Selenium

Get selenium from food (fish, eggs, meat, the occasional Brazil nut) rather than taking 100–200 µg supplements by default, even if you have Hashimoto's. Keep your total from all sources below 255 µg a day, and don't add a supplement on top of regular Brazil nuts.

What it is and why the thyroid cares

Selenium is an essential trace mineral. It sits at the working core of the enzymes that convert the thyroid hormone thyroxine (T4) into its active form, and of the enzymes that neutralise the hydrogen peroxide the gland produces while making hormone. Without enough of it, thyroid cells are left more exposed to oxidative damage.

The thyroid is one of the most selenium-rich tissues in the body, though in an autopsy study of 133 adults the kidney and the pituitary gland held more per gram. When intake runs low, the body ranks its organs and protects the thyroid's supply.^[1]

Hashimoto's thyroiditis, a slow autoimmune attack on the gland, is the main cause of an underactive thyroid in countries with enough iodine. It is marked by antibodies against thyroid peroxidase (TPO) and thyroglobulin, and women get it 4–10 times more often than men.^[2] That is where selenium supplements are most often recommended.

Antibodies fall; how people feel does not change

The antibody effect is real. Pooling 35 randomised trials, rated moderate-certainty, selenium supplements produced a moderate-to-large fall in TPO antibodies, whether or not people were already taking levothyroxine, and lowered thyroid-stimulating hormone (TSH) in those who were not. Thyroglobulin antibodies did not change significantly.^[3] A 2025 pooled analysis agrees on direction and adds a durability caveat: thyroglobulin antibodies fell at three months but not at six.^[4]

The patient outcome is not. In the largest trial with a patient-centred outcome, 412 people with Hashimoto's already on levothyroxine took 200 µg of selenium or a placebo every day for a year. Quality of life improved equally in both groups, including in those who started with low selenium, and the small drop in antibodies did not change how much levothyroxine they needed.^[5]

No trial has shown that selenium delays the point at which someone needs levothyroxine. Lower antibodies are a laboratory marker, not yet proof of a slower disease.

The safe range is narrow

Trials in Hashimoto's have used 50–200 µg a day. The ceiling is not far above that:

• Upper limit. Europe's food-safety agency puts the safe upper limit for adults at 255 µg a day from all sources, food and supplements combined (the US limit is 400 µg). That sits a margin below 330 µg a day, the lowest total intake at which extra hair loss appeared in a large trial. Chronic excess also brings brittle nails and digestive upset.^[6]
• Diabetes. In pooled trials, 200 µg a day raised the risk of type 2 diabetes by about 11%.^[7] In one randomised trial, 1,202 adults with a history of skin cancer, living in a low-selenium region of the eastern United States, took 200 µg a day or a placebo and were followed for an average of 7.7 years. The selenium group developed more type 2 diabetes, 12.6 against 8.4 cases per 1,000 person-years, and the excess was concentrated in the third who started with the highest blood selenium.^[8]

As a cancer-prevention pill: tested and failed

In a prostate-cancer prevention trial of 35,533 men, randomised to selenium (200 µg a day), vitamin E, both, or placebo, selenium did not protect the prostate: prostate cancer was marginally, and not significantly, more common than on placebo.^[9] A later analysis of the same trial suggests it may have harmed men who were already selenium-replete.^[10]

People in low-selenium regions, such as much of inland Europe, may benefit from a modest 50–100 µg a day; at 200 µg a day in people who already get enough, it is not protective. Other high-dose vitamins and minerals with a harm record are on Supplements to avoid.

Brazil nuts: rich but unpredictable

Two Brazil nuts a day raise selenium status about as well as a 100 µg supplement, but their content varies a lot: in one trial two nuts delivered anywhere from 20 to 84 µg (53 µg on average).^[11] That makes them a poor way to dose precisely, and a daily handful is not needed. If you already eat them regularly, don't add a selenium supplement on top.

What to do

1. Rely on food. Fish, eggs, meat and the occasional Brazil nut supply selenium.
2. Don't start a 100–200 µg supplement for Hashimoto's by default. It lowers antibodies but has not been shown to make people feel better or to delay treatment, and 200 µg a day carries a diabetes signal.
3. Count every source. Your diet, supplements, multivitamins and Brazil nuts together should stay below 255 µg a day.

The wider picture of thyroid testing and treatment is in Thyroid management.

Further reading

• Huwiler VV et al. Selenium Supplementation in Patients with Hashimoto Thyroiditis — systematic review and meta-analysis of randomized clinical trials. Thyroid 2024.^[12]
• Larsen C et al. Selenium supplementation and placebo are equally effective in improving quality of life in patients with hypothyroidism. Eur Thyroid J 2024.^[13]
• Turck D et al. Scientific opinion on the tolerable upper intake level for selenium. EFSA J 2023.^[14]
• Lippman SM et al. Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA 2009.^[15]
• Stranges S et al. Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Ann Intern Med 2007.^[16]
• Thomson CD et al. Brazil nuts: an effective way to improve selenium status. Am J Clin Nutr 2008.^[17]