Lab Reference Library / Selenium (Serum)
Nutritional & Micronutrient
Selenium (Serum)
Se · Serum Selenium · Selenium LevelReference range, optimal functional medicine levels, and why selenium is essential for thyroid hormone synthesis and activation, glutathione peroxidase antioxidant function, and immune regulation, and why both deficiency and excess produce significant clinical consequences.
Most SearchedThyroid Nutrient
Standard Range70 to 150 ng/mL
FM Optimal110 to 150 ng/mL
Toxicity RiskAbove 200 ng/mL
Unitsng/mL
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Category: Nutritional & Micronutrient | Also known as: Serum Selenium, Se, Plasma Selenium
1. What This Test Measures
Serum selenium measures the circulating concentration of selenium, an essential trace mineral that functions as a cofactor for the selenoprotein enzyme family. Key selenoproteins include glutathione peroxidases (GPx1 to GPx4, the primary intracellular antioxidant enzymes), thioredoxin reductases (regenerate antioxidant thioredoxin), and iodothyronine deiodinases (DIO1 to DIO3, which convert T4 to active T3 and regulate thyroid hormone metabolism throughout the body). The thyroid gland has the highest selenium concentration per gram of any organ, reflecting selenium's unique criticality for thyroid function.
Selenium is obtained from dietary sources and its concentration in food varies dramatically by geographic soil content. Brazil nuts can provide 70 to 90 mcg per nut from selenium-rich soils, or nearly zero from selenium-depleted soils. Selenomethionine from plant foods and animal products is the most bioavailable dietary form.
2. Optimal Range and Risk Thresholds
| Serum Selenium | Interpretation |
|---|---|
| Below 70 ng/mL | Deficient: impaired GPx and deiodinase activity; thyroid and antioxidant consequences |
| 70 to 90 ng/mL | Low: suboptimal; GPx activity partially reduced |
| 90 to 110 ng/mL | Low-normal: borderline adequate; improvement likely beneficial in thyroid patients |
| 110 to 150 ng/mL | Optimal: full selenoprotein enzyme activity; functional medicine target |
| 150 to 200 ng/mL | Adequate but approaching upper range; monitor if supplementing |
| Above 200 ng/mL | Excess: toxicity risk; reduce intake; selenosis signs at sustained above 400 ng/mL |
3. Selenium and Thyroid Function
Mechanisms
- DIO1 and DIO2 enzymes require selenium to convert T4 to active T3; selenium deficiency impairs T3 production even when T4 is normal
- Thyroid peroxidase (TPO) generates hydrogen peroxide during iodine organification; GPx in thyroid follicular cells requires selenium to neutralize this H2O2 and prevent oxidative thyroid cell damage
- Without adequate selenium, H2O2 accumulates and oxidizes thyroid proteins, exposing thyroid antigens and amplifying Hashimoto's autoimmune activity
- Selenium deficiency therefore simultaneously impairs hormone production and accelerates autoimmune thyroid destruction
Selenium and Hashimoto's
- Multiple RCTs demonstrate that selenomethionine 200 mcg daily reduces TPO antibodies by 30 to 60% in Hashimoto's patients
- Selenium should be optimized to 110 to 150 ng/mL before initiating iodine supplementation in any thyroid patient
- The selenium-iodine sequence matters: selenium first protects thyroid cells from oxidative damage during iodine organification
- Selenium optimization also improves Free T3 levels in patients with low-normal T3 by restoring deiodinase activity
4. Causes of Selenium Deficiency
- Geographic selenium-depleted soils: selenium content in food reflects soil content; New Zealand, Finland, and parts of China historically had very low soil selenium; selenium supplementation of fertilizers has improved population status in some regions
- Low dietary animal protein intake: selenomethionine in meat, poultry, and seafood is the most bioavailable dietary form; vegans and vegetarians are at higher risk
- Malabsorption: inflammatory bowel disease, celiac disease, and short bowel syndrome reduce selenium absorption
- Total parenteral nutrition without selenium supplementation: selenium is not always included in TPN formulations
- HIV and other chronic infections: selenium depletion is accelerated by chronic immune activation
- Dialysis: selenium is removed by hemodialysis
5. Supplementation Guidance
Best Forms
- Selenomethionine: the most bioavailable organic form; incorporated into proteins wherever methionine would be used; the form used in all major Hashimoto's clinical trials; preferred for Hashimoto's management and general selenium repletion
- Selenium-enriched yeast: mixture of selenomethionine and other organic selenium species; good bioavailability
- Sodium selenite: inorganic form; well-absorbed but can be pro-oxidant at higher doses; avoid at supplemental doses above 200 mcg daily
- Brazil nuts: 70 to 90 mcg per nut from selenium-rich soils; 1 to 2 nuts daily as a food source; soil content varies dramatically
Dosing and Monitoring
- Standard dose for Hashimoto's: 200 mcg selenomethionine daily
- Maintenance for general selenium adequacy: 100 to 200 mcg daily
