Selenium: The Trace Mineral That Protects Your Thyroid and Immune System
Written by &you Labs Team
Updated
Selenium is a mineral most people never think about. Your body needs only a tiny amount. But it plays a big role. A selenium blood test checks this small but mighty mineral. It powers your thyroid. It defends your cells from damage. It supports your immune system. Selenium levels vary a lot depending on where your food is grown. This makes deficiency more common than you might expect.
Quick answer
Selenium is a mineral your body needs in small amounts. It helps turn thyroid hormone into its active form. It also powers your body's main defense system against cell damage. It supports your immune system too. A healthy blood level sits around 120 to 150 nanograms per milliliter. Low soil selenium in some regions makes deficiency common. Fix it with food, like Brazil nuts. Or take a supplement of 200 micrograms a day if you are deficient. Do not go over 400 micrograms a day. Too much causes real harm.
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What selenium actually does
Selenium builds into special proteins throughout your body. One group of these proteins converts your thyroid hormone T4 into T3, its active form. Without enough selenium, this switch does not flip well. You can end up with thyroid symptoms even if your TSH and T4 look normal.
Another selenium protein is called glutathione peroxidase. It is your body's main defense against everyday cell damage. Doctors call this damage oxidative stress. Think of the protein as your body's cleanup crew. It clears out damaged molecules before they cause more harm.
Selenium also supports your immune system. It helps fight off viruses. It plays a role in sperm health too.
What counts as a healthy level
Result | Serum selenium |
|---|---|
Deficient | Under 70 ng/mL |
Common lab range | 70 to 150 ng/mL |
Optimal | 120 to 150 ng/mL |
Toxic | Over 400 ng/mL |
A blood test called red blood cell selenium reflects your levels over a longer stretch of time. It is a bit more stable than a single blood draw.
What happens when you run low
A low selenium level can quietly disrupt your thyroid. This happens even when your TSH looks fine. It also tends to make an autoimmune thyroid condition called Hashimoto's worse. It can weaken your immune defenses. This makes you more prone to infections. In severe, long cases, it can affect your heart muscle. It has also been linked to lower mood, weaker fertility in men, and changes in your hair, nails, and skin.
Who is most likely to run low
Where your food is grown matters a lot. Soil selenium levels vary widely from region to region. A vegan or vegetarian diet without much variety raises your risk. So does a diet heavy in processed food. Certain gut conditions, like Crohn's disease or celiac disease, make selenium harder to absorb. People on dialysis are also more likely to run low. So are pregnant or breastfeeding women, and some older adults.
The best food sources
Brazil nuts are by far the richest source. Just one or two a day can cover your daily needs. A single nut can hold 70 to 100 micrograms. Seafood is another strong source, like tuna, sardines, and shrimp. Organ meats and eggs help too. Meat, whole grains, beans, and sunflower seeds provide a moderate amount. Most fruits and vegetables offer very little. To see everything a nutrient focused panel can check, visit the &you Labs biomarker list.
How much you actually need
Group | Daily amount |
|---|---|
Adults | 55 micrograms |
Pregnant | 60 micrograms |
Breastfeeding | 70 micrograms |
Upper safe limit | 400 micrograms |
Common Hashimoto's dose | 200 micrograms |
When too much becomes a problem
Selenium toxicity is real. It usually comes from over supplementing or eating too many Brazil nuts at once. Watch for hair loss, brittle nails, and a garlic like smell on your breath. Skin rashes, stomach upset, fatigue, and irritability are signs too. Higher doses over time can also affect your liver. Two to three Brazil nuts a day is plenty for most adults. Four or more a day can push toward the upper safe limit.
If you need to supplement
Look for a form called selenomethionine, or selenium enriched yeast. Your body absorbs these well. A form called sodium selenite works less well. It is generally not the first choice. A typical dose is 100 to 200 micrograms a day. Doctors often recheck your levels after three to six months. Only supplement if you have a confirmed low level, or a specific reason like Hashimoto's. More is not automatically better.
Selenium and your thyroid
The clearest use for selenium testing is Hashimoto's thyroiditis. This is an autoimmune condition where your immune system attacks your thyroid. Some studies show that 200 micrograms of selenium daily lowers thyroid antibody levels in these patients. Evidence on other markers is more mixed. It is a reasonable, generally safe option to discuss with your doctor if you have this condition.
When testing is actually worth it
Testing makes sense in a few cases. One is having Hashimoto's. Another is thyroid symptoms despite normal TSH and T4. A third is a diet that puts you at real risk of deficiency. It is not something most people need as part of routine yearly screening if they eat a varied diet.
Curious about your own nutrient levels? Book a &you Labs panel and get results explained by a doctor.
Frequently asked questions
Should I just start taking selenium supplements?
Only if you are deficient or have a specific reason, like Hashimoto's. If your level is already healthy, extra selenium will not help, and Brazil nuts are an easy food based option.
How many Brazil nuts is too many?
Two to three a day is plenty for most adults. Four or more a day can start to approach the upper safe limit, depending on the nuts.
Will selenium fix my thyroid antibodies?
In Hashimoto's, it might help. Some studies show lower antibody levels with 200 micrograms a day, though results vary between studies.
Can I get too much selenium just from food?
It is rare. Overdoing Brazil nuts is the main risk. Most regular diets stay well below the toxic level.
Should selenium be part of my regular thyroid check?
For Hashimoto's or ongoing thyroid symptoms, yes, it is worth adding. For a routine thyroid check with a varied diet, it is not usually necessary.
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Key takeaways
- Selenium helps activate thyroid hormone, defends your cells from damage, and supports your immune system.
- A healthy blood level sits around 120 to 150 ng/mL.
- Low levels can worsen thyroid symptoms, weaken immunity, and affect fertility and mood.
- Brazil nuts are the easiest food source. Two to three a day covers most adults.
- Test and treat only when there is a clear reason. Too much selenium causes real harm.
Ready to check your nutrient and thyroid markers together? Book your &you Labs panel. Your results come back with both the standard range and the healthy target range, plus a doctor's written interpretation.
The &you Labs Team combines medical science with personal care. We help Filipinos understand their health markers with blood work analysis that is easy to follow, whole person support, and health expertise built for Filipino life.
This content is provided for educational purposes only and is not intended as medical advice. It should not replace professional medical consultation, diagnosis, or treatment. Please consult a qualified healthcare provider to discuss the risks and benefits of any treatment option.
