Selenium
Selenium is an essential trace mineral that your body needs for thyroid hormone production, antioxidant defense, immune function, and reproductive health. Selenium is required for the enzymes that convert T4 to active T3, and studies show selenium supplementation can reduce TPO antibodies in autoimmune thyroid disease. Testing reveals whether you have adequate levels to support these functions — or whether deficiency is silently impairing your thyroid and immune health.
Selenium is an essential trace mineral that your body needs for thyroid hormone production, antioxidant defense, immune function, and reproductive health. Though required in small amounts, selenium plays outsized roles in critical biological processes — and deficiency is more common than many realize.
Why does selenium testing matter for preventive health? Because selenium status directly affects thyroid function, and optimizing selenium may reduce thyroid autoantibodies in people with Hashimoto’s thyroiditis. Selenium also supports the enzymes that convert T4 to active T3. Testing reveals whether you have adequate levels to support these functions — or whether deficiency is silently impairing your thyroid and immune health.
Selenium testing is particularly valuable for those with thyroid conditions (especially autoimmune thyroid disease), people living in selenium-poor regions, vegetarians and vegans, those with GI disorders affecting absorption, and anyone wanting to optimize this critical micronutrient. Combined with thyroid testing, selenium assessment provides actionable information for thyroid health optimization.
Key Benefits of Selenium Testing
Selenium testing determines whether you have adequate levels of this essential mineral — information that guides targeted supplementation rather than guessing. Knowing your selenium status is especially valuable for thyroid optimization: selenium is required for T4-to-T3 conversion, and studies show selenium supplementation can reduce TPO antibodies in autoimmune thyroid disease.
The test identifies deficiency before symptoms become obvious. Selenium deficiency can impair thyroid function, weaken immunity, and reduce antioxidant protection — but these effects develop gradually and aren’t easily recognized without testing. It also prevents excess: selenium toxicity (selenosis) causes serious problems, so knowing your level prevents over-supplementation. Testing establishes your baseline and guides personalized optimization.
What Does Selenium Measure?
Selenium testing typically measures serum or plasma selenium concentration, reflecting your body’s selenium status. Selenium is incorporated into selenoproteins — specialized proteins that carry out selenium’s biological functions.
What Selenium Does in Your Body
Thyroid hormone metabolism: Selenium is essential for the deiodinase enzymes that convert T4 to active T3 and regulate thyroid hormone levels throughout the body. Without adequate selenium, T4-to-T3 conversion is impaired.
Antioxidant defense: Selenium is a key component of glutathione peroxidases — enzymes that neutralize harmful peroxides and protect cells from oxidative damage. The thyroid gland produces significant peroxide during hormone synthesis and relies heavily on selenium-dependent antioxidants for protection.
Immune function: Selenium supports both innate and adaptive immunity. Deficiency impairs immune cell function and increases susceptibility to certain infections.
Reproduction: Selenium is important for male fertility (sperm motility and formation) and may support healthy pregnancy outcomes.
Selenium and Thyroid Autoimmunity
The thyroid gland contains more selenium per gram than any other organ. Multiple studies show that selenium supplementation in people with Hashimoto’s thyroiditis reduces TPO antibody levels — suggesting selenium may help calm autoimmune thyroid inflammation. This makes selenium status particularly relevant for anyone with positive thyroid antibodies.
Why Selenium Testing Matters
Optimizes Thyroid Function
Selenium is required for the enzymes that convert T4 to T3. Low selenium can impair this conversion, contributing to low T3 symptoms even when T4 is adequate. Testing identifies whether selenium deficiency might be affecting your thyroid function — particularly important if you have persistent symptoms despite thyroid treatment.
May Reduce Thyroid Autoimmunity
Clinical trials show selenium supplementation reduces TPO antibodies in Hashimoto’s patients, particularly those who are selenium-deficient. Testing reveals who is most likely to benefit from supplementation — those with low levels have more to gain than those already replete.
Prevents Both Deficiency and Toxicity
Selenium has a relatively narrow therapeutic window — both too little and too much cause problems. Deficiency impairs thyroid and immune function; excess causes selenosis (hair loss, nail brittleness, GI symptoms, neurological effects). Testing guides supplementation to the optimal range without risking toxicity.
Identifies Those at Risk
Selenium status varies significantly by geography (soil selenium content), diet, and individual factors. Testing identifies who actually needs supplementation rather than assuming based on population averages. This is especially important for vegetarians, people with malabsorption, and those in selenium-poor regions.
What Can Affect Selenium Levels?
Causes of Low Selenium
Geographic location: Soil selenium content varies dramatically by region. Parts of China, Russia, and some European countries have selenium-poor soils, leading to lower dietary intake. Some regions of the US (particularly Pacific Northwest and parts of the East) have lower soil selenium than others.
