What does selenium do for your thyroid?

Selenium contributes to normal thyroid function and contributes to the protection of cells from oxidative stress, the everyday wear and tear on your cells. It also contributes to the normal function of your immune system. [1] [2] But selenium is one of the clearest examples in nutrition where more is not better.

How selenium works

Your body builds selenium into special proteins called selenoproteins. Some of them help produce active thyroid hormone, and others help protect your cells. [1]

How much selenium do you need?

The NHS puts adult needs at 75 µg a day for men and 60 µg a day for women, and says that if you eat meat, fish or nuts you should get enough from your diet. [5] Brazil nuts are a rich source, but the amount of selenium in food depends heavily on the soil it was grown in, so it varies a lot between countries. [3]

Why more is not better

Selenium follows a U-shaped pattern. If you are low, getting more helps. If you already have enough, extra may do harm. [1]

In one long study, people took 200 µg of selenium a day, the same amount as many supplements, for nearly eight years on average. It did not prevent type 2 diabetes, and more people in the selenium group developed it than in the dummy-tablet group. [4] That is why I never suggest selenium "just in case". I look at someone's diet first.

Why L-selenomethionine?

L-selenomethionine is the main form of selenium in foods like cereals, and your body absorbs it well. [2] Good absorption is helpful if you need it, and another reason to respect the dose.

How to take selenium safely

  • Check your diet first. Fish, eggs, meat and the occasional Brazil nut may already cover your needs.
  • Stay within UK guidance. The NHS says 350 µg a day or less from supplements is unlikely to cause harm. [5] Count every source, including multivitamins.
  • Do not double up. One selenium-containing product is enough.

Who should check with their GP first

Speak to your GP before taking selenium if you have a thyroid condition, diabetes or another medical condition, take regular medication, or are pregnant or breastfeeding. Too much selenium causes selenosis, which in its mildest form can lead to hair and nail loss. Other warning signs include a garlic smell on the breath and a metallic taste. [2] [5]

Frequently asked questions

How does selenium support the thyroid?

Selenium contributes to normal thyroid function. It is built into selenoproteins, which are involved in producing active thyroid hormone.

How much selenium do I need a day in the UK?

The NHS gives 75 µg a day for men and 60 µg a day for women aged 19 to 64, and says most people who eat meat, fish or nuts get enough from their diet.

What is the safe upper limit for selenium supplements?

The NHS says taking 350 µg or less a day of selenium supplements is unlikely to cause harm. Too much causes selenosis, which in its mildest form can lead to loss of hair and nails.

Should I take selenium for a thyroid condition?

Speak to your GP first. Selenium supplements are not a treatment for thyroid disease, and the right choice depends on your status and medication.

Can I get enough selenium from Brazil nuts?

Brazil nuts are a rich source, but the selenium content of foods varies with the soil they were grown in, so amounts differ between nuts and countries.

Related reading: Zinc picolinate benefits, Vitamin D3 and K2, Omega-3: EPA, DHA and heart health.

Make it part of a plan

Selenium is a good example of why supplements should match your diet, not sit on top of it. If you want training and nutrition built around you, online coaching includes nutrition and supplementation guidance. Apply for coaching and tell me what you are working on.

This article is general information, not medical advice. If you take medication, are pregnant or breastfeeding, or have a medical condition, check with your GP or pharmacist before starting a supplement.

Evidence reviewed: October 2026. This guide covers selenium needs, the research on higher doses and UK safety limits.

Research and further reading

  1. Rayman (2012). Selenium and human health. Lancet.
  2. NIH Office of Dietary Supplements. Selenium: health professional fact sheet.
  3. Rayman (2008). Food-chain selenium and human health: emphasis on intake. Br J Nutr.
  4. Stranges et al. (2007). Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Ann Intern Med.
  5. NHS. Vitamins and minerals: others (zinc and selenium guidance).