Selenium and Thyroid Nutrition: Food First

Selenium is one of those nutrients that sounds exotic and turns out to be ordinary: it is in tuna, eggs, chicken and bread, and most people in the United States get plenty. What makes it interesting is how narrow the sweet spot is. The thyroid holds more selenium than any other organ, a single Brazil nut can deliver more than a full day’s requirement, and the line between enough and too much is closer than for almost any other mineral. This guide covers selenium food sources, the numbers worth knowing, how the thyroid uses selenium alongside iodine, and why the smart move is to build from food and let your doctor decide about anything beyond that.

ENERGYbits® • Minerals & Thyroid Nutrition

Why selenium is having a moment

Search interest in selenium arrives from two directions: thyroid health, where people hear the gland is packed with the mineral, and the Brazil nut, which keeps resurfacing as a one-nut-a-day wellness trend. Both stories are real and both get simplified. The nut is extraordinarily rich, but the amount in any given nut swings enormously.

Here we cover the numbers, the foods, the selenium and iodine partnership, and why testing with your clinician beats guessing. Algae come in near the end in a small and honest role: they are not a major selenium source, and we show the numbers. For algae and thyroid specifically, see our guides to spirulina and the thyroid and chlorella and iodine.

What selenium does, and why the thyroid cares

Selenium works through proteins. The body builds 25 selenoproteins, each with selenium built into its structure, and they handle some of the cell’s most important housekeeping: managing oxidative stress, supporting DNA synthesis, and helping regulate thyroid hormone [1]. You cannot get these functions from a different mineral, which is why selenium is classed as essential even though the amounts involved are tiny, measured in micrograms.

The thyroid is the selenium capital of the body

The thyroid contains the highest concentration of selenium of any organ [1][2]. That is not a coincidence. Making thyroid hormone is chemically messy work. The gland produces hydrogen peroxide as part of the process, and selenoproteins such as glutathione peroxidases and thioredoxin reductases help protect thyroid tissue from that oxidative load [1]. Other selenoproteins, the deiodinases, help convert the storage hormone T4 into the active hormone T3 [1].

Deficiency is rare, but it has a pattern

Selenium deficiency on its own rarely causes illness. It tends to matter when other stressors are present [1]. The best-known examples are regional. Keshan disease, a cardiomyopathy first identified in China in 1935, occurred in areas where adults were getting about 10 mcg a day or less. Kashin-Beck disease, a bone and joint condition, has been linked to areas low in both selenium and iodine [1]. These are conditions of extreme soil depletion and limited food variety. They are not a description of a typical diet in the United States, where average selenium intake from food and beverages is about 108 mcg a day across all ages, and about 132 mcg for adult men and 94 mcg for adult women [1].

How much you need, and how much is too much

These are the NIH Office of Dietary Supplements figures [1]. The RDA is the amount that meets the needs of nearly all healthy people. The tolerable upper intake level is the highest daily intake unlikely to cause harm over time. The NIH notes that the upper limit does not apply to people taking selenium under a physician’s care [1].

Where the typical person lands

Average intake from food and beverages sits above the RDA, and roughly 17 to 19 percent of U.S. adults also take a selenium-containing supplement [1]. Many people are already past 55 mcg before any supplement enters the picture.

The food sources, ranked

Selenium food sources fall into a few families: Brazil nuts in a class of their own, then seafood, then meats and organ meats, then eggs and dairy, then grain foods. The table below comes from the NIH fact sheet, using its serving sizes [1]. The percent of the 55 mcg daily value is simple division.

Ordinary foods do real work: one egg and a cup of cottage cheese together supply more than half the day’s requirement. Grains add up across a day, and seafood is the dependable heavyweight because its selenium does not swing with the soil.

A note for plant-based eaters

Plant foods vary widely in selenium, so people eating mostly plants can land lower. A 2025 randomized trial in the European Journal of Nutrition found that vegans had significantly lower estimated selenium intake than omnivores, and that adding 55 mcg a day for two weeks, either as Brazil nut butter or as a supplement, raised blood selenium and selenoprotein P in both groups [5]. The authors concluded that Brazil nuts can be a useful plant-based source when eaten once in a while. That qualifier, once in a while, is the practical lesson of the next section.