Dietary patterns: Vegetarians and vegans may have lower selenium intake since the best sources are animal products (Brazil nuts being an exception). Highly processed diets may also be low in selenium.
Malabsorption: GI conditions affecting absorption (Crohn’s disease, celiac disease, short bowel syndrome) can cause selenium deficiency regardless of intake.
Dialysis: Patients on hemodialysis often have low selenium due to losses during dialysis.
Pregnancy and lactation: Increased demands can deplete maternal selenium stores.
Causes of High Selenium
Over-supplementation: The most common cause. Taking high-dose selenium supplements without monitoring can lead to toxicity.
Excessive Brazil nut consumption: Brazil nuts are extremely high in selenium. Eating several daily can cause excess.
Occupational exposure: Certain industrial exposures can elevate selenium.
Testing Considerations
No fasting required. Serum selenium reflects recent intake; it may not fully capture long-term status. Recent selenium supplementation will elevate levels. For baseline assessment, test before starting supplements or after a washout period.
When Should You Test Selenium?
Thyroid Conditions
Selenium testing is particularly valuable for anyone with thyroid disease — especially autoimmune thyroid disease (Hashimoto’s, Graves’). If you have positive TPO or TgAb antibodies, knowing your selenium status guides whether supplementation might help reduce autoimmune activity. If you have persistent hypothyroid symptoms despite treatment, low selenium could be impairing T4-to-T3 conversion.
Preventive Baseline
For comprehensive nutritional assessment, selenium testing establishes your baseline status. This is particularly relevant if you have risk factors for deficiency: vegetarian/vegan diet, GI disorders, living in selenium-poor regions, or limited dietary variety.
Before Starting Supplementation
If considering selenium supplements (especially for thyroid support), test first. This determines whether you actually need supplementation and provides a baseline to monitor response. Given selenium’s toxicity risk with excess, testing before and during supplementation is prudent.
Symptoms Suggesting Deficiency
Selenium deficiency symptoms are non-specific: fatigue, muscle weakness, impaired immunity (frequent infections), and thyroid dysfunction. If you have these symptoms plus risk factors for deficiency, testing clarifies whether selenium is a contributing factor.
Monitoring Supplementation
If taking selenium supplements, periodic testing ensures you’ve achieved optimal levels without excess. This is especially important with higher-dose supplementation.
Understanding Your Selenium Results
Results are typically reported in micrograms per liter (μg/L) or micrograms per deciliter (μg/dL). Reference ranges vary by laboratory but generally indicate:
Low/deficient: Below the reference range. Supplementation likely beneficial, especially for thyroid support. Investigate causes (dietary, absorption).
Optimal range: Within normal reference range. Adequate for selenoprotein function. Studies suggest levels around 100-120 μg/L may be optimal for thyroid and immune health.
High/excess: Above reference range. Risk of selenosis if significantly elevated. Reduce supplementation or dietary sources (Brazil nuts). Retest to confirm normalization.
Optimal vs. Adequate
Being within the “normal” range doesn’t necessarily mean optimal. Some research suggests higher levels within the normal range may provide better thyroid and antioxidant protection. Your healthcare provider can help interpret where you fall within the range and whether optimization is appropriate.
Next Steps Based on Results
If low: Increase dietary selenium (Brazil nuts, seafood, organ meats, eggs) and/or supplement. Typical supplementation is 100-200 mcg daily, though individualization is important. Retest in 2-3 months.
If optimal: Maintain current intake. Consider dietary selenium rather than supplements if adequate.
If high: Reduce or stop selenium supplements. Limit Brazil nuts. Retest to confirm levels decrease.
What to Do About Low Selenium
Dietary Sources
Brazil nuts: The richest food source — just 1-2 nuts daily can meet requirements. However, content varies dramatically by origin.
Seafood: Fish (tuna, halibut, sardines), shellfish (oysters, shrimp) are good sources.
Organ meats: Liver and kidney are selenium-rich.
Eggs and poultry: Moderate sources.
Grains and vegetables: Content depends on soil selenium where grown.
Supplementation
If dietary improvement is insufficient or for therapeutic doses (such as autoimmune thyroid disease), selenium supplements are effective. Common forms include selenomethionine and sodium selenite. Typical doses range from 50-200 mcg daily depending on status and goals.
For thyroid autoimmunity: Studies showing TPO antibody reduction typically used 200 mcg daily. Start with testing, supplement if low, and monitor response.
Caution: The tolerable upper limit is 400 mcg daily from all sources. Toxicity begins above this level. Do not megadose — more is not better with selenium.
Monitoring
Retest selenium 2-3 months after starting supplementation to confirm improvement and ensure you’re not exceeding optimal range. Adjust dose accordingly.
