What is selenium used for in the body?

Selenium is incorporated into selenoproteins involved in redox control, thyroid hormone metabolism, selenium transport and other normal functions. That is why inadequate intake matters. It does not mean a high-dose supplement improves a person who already has adequate intake.

Should everyone take selenium for cancer prevention?

No. SELECT tested 200 micrograms/day of L-selenomethionine in more than 35,000 men and did not show prostate-cancer prevention. Cochrane concluded that well-designed trials did not show selenium prevents cancer, so a supplement should not replace screening or treatment.

Can selenium improve Hashimoto thyroiditis?

Some selected trials found lower TPO antibodies or a small TSH change, especially in people not taking thyroid replacement, but the evidence is heterogeneous and does not prove symptom relief, normal thyroid hormones, remission or prevention. Discuss any trial with the treating clinician.

How much selenium do adults need?

The US RDA is 55 micrograms/day for adults, 60 micrograms/day in pregnancy and 70 micrograms/day during lactation. These are total-intake reference values, not automatic supplement doses. The US adult UL is 400 micrograms/day, while EFSA sets 255 micrograms/day for adults including pregnancy and lactation.

Are Brazil nuts a good selenium source?

They can be, but their content varies dramatically by origin and nut. NIH ODS lists a US average of 544 micrograms in 1 ounce, about 6 to 8 nuts, while EFSA reports very wide values. Regular large servings can exceed an adult upper limit, so do not treat one nut as a reliably measured dose.

Is 200 micrograms of selenium a safe daily dose?

200 micrograms was a research dose in SELECT and many thyroid trials, but safety depends on total intake, duration, form, jurisdiction and individual risk. It is not a universal recommendation, and it can combine with food or other supplements. Ask a clinician when a treatment reason exists.

What are signs of too much selenium?

Garlic-smelling breath, metallic taste, hair loss, brittle or lost nails, rash, nausea, diarrhoea, fatigue, irritability and neurologic symptoms can signal chronic excess. Severe gastrointestinal, neurologic, breathing, heart or kidney symptoms after a large exposure require urgent medical help.

Can I take selenium during chemotherapy?

Do not start or change selenium during cancer treatment without the oncology team's advice. Cisplatin can change selenium measures, but evidence that supplementation prevents chemotherapy toxicity is insufficient, and antioxidant decisions can depend on the treatment plan.

Does selenium prevent ageing or diabetes?

No established selenium supplement dose prevents ageing, extends life or prevents diabetes in a selenium-replete adult. Antioxidant mechanisms and observational associations are weaker than patient-important randomized outcomes. Use evidence-based nutrition and indicated clinical care instead.

An essential trace mineral with a narrow safety margin

Selenium supplements: evidence, studied doses, food sources and safety

Selenium is essential for selenoproteins, but more is not automatically better.

Most well-nourished adults should focus on food and total intake rather than routine high-dose supplements. Large randomized trials did not show cancer or cardiovascular prevention, and thyroid studies mainly show selected biomarker changes rather than a proven clinical benefit. Food and supplement amounts vary, Brazil nuts can be unexpectedly high, and chronic excess can cause selenosis.^{1,2,3,4,7,8,10}

Strong

Updated September 4, 202616 min readHuman evidence first

At a glance

Selenium

Overall evidence: Essential nutrient; routine high-dose supplementation has limited or no established patient-important benefit in replete adults^1,3,4,7,10StrongFinding: mixed results
Best-supported form: No form has proven superior for general outcomes; common products use selenomethionine, selenium yeast, selenite or selenate^1,4LimitedFinding: still unknown
US adult RDA: 55 micrograms/day; pregnancy 60 micrograms/day and lactation 70 micrograms/day¹StrongFinding: established use
Adult upper limits: US: 400 micrograms/day. EFSA: 255 micrograms/day from all sources, including pregnancy and lactation^1,2StrongFinding: established use
Cancer prevention: Large randomized evidence does not show selenium prevents prostate cancer or other cancers^3,4,5,6,7StrongFinding: no meaningful benefit
Thyroid evidence: Hashimoto trials show some antibody or TSH changes, without established disease-modifying or symptom benefit^1,8,9ModerateFinding: mixed results
Food caution: Brazil-nut selenium varies widely; an average serving can exceed an adult upper limit^1,2StrongFinding: established use

Study findings describe groups, not a guaranteed result for one person.

Research and writing

LongevityMate Editorial Team

Editorial oversight

Lukas Dvorsky, Founder

Clinical review

Not yet assigned or claimed

Evidence and editorial policy How health information is prepared

Educational information, not medical advice

The answer first

Should you take selenium?

Start with the reason, not the product. A supplement is most useful when it closes a real gap or serves a defined, measurable goal.

LongevityMate verdict

For a generally nourished adult, routine selenium supplementation has no established cancer, heart, diabetes, thyroid or antiageing benefit. A supplement may be reasonable when a clinician identifies a likely inadequacy or a specific clinical indication, but evidence and dosing must be individualized. Do not infer a personal need from a wellness claim, a single food value or a trial dose. Keep total intake below the applicable jurisdictional upper limit and stop self-directed use if toxicity symptoms appear.^{1,2,3,4,7,8,9}

Strong

Yes, selenium is essential

Selenium is built into selenoproteins involved in thyroid hormone metabolism, redox biology and transport. Nutritional need does not imply benefit from extra intake when requirements are met.

Food	Serving	Selenium
Brazil nuts	1 ounce, about 6–8 nuts	544 micrograms selenium on a US average; individual nuts vary widely
Yellowfin tuna, cooked	3 ounces	92 micrograms selenium
Egg, hard-boiled	1 large egg	15 micrograms selenium
Lentils, boiled	1 cup	6 micrograms selenium

Yes, selenium is essential

Related guides

Related guides

Yes, selenium is essential

No established cancer or heart prevention

Maybe in a clinician-defined deficiency or selected context

Safety depends on total intake

How it may work, in plain English

Selenoprotein biology

Thyroid hormone metabolism

Strong evidence

Moderate evidence

Limited evidence

Insufficient evidence

Do not self-start when safety is uncertain

Who may benefit

Who should avoid it or get advice first

Common side effects

Drug and supplement interactions

Cisplatin and other cancer treatment

Thyroid medicines and thyroid care

US dietary reference for most adults

Common multivitamin context

Standalone supplement label context

SELECT cancer-prevention trial

Hashimoto research

How to take it

No form has proven superior for general outcomes; common products use selenomethionine, selenium yeast, selenite or selenate

Food context

Correct a likely inadequate intake

Change a blood selenium or selenoprotein marker

Change Hashimoto thyroid antibodies

Prevent cancer, heart disease or ageing

Explore by health goal

Similar and related supplements

Already have blood-test results?

Anecdote can start a question—not answer it

Evidence method

Commercial independence

Editorial status

Corrections

A narrow exposure range

Rare but serious effects

Other selenium-containing supplements

Pregnancy, lactation, childhood or kidney disease

Choosing a quality product