Deformity in Children
Kashin-Beck disease is a chronic joint condition found in children and adolescents living in selenium-poor areas, primarily across 15 provinces in China. The deficiency destroys cartilage cells on growth plates and joint surfaces, leading to stunted growth, enlarged joints, bone deformities, and in advanced cases, permanent disability. Because it targets growing cartilage, the disease primarily affects young people whose bones are still developing. Both selenium and iodine deficiency contribute to the condition, and the damage, once done, is largely irreversible.
Reduced Fertility
Selenium plays a dose-dependent role in male reproductive health. Sperm cells are highly vulnerable to oxidative damage, and the selenium-based antioxidant enzymes in reproductive tissue are their primary protection. When selenium is too low, sperm cells undergo increased programmed cell death, and motility drops. Clinical trials have found that supplementing with 100 micrograms of selenium daily improved sperm motility and led to an 11% increase in successful fathering. That said, more is not better: intake above 300 micrograms per day actually impaired motility, illustrating the narrow window for this mineral.
Cognitive Decline and Low Mood
The link between low selenium and mental health is real but nuanced. Studies in elderly populations have found that higher selenium levels correlate with fewer depressive symptoms and better cognitive performance. However, when researchers controlled for cognitive function, the association with depression disappeared, suggesting that selenium’s primary effect on mood may operate through its impact on the brain’s overall processing ability rather than through a direct effect on mood chemistry. Long-term exposure to low selenium appears to be necessary before cognitive effects emerge, which may explain why results across shorter studies have been inconsistent.
Who Is Most at Risk
Selenium deficiency is rare in North America and most of Europe, where soil selenium levels are adequate and diets are varied. The people most at risk include:
- People living in selenium-depleted regions, particularly parts of China, Tibet, Siberia, and some areas of New Zealand and parts of Europe
- People with gastrointestinal conditions that impair nutrient absorption, such as Crohn’s disease or short bowel syndrome
- People on kidney dialysis, which removes selenium from the blood
- People on highly restricted diets, particularly those that exclude major selenium sources like seafood, meat, and Brazil nuts
- Pregnant women with low selenium, who face elevated risks: blood selenium below 7 mcg/dL in early to mid-pregnancy has been associated with nearly eight times the risk of preterm birth
How Much You Need and Where to Get It
The recommended daily intake is 55 micrograms for adults, 60 micrograms during pregnancy, and 70 micrograms while breastfeeding. Children need between 20 and 40 micrograms depending on age. Blood selenium concentrations of 8 mcg/dL or higher are considered sufficient for selenoprotein production in healthy people.
A single Brazil nut contains roughly 70 to 90 micrograms of selenium, making it the most concentrated food source by a wide margin. Yellowfin tuna, halibut, sardines, shrimp, and turkey are also rich sources. Eggs, cottage cheese, and brown rice provide moderate amounts. For most people eating a varied diet that includes some animal protein or seafood, meeting the daily target is straightforward without supplementation. If you suspect a deficiency, a simple blood test can confirm your selenium status.