Selenium lowers thyroid antibodies in trials, but most were run in people low on it. Why that matters in North America, and the Brazil nut limit.
Selenium helps your thyroid turn its stored hormone into the active form, and in Hashimoto's trials it lowers thyroid antibodies. But most of those trials were run in people short on selenium. Women in the US and Canada usually get well above their needs from food.
Maybe your TPO antibody result came back high and someone told you to try selenium. Or you already take it, plus a Brazil nut or two, and can't tell whether it's doing anything. The useful question isn't whether selenium works. It's whether the people in the studies looked like you.
What Does Selenium Do for Your Thyroid?
It keeps thyroid hormone working. According to the NIH Office of Dietary Supplements, selenium concentration is higher in the thyroid than in any other organ. Selenium-containing proteins help convert T4, the storage form of thyroid hormone, into T3, the active form.
They also protect the gland from the hydrogen peroxide it produces while making hormone. That's why selenium keeps coming up whenever thyroid health does. Needing a nutrient, though, isn't the same as needing more of it.
Does Selenium Help Hashimoto's?
It lowers antibodies on a lab report. Whether it makes you feel better is far less certain.
A Cochrane review found four trials with 463 adults, all at unclear to high risk of bias. Selenomethionine at 200 mcg a day lowered TPO antibodies in three of them, but the reviewers said the "clinical relevance is unclear" for patients. None of the trials measured health-related quality of life, and the review found not enough evidence to support selenium for Hashimoto's.
A larger 2024 review in the journal Thyroid pooled 35 trials, and nearly all had mostly women in them. Selenium lowered TPO antibodies, and it lowered TSH slightly in people not taking thyroid hormone. Side effects were similar to those in the control groups over the trial periods.
Feeling better was a different story. Two groups in that review reported more improvement in well-being with selenium, and four found no difference in quality of life. Every trial raised at least some concern about bias.
The American Thyroid Association's patient summary of that review notes that TSH fell from 3.24 to 2.4 mIU/L in the strongest trial. It also says data is still limited and further studies are needed. It doesn't tell you to take selenium.
Who Was Studied in the Selenium Trials?
Mostly women in places where selenium intake runs low. That detail rarely makes it into the advice, and it matters if you live in North America.
Of the 35 trials in the 2024 review, 19 were in Europe, 11 in Asia, four in the Middle East and one in South America. Only 18 measured selenium at the start. In nine of those, people were severely deficient, in seven mildly deficient, and in just two sufficient.
Most studies were in "populations most at risk for selenium deficiency," the authors wrote. Data on people with enough selenium was too scarce to say whether they benefit.
| Question | Hashimoto's trials | A typical US or Canadian woman |
|---|
| Where | Europe, Asia, Middle East, South America | US or Canada |
| Selenium status | Mostly deficient, where measured | Deficiency is "very rare," per the NIH |
| Usual intake | Not reported | About 94 mcg a day from food (US data), against a target of 55 |
| Dose tested | 200 mcg a day in many | 200 mcg plus about 94 from food comes to roughly 300, by simple addition |
The NIH says selenium deficiency is very rare in the US and Canada. Intakes there, even in low-selenium regions, are well above the recommended amount. In the US, adult women average 94 mcg a day from food and drink. A trial result from a population that was short on selenium may not carry over to one that isn't.
Low intake is more common elsewhere. The NIH notes low average intakes in some European countries, especially among vegans, and the lowest intakes in parts of China. If you live in or eat mostly food grown in a low-selenium region, the trials may fit you better.
Can Too Much Selenium Hurt You?
Yes, and the margin is narrower than with many supplements. The NIH sets the upper limit at 400 mcg a day for adults, counting food and supplements together.
Chronic excess, called selenosis, most often shows up as hair loss and brittle or lost nails. Other signs include garlic breath, a metallic taste, skin rash, nausea, diarrhea, fatigue and irritability. In 2008, 201 people had severe reactions to a liquid supplement that held 200 times the selenium on its label.
The longer trials raise a separate question. A trial of 1,202 people in low-selenium areas of the eastern US gave 200 mcg a day or placebo for an average of 7.7 years. Type 2 diabetes developed in 58 people on selenium and 39 on placebo, and the authors said selenium "may increase risk for the disease."