Brazil nuts: the powerful, unpredictable one

The NIH lists Brazil nuts at 68 to 91 mcg of selenium per nut, and warns that eating them regularly in quantity could cause toxicity [1][2]. Do the arithmetic and the issue is obvious. One nut meets or exceeds the 55 mcg RDA. Somewhere around five nuts, using that range, would reach the 400 mcg upper limit (a calculation from the NIH figures). Because the upper limit is about chronic intake, a handful now and then is not the concern. A handful every day is.

Why one nut and the next are so different

The variation is not small. A study of nuts from across the Brazilian Amazon, published in Chemosphere in 2017, found median selenium concentrations ranging from 2.07 mg/kg in nuts from Mato Grosso state to 68.15 mg/kg in nuts from Amazonas state. Depending on origin, the authors calculated, a single nut could supply anywhere from 11 percent to 288 percent of the daily requirement for an adult man, using their 70 mcg reference [4]. Selenium uptake tracked total selenium in the soil and fell when the soil was more acidic.

Using the study’s 70 mcg reference, 11 percent is about 8 mcg and 288 percent about 200 mcg per nut: a 25-fold spread inside one species (calculation). Packaging rarely says which end you are holding.

A reasonable way to enjoy them

• Treat Brazil nuts as an occasional food, a few nuts in a week, rather than a daily supplement.
• If you eat them often, count them the way you would count a supplement, and tally the rest of your selenium from other foods.
• Do not stack them on top of a selenium-containing multivitamin or standalone selenium product without checking the totals.
• Pregnant or nursing, on dialysis, or managing a thyroid condition? Ask your clinician what is right for you before making nuts a habit.

Soil, geography and why labels cannot tell you much

Selenium is a strange nutrient in that its presence in food depends on geology. Plants take up selenium from soil, and the amount available depends on how much selenium the soil holds, the form it is in, the soil’s pH and its organic matter [1]. The Winther review makes the same point: intake varies across the world largely because of soil content and factors that affect how available selenium is to plants [3].

Animal foods are steadier. Animals maintain relatively stable tissue selenium levels and commercially formulated feeds are consistent, so soil swings are damped out in meat, eggs and dairy [1]. This is one reason a mixed diet is more reliable than a single plant source. It is also why the plant-based and low-selenium-soil combination is the one the NIH flags as higher risk for deficiency, alongside people on kidney dialysis and people living with HIV [2].

Selenium and iodine: a working partnership

Thyroid hormone is made from iodine. Selenium does not replace iodine and iodine does not replace selenium, and each is needed for the process to run well. Iodine is the raw material. Selenium builds the enzymes that activate the hormone and protect the gland while it works [1].

The NIH points out that selenium deficiency may worsen iodine deficiency, and that in infants this combination could raise the risk of congenital hypothyroidism [1]. Endemic myxedematous hypothyroidism, which develops during gestation or early childhood, occurs in very low-selenium regions such as Central Africa [1]. The practical interpretation for most readers is simple: the two nutrients are linked, so thyroid nutrition is about both rather than one in isolation.

Why more is not better

The usual instinct is to take more of a nutrient that supports the thyroid. Selenium is a good case against it, for three reasons.

1. The window is narrow

Chronic intake above 400 mcg a day can cause selenosis. The early signs are a garlic odor on the breath and a metallic taste. The most common clinical signs are hair loss and brittle or lost nails. Skin rash, nausea, diarrhea, fatigue, irritability and nervous system changes can follow. Acute toxicity, for example from a misformulated supplement, can be severe [1]. The gap between the 55 mcg RDA and the 400 mcg limit is wide on paper, but supplements, fortified products and Brazil nuts can close it quickly.