Selenium and Related Health Conditions
Thyroid Health
Hashimoto’s Thyroiditis: Selenium supplementation may reduce TPO antibodies. Testing guides who benefits most.
Graves’ Disease: Some evidence suggests selenium may help with Graves’ ophthalmopathy (eye disease).
Hypothyroidism: Selenium supports T4-to-T3 conversion. Low selenium may contribute to persistent symptoms.
Immune Function
Infection susceptibility: Selenium deficiency impairs immune cell function. Adequate selenium supports resistance to viral and bacterial infections.
Autoimmune conditions: Beyond thyroid, selenium’s antioxidant and immunomodulatory effects may influence other autoimmune processes.
Cardiovascular Health
Selenium’s antioxidant role may support cardiovascular health, though evidence is mixed. Severe deficiency is associated with Keshan disease, a form of cardiomyopathy.
Fertility
Male fertility: Selenium is essential for sperm formation and motility. Deficiency can impair male fertility.
Pregnancy: Adequate selenium supports healthy pregnancy outcomes. Severe deficiency may increase preeclampsia risk.
Why Regular Selenium Testing Matters
Selenium status can change with dietary patterns, supplementation, and health conditions. Testing establishes your baseline, guides targeted supplementation, and monitors the effect of interventions. For those with thyroid autoimmunity, periodic testing tracks whether selenium optimization is reducing antibodies.
Given selenium’s narrow therapeutic window — where both deficiency and excess cause problems — testing is the responsible approach to optimization. It transforms supplementation from guesswork to precision.
Related Biomarkers Often Tested Together
TSH — Thyroid function screening. Interpret selenium in context of thyroid status.
Free T3 — Active thyroid hormone. Low selenium can impair T4-to-T3 conversion affecting Free T3.
TPO Antibodies — Thyroid autoimmunity marker. Selenium supplementation may reduce TPO antibodies in deficient individuals.
Zinc — Another essential trace mineral for thyroid and immune function. Often tested alongside selenium for comprehensive micronutrient assessment.
Vitamin D — Commonly co-deficient with selenium; both affect thyroid and immune health.
Note: Information provided in this article is for educational purposes and doesn’t replace personalized medical advice.
Frequently Asked Questions
Selenium has multiple thyroid roles: it’s required for the enzymes (deiodinases) that convert T4 to active T3; it’s essential for antioxidant enzymes that protect the thyroid from oxidative damage during hormone production; and it may help modulate thyroid autoimmunity. The thyroid contains more selenium per gram than any other organ.
Multiple clinical trials show selenium supplementation (typically 200 mcg daily) reduces TPO antibody levels in people with Hashimoto’s thyroiditis, particularly those who are selenium-deficient. The effect is most pronounced in those starting with low selenium. It doesn’t cure autoimmune thyroid disease, but may reduce autoimmune activity.
The recommended daily allowance (RDA) is 55 mcg for adults. However, optimal levels for thyroid and immune function may be higher — some research suggests 100-200 mcg daily. The upper limit is 400 mcg; above this, toxicity risk increases. Testing guides personalized targets.
Yes, for most people — if diet includes good selenium sources. Just 1-2 Brazil nuts daily can provide adequate selenium. Seafood, meat, and eggs are also good sources. However, if you’re vegetarian, live in a selenium-poor region, or have absorption issues, dietary sources may be insufficient.
Selenium toxicity (selenosis) causes GI symptoms (nausea, diarrhea), garlic breath odor, hair loss, brittle nails, and neurological symptoms. It typically occurs with chronic intake above 400 mcg daily. This is why testing before and during supplementation is important — to achieve optimal levels without excess.
Not necessarily. Selenium supplementation is most beneficial for those who are deficient. If you’re already selenium-replete, additional supplementation is unlikely to help and could risk excess. Test first, then supplement if indicated. This targeted approach is more effective than universal supplementation.
For baseline assessment: once to establish status. If supplementing: retest in 2-3 months to confirm improvement and ensure not exceeding optimal range. For ongoing monitoring: annually is usually sufficient once stable. More frequent testing if changing supplementation doses.
Selenomethionine is well-absorbed and commonly used. Sodium selenite is another effective form. Both work; selenomethionine may be slightly better absorbed. Quality and dose accuracy matter — choose reputable brands.
References
Key Sources:
- Ventura M, et al. Selenium and Thyroid Disease: From Pathophysiology to Treatment. Int J Endocrinol. 2017;2017:1297658.
- Wichman J, et al. Selenium Supplementation Significantly Reduces Thyroid Autoantibody Levels in Patients with Chronic Autoimmune Thyroiditis. Thyroid. 2016;26(12):1681-1692.
- Rayman MP. Selenium and human health. Lancet. 2012;379(9822):1256-1268.
