Selenium
Selenium is a trace mineral that plays important roles in thyroid health, antioxidant protection and immune function. Although we only need tiny amounts, it has gained popularity online — especially in discussions around energy, metabolism and the thyroid.
However, selenium is one of the nutrients where too much can be just as problematic as too little. Supplements vary widely in dose, and some foods — particularly Brazil nuts — contain very high levels already. For this reason, selenium should not be taken casually or based on symptoms alone.
A food-first approach, supported by testing and professional guidance, is the safest way to ensure selenium is used appropriately.
What It Is
Selenium is a trace mineral needed in very small amounts. It supports:
antioxidant activity (protecting cells from damage)
thyroid hormone production
immune function
reproductive health
Different soils contain different levels of selenium, which is why food content varies between countries.
The body does not need large amounts, and excess intake can build up over time.
What It’s Used For
Selenium supplements are sometimes considered to support:
thyroid hormone conversion
antioxidant protection
immune function
reproductive health (sperm quality, for example)
However, selenium is not a supplement to take for tiredness, metabolism or immunity without clear evidence of low intake. Many symptoms thought to relate to selenium can have many other causes.
What the Evidence Says
Research shows that selenium:
contributes to normal thyroid hormone metabolism
supports antioxidant enzymes such as glutathione peroxidase
plays a role in immunity and inflammation balance
may support reproductive health in men
But evidence also shows:
deficiency is uncommon in the UK
excess selenium can lead to hair loss, brittle nails and digestive upset
too much selenium may worsen thyroid function for some individuals
a single Brazil nut can contain more than the daily requirement
supplements often contain much higher amounts than food
This is why selenium is considered a “handle with care” supplement.
When It Can Be Helpful
Selenium may be considered when:
a recent blood or urine test suggests low selenium status
someone avoids most selenium-rich foods (fish, eggs, grains, nuts)
dietary intake is consistently low due to restriction or food preferences
thyroid testing indicates it may play a supportive role and is reviewed by a GP or practitioner
Selenium should not be used based on guesswork or symptoms alone.
It is not a safe “general wellness” supplement.
When Caution Is Needed
Selenium requires special care because the difference between a helpful amount and a harmful amount is small.
Caution is especially important if you:
eat Brazil nuts regularly (they can be extremely high in selenium)
take multivitamins or thyroid-support blends that already contain selenium
have been diagnosed with a thyroid condition
take thyroid medication (selenium may change how your body processes it)
are pregnant or breastfeeding (there are strict upper limits)
consume supplement powders or “superfood blends” containing hidden selenium
have symptoms such as metallic taste, brittle nails or hair loss — which can indicate high selenium, not low
Why It’s Not Always the First Step
Before thinking about selenium supplements, it’s important to consider:
overall diet diversity
intake of fish, eggs, nuts and whole grains
whether you eat Brazil nuts already
thyroid testing and GP assessment
whether your symptoms may relate to stress, sleep, iron status or general nourishment
if other supplements you take already include selenium
Most people meet their selenium needs through food, and fatigue or mood changes are more often linked to other factors.
Nutrition Support
Selenium-rich foods include:
Brazil nuts (very high — usually 1–2 nuts is enough)
eggs
fish (tuna, sardines, salmon)
whole grains
sunflower seeds
poultry
Food sources generally provide safer amounts than supplements, as they are less likely to push intake above the upper limit.
Supporting digestion, eating balanced meals and maintaining overall nutrient diversity also help the body use selenium efficiently.
If You’re Unsure Whether Selenium Is Right for You
If you're unsure whether your selenium intake is appropriate — or you’re considering a supplement because of something you’ve read online — we can explore your diet, symptoms and any recent tests together. Selenium is a nutrient where clarity is important before making changes.
Prefer to read at your own pace?
I share occasional, practical updates like this, and let people know when something new is published. You’re welcome to leave your email below