Are Brazil nuts good for your thyroid?

For thyroid hormones, the trials say no measurable effect. A meta-analysis of 7 randomized trials in 315 adults eating Brazil nuts, on average 9.42 g a day for about 11 weeks, found plasma selenium rose by 87.63 mcg per liter while T3, T4 and TSH did not change. Those trials were mostly in people without thyroid disease, and we found no trial of Brazil nuts in Hashimoto's or any other thyroid condition.

Myth. Eating Brazil nuts to tune up the thyroid has been measured, in trials of mostly healthy people, and the hormones stayed where they were. The TSH difference was 0.01 ng/ml, with a confidence interval from -0.03 to 0.05.

Unsettled. Whether Brazil nuts help people who already have autoimmune thyroid disease is open. The only evidence comes from selenium pills, where antibodies fall but the things patients feel were rarely measured, and the Cochrane review called the evidence too weak for confident decisions. Nobody has run the trial with the nuts.

What the trials found

Brazil nuts do deliver selenium. In a 12-week randomized trial in 59 New Zealand adults, two nuts a day raised plasma selenium by 64.2 percent, against 61.0 percent on 100 mcg of selenomethionine and 7.6 percent on placebo. The nuts in that trial varied widely in selenium content, and the trial did not measure the thyroid.

The thyroid evidence is about selenium supplements in people with Hashimoto's, also called autoimmune thyroiditis. The largest meta-analysis pooled 35 randomized trials. In 869 people not taking thyroid hormone, selenium lowered TSH with a standardized difference of -0.21, a small effect, on moderate certainty evidence. Free T4 and T3 did not change. The same review's abstract reports a larger fall in thyroid peroxidase antibodies, a standardized difference of -0.96 across 2,358 people.

Other reviews are less encouraging. A meta-analysis of 16 controlled trials also found antibodies fell, but the authors say the clinical relevance remains to be shown, and people on selenium reported adverse effects more often than controls (p = 0.036). The 35-trial review found no difference in adverse effects, so the two disagree on that point. A meta-analysis of 9 trials in people not on levothyroxine found no effect on TSH, thyroid ultrasound or quality of life, on very low to moderate certainty evidence. The 2013 Cochrane review found 4 trials in 463 people, at unclear to high risk of bias, and said they did not allow confident decision making.

Antibody levels are a laboratory marker. A lower number on a blood test is not the same as feeling better or needing less medicine, and those outcomes are the ones the trials rarely measured.

Who should be careful

The real risk with Brazil nuts is too much selenium. The NIH says one nut holds 68 to 91 mcg and that eating them regularly could cause selenium toxicity. It gives an average of 544 mcg per ounce from USDA data, while noting that other analyses vary widely. The adult upper limit is 400 mcg a day from food and supplements together, and for children it ranges from 45 to 400 mcg by age. In one non-randomized trial in 56 women with overweight, just 8 g of nuts a day supplied 347.2 mcg, from that particular batch.

More selenium has costs of its own. A Cochrane review of 10 randomized trials in 27,232 people found selenium supplements did not prevent cancer and slightly raised the risk of type 2 diabetes. Those trials used supplements, so the finding applies to a daily habit of Brazil nuts only by analogy.

Pregnancy is the case where the experts have spoken directly. The NIH reports that the American Thyroid Association advises against selenium supplements for pregnant women with thyroid autoimmunity. The trials are mixed, as the NIH notes. In a randomized trial in 151 pregnant women with thyroid antibodies, 200 mcg of selenomethionine a day cut postpartum thyroid dysfunction from 48.6 to 28.6 per cent. In a UK trial in 230 pregnant women with mild iodine deficiency, 60 mcg a day did not lower antibodies more than placebo, and in the women who had antibodies, free T4 fell more on selenium.

Not for everyone. If you are pregnant and have thyroid antibodies, the expert advice is against selenium supplements, and a daily handful of Brazil nuts delivers selenium in supplement-sized amounts. Anyone already taking a selenium or multivitamin supplement can pass the adult upper limit with a few nuts on top. Children have lower limits, from 45 mcg a day depending on age.

What expert bodies say

We found no expert body position on Brazil nuts and the thyroid. The NIH Office of Dietary Supplements sets the adult upper limit for selenium at 400 mcg a day and warns that regular Brazil nut eating could cause toxicity. It relays the American Thyroid Association advice against selenium supplements in pregnant women with thyroid autoimmunity.

How we searched

What would change this answer

• A randomized trial of Brazil nuts against a selenium supplement in people with thyroid antibodies, measuring TSH, levothyroxine dose and quality of life. We found none registered.
• Trials of selenium in Hashimoto's that report how people feel and how much medicine they need, rather than antibody levels alone.

More on the nut itself is on the Brazil nuts page, and the nutrient behind the claim has its own selenium guide.

Sources

1. ev-bnth-01 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = 315
   Furthermore, Brazil nut had increasing effect on GPx levels too (WMD: 8.05U/gHb, 95% CI:0.65, 15.45, I2 = 96%) but brazil nut had no significant effect on T3 (WMD: 0.06 pg/ml, 95% CI: −0.50, 0.39, I2 = 74%), T4 (WMD: −0.01 pg/ml, 95% CI: −0.46, 0.44, I2 = 82%), and TSH levels (WMD: 0.01 ng/ml, 95% CI: −0.03, 0.05, I2 = 0.67%).
   
