What is selenium good for, and can you take too much?

Last updated: June 2026 · Reviewed by the FactoWiki Editorial Team for clarity and source accuracy

Selenium's roles in thyroid and antioxidant defence, why deficiency is uncommon, and its surprisingly narrow safe range.

What selenium is

Selenium is an essential trace mineral the body uses to build selenoproteins — molecules involved in antioxidant defence, thyroid hormone metabolism and immune function. You need only a small amount, and most people get it from food (Brazil nuts are famously rich in it, along with seafood, meat and grains). Like several minerals, selenium is genuinely important in the right amount and genuinely harmful in excess, which defines how to think about supplementing it.

Thyroid and antioxidant roles

Selenium's clearest roles are in the thyroid, which is particularly selenium-dependent, and in antioxidant systems. It's sometimes used in specific thyroid conditions under medical guidance, where there's some supporting evidence. For the general population with adequate intake, though, taking extra selenium for thyroid or antioxidant 'support' offers little benefit — the body's needs are already met.

Immune, heart and cancer claims

Selenium has been marketed for immunity, heart health and even cancer prevention. The evidence here is mixed and, in some large trials, disappointing — notably, selenium supplementation did not deliver the cancer-prevention benefits once hoped for, and some research even raised concerns at higher intakes. This is a good example of an antioxidant mineral where 'more' did not translate into the broad protection the marketing suggested.

Deficiency is uncommon

In most regions with varied diets, genuine selenium deficiency is uncommon, because the mineral is widely available in food. Deficiency does occur in areas with low-selenium soils or in specific medical situations, but for the average well-fed person, there's usually no shortfall to correct — which means routine supplementation often adds risk without benefit.

The narrow safe range

This is the crucial part: selenium has a surprisingly narrow gap between enough and too much. Chronic excess causes selenosis, with symptoms like hair and nail loss or brittleness, digestive upset, a garlic-like breath odour, and nerve problems. Upper limits for adults are around 400 micrograms a day from all sources — and because some foods (and a few Brazil nuts) are very high, it's genuinely possible to overshoot by stacking supplements on top of a selenium-rich diet.

The verdict

Selenium is essential for thyroid and antioxidant function, but most people get enough from food, broad disease-prevention claims haven't held up, and its safe range is narrow enough that excess causes real harm. Unless you have a confirmed deficiency or a specific medical reason (guided by a doctor), routine high-dose selenium supplements are a case where the risks can outweigh the benefits — and a couple of Brazil nuts already deliver plenty.

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This article is general information, not medical advice. See our medical disclaimer, and talk to a qualified healthcare professional about your own situation.