- Recheck serum selenium at 3 months to confirm target 110 to 150 ng/mL without excess
- Do not exceed 400 mcg daily; the therapeutic window for selenium is narrower than most minerals
- Signs of selenosis (toxicity): garlic breath (dimethylselenide exhalation), hair loss, brittle nails, gastrointestinal distress, peripheral neuropathy; typically above sustained 800 mcg daily intake
Food Sources (mcg per serving)
- Brazil nuts: 70 to 90 mcg per nut (highly variable by soil)
- Tuna, halibut, sardines: 40 to 60 mcg per 3 oz serving
- Shrimp, oysters: 30 to 40 mcg per 3 oz serving
- Beef and pork: 20 to 35 mcg per 3 oz serving
- Turkey: 25 to 30 mcg per 3 oz serving
- Eggs: 15 to 20 mcg per large egg
- Whole wheat bread: 10 to 15 mcg per slice
6. Related Lab Tests
IodineSelenium Must Be Adequate Before Iodine Supplementation
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TSHSelenium Supports Thyroid Function
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TPO AntibodiesSelenium Reduces Hashimoto's Antibody Titers
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Free T3Selenium Required for T4-to-T3 Conversion
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Vitamin DCo-nutrient for Thyroid Autoimmune Management
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ZincZinc Also Required for Thyroid Hormone Synthesis
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7. Clinical Perspective
Clinical Perspective
Selenium is the nutrient I check on every new Hashimoto's patient before I do anything else with their thyroid protocol. Without adequate selenium, the hydrogen peroxide generated during thyroid hormone synthesis is not neutralized, which accelerates thyroid cell destruction and keeps antibody titers elevated regardless of other interventions. I have seen patients who had been treated for Hashimoto's for years with persistently high TPO antibodies, where a selenium of 74 ng/mL explained the entire picture. Three months of selenomethionine 200 mcg daily brought their selenium to 128 ng/mL and their TPO antibodies dropped by more than half. Selenium optimization is often the single most impactful intervention available for Hashimoto's management, and it costs pennies per day.
Brian Lamkin, DO | Founder, The Lamkin Clinic | Edmond, Oklahoma
8. Frequently Asked Questions
What is the optimal selenium level?
In functional medicine, optimal serum selenium is 110 to 150 ng/mL, required for full activity of glutathione peroxidase and iodothyronine deiodinase enzymes. Values below 90 ng/mL impair thyroid T4-to-T3 conversion and antioxidant defense. Values above 200 ng/mL enter the toxicity range.
Does selenium lower TPO antibodies in Hashimoto's?
Yes. Multiple randomized controlled trials demonstrate that selenomethionine 200 mcg daily reduces TPO antibody titers by 30 to 60% in Hashimoto's patients over 3 to 12 months. The mechanism involves selenium-dependent glutathione peroxidase protecting thyroid follicular cells from hydrogen peroxide-mediated oxidative damage that would otherwise expose thyroid antigens and amplify autoimmune activity.
Why must selenium be adequate before supplementing iodine?
Selenium is required for the glutathione peroxidase enzymes that neutralize hydrogen peroxide generated during iodine organification in thyroid follicular cells. Without adequate selenium, excess iodine increases H2O2 accumulation, causing thyroid cell oxidative damage, amplifying antigen exposure, and worsening Hashimoto's autoimmunity. Selenium optimization before iodine supplementation is protective at multiple levels.
What is the best form of selenium to supplement?
Selenomethionine is the most bioavailable form and the one used in all major Hashimoto's clinical trials. The standard dose is 200 mcg daily for Hashimoto's management. Recheck serum selenium at 3 months to confirm the target range of 110 to 150 ng/mL is achieved without exceeding 200 ng/mL.
Content authored and clinically reviewed by Brian Lamkin, DO, founder of The Lamkin Clinic in Edmond, Oklahoma. Brian Lamkin, DO has 25+ years of experience in functional and regenerative medicine. This page reflects current functional medicine practice standards and is updated as new clinical evidence becomes available.
Selenium below 90 ng/mL quietly impairs thyroid hormone activation and allows autoimmune thyroid damage to accelerate unchecked.
Selenium is the most impactful single nutrient for Hashimoto's management. Schedule a consultation for thyroid nutrient assessment including selenium, iodine, vitamin D, and zinc.
Schedule a ConsultationMedical Disclaimer: This content is provided for educational purposes only and is not intended as a substitute for professional medical advice, diagnosis, or treatment. Lab interpretation should always be performed in clinical context by a qualified healthcare provider. Reference ranges and optimal targets may vary based on individual patient history, clinical presentation, and laboratory methodology. Schedule a consultation to discuss your specific results with Dr. Lamkin.