   Who: adults in randomized trials of Brazil nuts, mean 9.42 g/day (5 to 15 g) for a mean 11 weeks (8 to 24)
   Effect: TSH WMD 0.01 ng/ml (95% CI -0.03 to 0.05); T3 WMD 0.06 pg/ml (-0.50 to 0.39); T4 WMD -0.01 (-0.46 to 0.44); plasma selenium +87.63 mcg/l (36.02 to 139.24)
   Certainty: Trials were mostly in people without thyroid disease; small, heterogeneous; no trial of Brazil nuts in Hashimoto's found.
   Journal of King Saud University - Science, 2020, 32(3):1845-1852 (Li et al.) · checked 2026-10-07 · we read the abstract
2. ev-bnth-02 · Randomized controlled trial(s) · randomized placebo-controlled trial, 3 arms, 12 weeks · n = 59
   Consumption of 2 Brazil nuts daily is as effective for increasing selenium status and enhancing GPx activity as 100 mug Se as selenomethionine.
   
   Who: adults in New Zealand; 2 Brazil nuts/day (actual intake about 53 mcg selenium, range 20 to 84), 100 mcg selenomethionine, or placebo for 12 weeks
   Effect: plasma selenium +64.2% (nuts), +61.0% (selenomethionine), +7.6% (placebo); whole blood GPx +13.2%, +5.3%, +1.9%
   Certainty: Selenium status, not thyroid outcomes; selenium per nut varied widely.
   Am J Clin Nutr, 2008 · checked 2026-10-07 · we read the abstract
3. ev-bnth-03 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized controlled trials · n = 869
   Our meta-analysis found that selenium supplementation decreased TSH in patients without THRT (SMD -0.21 [confidence interval, CI -0.43 to -0.02]; 7 cohorts, 869 participants; I2 = 0%).
   
   Who: people with Hashimoto thyroiditis without thyroid hormone replacement therapy (primary outcome)
   Effect: TSH SMD -0.21 (95% CI -0.43 to -0.02), 7 cohorts, I2 0%; TPOAb SMD -0.96 (-1.36 to -0.56), 29 cohorts, 2,358 people; adverse effects OR 0.89 (0.46 to 1.75)
   Certainty: Selenium supplements (selenomethionine, selenite), not Brazil nuts; small standardized effect; fT4 and fT3 unchanged.
   Thyroid, 2024 · checked 2026-10-07 · we read the abstract
4. ev-bnth-04 · Meta-analysis or systematic review · systematic review and meta-analysis of controlled trials · n = 16 trials
   Study participants receiving selenium had a significantly higher risk than controls of reporting adverse effects (p = 0.036).
   
   Who: adults with chronic autoimmune thyroiditis, with or without levothyroxine, selenium vs placebo or levothyroxine
   Effect: TPOAb at 3 months, on levothyroxine: WMD -271 (95% CI -366 to -175), 7 studies; untreated: WMD -512 (-626 to -398), 3 studies; adverse effects higher on selenium (p = 0.036); GRADE generally low
   Certainty: Antibody levels are a lab marker; the authors say clinical relevance remains to be demonstrated.
   Thyroid, 2016 · checked 2026-10-07 · we read the abstract
5. ev-bnth-05 · Meta-analysis or systematic review · systematic review and meta-analysis of controlled trials · n = 9 trials
   We found no effect of selenium supplementation on thyroid stimulating hormone, health-related quality of life or thyroid ultrasound, in levothyroxine substitution-untreated individuals, and sporadic evaluation of clinically relevant outcomes in levothyroxine substitution-treated patients.
   
   Who: adults with chronic autoimmune thyroiditis not taking levothyroxine
   Effect: no change in TSH, thyroid echogenicity or health-related quality of life; GRADE very low to moderate
   Certainty: Small trials; outcomes that matter to patients were rarely measured.
   Endocrine, 2017 · checked 2026-10-07 · we read the abstract
6. ev-bnth-06 · Meta-analysis or systematic review · Cochrane systematic review of randomized controlled trials · n = 463
   The current level of evidence for the efficacy of selenium supplementation in the management of people with Hashimoto's thyroiditis is based on four randomised controlled trials assessed at unclear to high risk of bias; this does not at present allow confident decision making about the use of selenium supplementation for Hashimoto's thyroiditis.
   
   Who: adults diagnosed with Hashimoto's thyroiditis; selenomethionine or sodium selenite 200 mcg, mean 7.5 months
   Effect: TPOAb fell with selenomethionine 200 mcg in 3 trials (MD -917, -345 and -235 U/mL), no pooling (I2 99%); risk of bias unclear to high
   Certainty: 2013 review; newer meta-analyses (cards 03 to 05) disagree on clinical value.
   Cochrane Database Syst Rev, 2013 · checked 2026-10-07 · we read the abstract
7. ev-bnth-07 · Position of an expert body · agency fact sheet · n = —
   As discussed earlier, Brazil nuts contain very high amounts of selenium (68–91 mcg per nut) and could cause selenium toxicity if consumed regularly.
   