2. The thyroid trials are modest

In people with chronic autoimmune thyroiditis who are treated with levothyroxine, selenium supplementation lowered thyroid peroxidase antibody (TPOAb) levels in some studies. A systematic review found no effect on TSH, thyroid ultrasound appearance or quality of life [1]. The Winther review reaches a similar conclusion: supplementation has generally reduced autoantibody titers without clear improvement in the clinical course, and recommendations extend only to Graves orbitopathy, even though supplements are widely used for other thyroid conditions [3]. The NIH adds that a trial in pregnant women with positive TPOAb reported fewer postpartum thyroid disorders with selenium (28.6 percent versus 48.6 percent with placebo), while the American Thyroid Association gave a weak recommendation in 2017 against selenium supplementation for TPOAb-positive pregnant women [1]. In short, the evidence is mixed and still developing.

3. Benefit appears to depend on where you start

Epidemiological studies link low selenium status with higher thyroid disease risk in women, though not in men [1]. Future trials may identify who benefits based on selenium status or genetics [3]. That argues for knowing your baseline, not taking a standard dose on spec.

Where algae fit, honestly

Since this article lives on a site about algae, an honest paragraph is owed. Spirulina and chlorella are whole foods valued for protein, chlorophyll, phycocyanin, vitamins and minerals. Selenium is not what they are known for, and the numbers show why. USDA FoodData Central lists dried spirulina at 7.2 mcg of selenium per 100 g [6]. An ENERGYbits serving is 30 tablets at 250 mg each, or 7.5 g. By calculation, that serving provides about 0.54 mcg of selenium (7.2 mcg per 100 g x 0.075), roughly 1 percent of the 55 mcg RDA.

Algae are a fine part of a nutrient-dense routine, but they should not be counted on for this mineral. The flip side is that a daily algae serving will not push you toward the upper limit. For servings, see our spirulina dosage guide, and for who should check with a provider first, our spirulina safety overview.

A food-first week

You do not need a supplement protocol. You need a pattern, and ordinary meals build it.

Two eggs at breakfast (about 30 mcg) and 3 oz of roasted chicken with a cup of cooked brown rice at dinner (about 34 mcg) already total roughly 64 mcg, above the RDA, before lunch or any nut (a calculation from the NIH values [1]). Real numbers shift with the product and the cooking method.

• Aim for variety across seafood, eggs, poultry, dairy and grains rather than a single hero food.
• If you eat mostly plants, include whole grains and consider a few Brazil nuts per week, rather than daily.
• Check labels on multivitamins, thyroid “support” blends and hair-and-nail formulas, which often contain selenium.

Testing and talking with your doctor

If you are wondering about your own status, the useful move is a conversation with your clinician, not a larger dose. The NIH describes a serum selenium of 8 mcg/dL or higher as sufficient in healthy people; the mean in a national survey was 12.7 mcg/dL [1]. Whether a test makes sense for you depends on your diet, your health history and any thyroid diagnosis. Bring your supplement list, including multivitamins and any Brazil nut habit. The NIH also notes that selenium may interact with some medications, and that the cancer drug cisplatin may lower selenium levels [2].

Good questions to ask

• Is my diet likely to be low in selenium, given what I eat and where I live?
• Does my thyroid diagnosis or medication change how much selenium or iodine I should aim for?
• If I take a multivitamin or eat Brazil nuts regularly, what is my total intake?

Conclusion

Selenium rewards a calm approach. The thyroid really does depend on it, and the nutrient really is concentrated in a few foods, but the people best served by this knowledge are the ones who use it to eat a little more variety, not to add a pill. A diet with eggs, fish, poultry, whole grains and the occasional Brazil nut gets most people to the RDA with room to spare. Beyond that, the evidence for extra selenium is thin, the margin for error is narrow, and the right dose, if any, is a question for a clinician who knows your history.

Algae belong in a food-first life for what they do offer: whole-food protein, chlorophyll, phycocyanin and a spread of vitamins and minerals, in a single-ingredient form. If you would like to add them, the ENERGYbits spirulina page is a good place to read about sourcing, and the VITALITYbits spirulina and chlorella blend brings both algae together. ENERGYbits is a single ingredient with no fillers or additives, freshwater-grown, gently dried at low temperatures and third-party tested for purity and contaminants.