The much larger SELECT trial, of 35,533 men in the US, Canada and Puerto Rico, tested the same 200 mcg dose against placebo. It found a small increase in diabetes that wasn't statistically significant, and no protection against prostate cancer. Neither trial studied women with Hashimoto's, but both tested the dose most Hashimoto's trials use, for years rather than months.
How Much Selenium Do You Need a Day?
Women need 55 mcg a day, according to the NIH. That rises to 60 mcg in pregnancy and 70 mcg while breastfeeding.
Brazil nuts are the shortcut and the trap. One nut holds 68 to 91 mcg, more than a day's need. An ounce, about six to eight nuts, averages 544 mcg, over the upper limit, and the NIH says values vary widely between analyses.
Seafood and meat give steadier amounts. Per 3 ounces (85 g), cooked yellowfin tuna has about 92 mcg, canned sardines 45 and cooked shrimp 42. A large hard-boiled egg has about 15. Plant foods vary with the soil they grew in.
The NIH lists people on kidney dialysis, people living with HIV and people in selenium-deficient regions as more likely to run low. It notes that vegetarians and vegans may take in less. Iodine is the other mineral that comes up with thyroid health. Iodine and thyroid symptoms that won't go away covers why extra can backfire there too.
Is Selenium Used for Thyroid Eye Disease?
Yes, and it's the one thyroid use guidelines back. Thyroid eye disease, also called Graves' orbitopathy, can cause dry, red or bulging eyes and double vision.
One trial of 159 people with mild eye disease compared 100 mcg twice a day for six months with placebo (and a second drug). The ATA's summary says eyes improved in 61%, against 36% on placebo. The 2021 European guidelines list selenium for mild, active disease, with a qualifier: in selenium-deficient areas. The guidelines add that early referral to a specialist center matters for most people with the condition.
A 2023 survey of endocrinologists in Portugal pointed out that recommendations cover only mild Graves' orbitopathy. More than a third still used it for Hashimoto's at least occasionally. The authors suspected studies showing better lab markers were driving that.
Should You Take Selenium for Hashimoto's?
Not on your own. If you eat fish, meat, eggs or grains and live in North America, a supplement is unlikely to fix a shortage you probably don't have.
- Bring every supplement to your doctor, including multivitamins.
- Count Brazil nuts as a supplement. A handful alone can pass the 400 mcg limit, and even one or two on top of a 200 mcg pill can.
- Ask whether lower antibodies would change anything about your treatment.
- If you're pregnant with thyroid antibodies, the NIH notes the American Thyroid Association recommends against selenium supplements.
- If your diet is mostly vegan, or your food is grown in a low-selenium region, ask whether your intake is worth checking.
Tiredness, low mood and thinning hair have many causes besides selenium. Thyroid-related hair loss and the nutrition reasons you're always tired go through the likelier ones.
When to See a Doctor
Mayo Clinic lists fatigue, feeling cold, dry skin, constipation, heavy or irregular periods, brittle nails, hair loss, low mood and memory problems among Hashimoto's symptoms. Women are much more likely to get it, most often in middle age. Because these signs aren't specific, Mayo advises seeing a doctor promptly for a diagnosis.
See your doctor if you take selenium and notice hair shedding, brittle nails, garlic breath or a metallic taste. Stop the supplement and any Brazil nuts until you've spoken to them. Get prompt care for bulging or painful eyes, or double vision.
Thyroid changes can affect mood too. Thyroid problems, anxiety and depression explains the link. If you or a child swallowed a large amount of a selenium supplement, call Poison Control at 1-800-222-1222 (US). In an emergency, call 911 (999 in the UK) or your local emergency number.
Frequently Asked Questions
Can Selenium Lower Thyroid Antibodies?
In trials, yes. Selenium lowered TPO antibodies in the Cochrane review and the 2024 review. Evidence that people feel better is mixed and weak, and most trial participants started out short on selenium.
How Many Brazil Nuts a Day for Selenium?
One nut has 68 to 91 mcg, which already covers a woman's daily 55 mcg. An ounce of six to eight nuts can pass the 400 mcg upper limit, so keep them to one or two a day. Skip them if you take a selenium supplement.
Is 200 mcg of Selenium Too Much?
It's below the 400 mcg upper limit, and it's the dose most Hashimoto's trials used. Added to the 94 mcg US women average from food, that's roughly 300 mcg a day by simple addition. One long trial at this dose found more type 2 diabetes. A larger one found a small rise that wasn't statistically significant, so ask your doctor first.
This article is for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making changes to your health routine.