   Who: general population
   Effect: 68 to 91 mcg selenium per nut
   Certainty: Selenium in Brazil nuts varies widely with soil.
   NIH Office of Dietary Supplements, Selenium Fact Sheet for Health Professionals · checked 2026-10-07 · we read the section
8. ev-bnth-08 · Position of an expert body · agency fact sheet · n = —
   The Tolerable Upper Intake Level for selenium is 400 mcg for adults, and it ranges from 45 mcg to 400 mcg for infants, children, and adolescents, depending on age.
   
   Who: adults; children and adolescents 45 to 400 mcg by age
   Effect: UL 400 mcg/day for adults
   Certainty: UL covers food plus supplements; a handful of Brazil nuts can approach it.
   NIH Office of Dietary Supplements, Selenium Fact Sheet for Health Professionals · checked 2026-10-07 · we read the section
9. ev-bnth-09 · Position of an expert body · agency fact sheet · n = —
   For example, according to FoodData Central from the U.S. Department of Agriculture (USDA), Brazil nuts have an average of 544 mcg selenium/ounce, but values from other analyses vary widely [16-18].
   
   Who: Brazil nuts in USDA FoodData Central and other analyses
   Effect: 544 mcg selenium per ounce (28 g), wide variation
   Certainty: One ounce is about 6 to 8 nuts; content depends on soil selenium.
   NIH Office of Dietary Supplements, Selenium Fact Sheet for Health Professionals · checked 2026-10-07 · we read the section
10. ev-bnth-10 · Observational data · nonrandomized controlled trial, 8 weeks · n = 56
    The control group (CO) consumed a nut-free diet, while the BN group consumed 8 g BN/d, providing 347.2 μg selenium (Se).
    
    Who: women with overweight or obesity on an energy-restricted diet; 8 g Brazil nuts/day vs nut-free diet
    Effect: 8 g Brazil nuts = 347.2 mcg selenium per day, close to the 400 mcg adult UL
    Certainty: Non-randomized; selenium content of that batch; inflammation outcomes, not thyroid.
    J Nutr, 2024 · checked 2026-10-07 · we read the abstract
11. ev-bnth-11 · Position of an expert body · agency fact sheet · n = —
    The results of the clinical trials that have examined the effects of selenium supplementation on thyroid antibodies in pregnant women have also been mixed, and the American Thyroid Association recommends against selenium supplementation in pregnant women who have thyroid autoimmunity.
    
    Who: pregnant women who are TPOAb positive
    Effect: ATA 2017 weak recommendation against selenium supplementation, moderate quality evidence
    Certainty: About supplements; ATA position relayed by NIH ODS.
    NIH Office of Dietary Supplements, Selenium Fact Sheet for Health Professionals · checked 2026-10-07 · we read the section
12. ev-bnth-12 · Randomized controlled trial(s) · randomized placebo-controlled trial · n = 151
    PPTD and permanent hypothyroidism were significantly lower in group S1 compared with S0 (28.6 vs. 48.6%, P<0.01; and 11.7 vs. 20.3%, P<0.01).
    
    Who: euthyroid TPOAb-positive pregnant women, selenomethionine 200 mcg/day vs placebo during pregnancy and postpartum
    Effect: postpartum thyroid dysfunction 28.6% vs 48.6%; permanent hypothyroidism 11.7% vs 20.3% (P<0.01)
    Certainty: Single trial; not replicated with 60 mcg (card 13).
    J Clin Endocrinol Metab, 2007 · checked 2026-10-07 · we read the abstract
13. ev-bnth-13 · Randomized controlled trial(s) · double-blind randomized placebo-controlled trial, secondary analysis · n = 230
    Se supplementation had no more effect than placebo in decreasing TPO-Ab concentration or the prevalence of TPO-Ab positivity during the course of pregnancy.
    
    Who: pregnant women from 12 weeks gestation, UK antenatal clinic, mild-to-moderate iodine deficiency; 60 mcg/day selenium vs placebo
    Effect: no effect on TPOAb; in antibody-positive women FT4 fell more on selenium (P = 0.029 at 35 weeks)
    Certainty: Low dose; secondary analysis of SPRINT.
    Eur J Nutr, 2016 · checked 2026-10-07 · we read the abstract
14. ev-bnth-14 · Meta-analysis or systematic review · Cochrane systematic review and meta-analysis of RCTs and cohorts · n = 27232
    RCTs showed a slightly increased risk of type 2 diabetes associated with supplementation.
    
    Who: adults in randomized trials of selenium supplements vs placebo, 88% male
    Effect: any cancer RR 1.01 (95% CI 0.93 to 1.10), high certainty; slightly increased type 2 diabetes risk; alopecia and dermatitis in SELECT
    Certainty: Supplements at about 200 mcg/day; applies to habitual high intake from Brazil nuts only by analogy.
    Cochrane Database Syst Rev, 2018 · checked 2026-10-07 · we read the abstract