Frequently asked questions

What foods are highest in selenium? +

Brazil nuts are highest, at about 68 to 91 mcg per nut. Yellowfin tuna (92 mcg in 3 oz), sardines, shrimp, pork, beef, pasta, turkey, cod, chicken, cottage cheese and eggs follow [1].

How much selenium do I need per day? +

Adults need 55 mcg a day. Pregnancy raises it to 60 mcg and lactation to 70 mcg. The upper limit for adults is 400 mcg a day from all sources [1].

Is selenium good for the thyroid? +

The thyroid needs selenium to make and activate hormone, and it holds more selenium than any other organ. Adequate intake supports normal function, but trials of extra selenium in thyroid disease show mixed results [1][3].

How many Brazil nuts can I eat a day? +

There is no single safe number because content varies so much. At 68 to 91 mcg per nut, around five nuts a day could reach the 400 mcg limit over time, so most people do best treating them as an occasional food [1][4].

What are the symptoms of too much selenium? +

Early signs are garlic breath and a metallic taste. Hair loss and brittle or lost nails are the most common clinical signs, and nausea, diarrhea, rash, fatigue and nerve symptoms can occur [1].

Can you get enough selenium from food alone? +

Usually yes. Average U.S. intake from food and beverages is about 108 mcg a day, nearly double the RDA. People eating mostly plants grown in low-selenium soil are at higher risk of falling short [1][2].

Do I need a selenium supplement for thyroid health? +

Not necessarily, and not without guidance. Current recommendations extend to specific situations, such as Graves orbitopathy, under clinician direction. For others the benefit is unclear [3]. Ask your doctor before starting one.

Is spirulina high in selenium? +

No. USDA lists dried spirulina at 7.2 mcg per 100 g, so a 7.5 g serving provides about 0.54 mcg, roughly 1 percent of the RDA (calculation) [6].

Do vegans need to worry about selenium? +

They may be lower on average. A 2025 trial found vegans had lower estimated intake, and that Brazil nut butter or a supplement providing 55 mcg a day raised selenium markers over two weeks [5]. Whole grains and occasional Brazil nuts help.

Who should check with a provider before taking selenium or algae? +

People who are pregnant or nursing, or who have a thyroid condition, kidney disease, phenylketonuria, an autoimmune condition or a bleeding disorder, and anyone taking blood thinners, immunosuppressants or thyroid medication, should check first.

This article is for educational purposes only and is not medical advice. These statements have not been evaluated by the FDA. ENERGYbits® products are whole-food supplements and are not intended to diagnose, treat, cure, or prevent any disease. Talk with your doctor before starting any supplement, especially if you are pregnant, nursing, taking medication, or managing a medical condition.

References

1. National Institutes of Health, Office of Dietary Supplements. Selenium: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/
2. National Institutes of Health, Office of Dietary Supplements. Selenium: Fact Sheet for Consumers. https://ods.od.nih.gov/factsheets/Selenium-Consumer/
3. Winther KH, Rayman MP, Bonnema SJ, Hegedus L. Selenium in thyroid disorders - essential knowledge for clinicians. Nat Rev Endocrinol. 2020. https://doi.org/10.1038/s41574-019-0311-6
4. Silva Junior EC, Wadt LHO, Silva KE, et al. Natural variation of selenium in Brazil nuts and soils from the Amazon region. Chemosphere. 2017;188:650-658. https://doi.org/10.1016/j.chemosphere.2017.08.158
5. Simon R, Lossow K, Pellowski D, et al. Improving the selenium supply of vegans and omnivores with Brazil nut butter compared to a dietary supplement in a randomized controlled trial. Eur J Nutr. 2025. https://doi.org/10.1007/s00394-025-03587-z
6. U.S. Department of Agriculture, FoodData Central. Seaweed, spirulina, dried (FDC ID 170495, SR Legacy). https://fdc.nal.usda.gov/food-details/170495/nutrients