Selenium Deficiency: Regional Soil Causes and the Signs to Watch

Selenium is an essential trace mineral that most people in the United States get in adequate amounts from food, according to the NIH Office of Dietary Supplements (ODS). However, people living in low-selenium regions or with certain medical conditions may fall short of the adult target of 55 micrograms per day.

Why Soil Geography Matters

Unlike many nutrients, the selenium content of plant foods depends almost entirely on the selenium concentration in the soil where those plants were grown. Crops grown in selenium-poor soil absorb less of the mineral, and animals raised on those crops also carry lower amounts in their meat and organs. This creates a direct link between regional geology and the selenium status of local populations.

Selenium-poor soils are found across parts of China, New Zealand, and much of Europe. Within the United States, the Pacific Northwest, parts of the Northeast, and the Great Lakes region tend to have lower soil selenium compared to the Great Plains states, where selenium levels are generally higher. People whose diets rely heavily on locally grown food in low-selenium areas face a greater risk of inadequate intake than those in selenium-rich regions or those who eat a varied diet sourced from multiple geographic areas.

What Selenium Does in the Body

The NIH ODS notes that selenium serves as a component of 25 selenoproteins, a group that includes glutathione peroxidases, thioredoxin reductases, and selenoprotein P. These proteins carry out several important physiological roles.

• Antioxidant protection: Selenoproteins help protect cells from oxidative damage. The evidence supporting this role is graded strong by the NIH.
• Thyroid hormone metabolism: Selenium is required for normal thyroid hormone metabolism, including the conversion of the prohormone T4 to the active form T3. This role is also supported by strong evidence.
• DNA synthesis and reproduction: Selenium supports DNA synthesis and normal reproduction, again rated as strong evidence by the NIH ODS.

Understanding these roles helps explain why prolonged inadequate intake can affect multiple body systems over time.

Signs That May Suggest Low Selenium

Mild or moderate selenium insufficiency often produces no obvious symptoms, which is one reason it can go undetected. More pronounced deficiency, particularly in regions with severely depleted soils, has been associated with the following signs and conditions.

• Muscle weakness or myopathy: Skeletal muscle relies on selenoproteins for normal function, and weakness has been reported in populations with very low intake.
• Thyroid changes: Because selenium is required for T4-to-T3 conversion, inadequate selenium may affect thyroid hormone balance. Symptoms tied to thyroid disruption, such as fatigue, cold sensitivity, or changes in weight, can sometimes be traced in part to selenium status, though thyroid conditions have many causes and require clinical evaluation.
• Immune function changes: Selenoproteins play a role in immune response, and deficiency has been linked to impaired immune activity in research settings.
• Mood and cognitive effects: Some observational studies have noted associations between low selenium status and mood changes, though the NIH rates evidence for selenium supplementation reducing cognitive decline as insufficient at this time.

If you recognize any of these signs, the right first step is to speak with a clinician who can order a blood or urine selenium test and interpret it in the context of your full health picture.

Typical Intake and the Upper Limit

According to the NIH ODS, the average daily selenium intake from foods and beverages for people age 2 and older in the United States is about 108 micrograms, which is well above the adult target of 55 micrograms per day. This means most Americans are not deficient.

However, the Tolerable Upper Intake Level for adults is 400 micrograms per day from food and supplements combined. The NIH set this limit based on evidence of hair and nail brittleness and loss at higher intakes. A condition called selenosis, caused by chronically excessive selenium, can also produce nausea, irritability, and neurological effects. Staying below the upper limit is advised unless selenium is taken under the supervision of a health care provider.

What the Research Says About Supplementation

It is important to be clear about what current evidence does and does not support. Selenium supplements are not intended to diagnose, treat, cure, or prevent any disease, and these statements have not been evaluated by the FDA.

With that context, here is how the NIH ODS grades the evidence for common claims:

• Cancer prevention: Evidence that selenium supplementation reduces the overall risk of cancer or specific cancers is rated insufficient.
• Cardiovascular disease: Evidence for a cardiovascular benefit from supplementation is rated insufficient.
• Cognitive decline or Alzheimer's disease: Evidence is rated insufficient.
• HIV outcomes: Evidence is rated insufficient.
• Hashimoto's thyroiditis: Evidence that selenium supplementation improves outcomes in chronic autoimmune thyroiditis is rated limited, meaning some signal exists but the research is not yet conclusive.
• Male fertility: Evidence that supplementation improves male fertility or sperm quality is rated insufficient.

These ratings do not mean selenium is unimportant. They mean that taking extra selenium beyond what is needed to meet the 55 microgram target has not been shown to provide additional health benefits in well-nourished populations.

Medications and Interactions

Selenium supplements can interact with certain medications, and some medications can affect selenium levels in the body. For example, the NIH ODS notes that cisplatin, a platinum-based chemotherapy agent, can reduce selenium levels in hair and serum, though whether this reduction is clinically significant is not yet known. A Cochrane Review found insufficient evidence that selenium supplementation relieves chemotherapy side effects. If you take any medications regularly, discuss your selenium status with your health care provider or pharmacist before adding a supplement.

Practical Takeaways

• Most adults in the United States meet the 55 microgram daily target through food alone, based on NIH ODS data.
• People in low-selenium soil regions, or those with restricted diets, may be at higher risk and should discuss testing with a clinician.
• Brazil nuts, seafood, organ meats, and whole grains grown in selenium-rich soil are among the better dietary sources.
• Do not exceed 400 micrograms per day from food and supplements combined without medical supervision.
• Always consult a clinician or pharmacist for personal dosing guidance, especially if you have a thyroid condition or take prescription medications.