Selenium dosage and evidence
Written by Aaron CuhaReviewed Sep 2026
The short answer
SELECT, a trial of 35,533 men, tested selenium, vitamin E, both, or a dummy pill. Selenium did not prevent prostate cancer. That is the honesty bar. Thyroid questions exist, and they belong with your doctor. Do not pair selenium with iodine because a Hashimoto's forum said so.
Evidence grade for Selenium: Moderate. The rule: at least one finished study in people, and at least 100 people who took it. The sum on this page: 1 completed trial, 35,533 people given it. A grade counts how much has been done to people, not whether it works.
- People given it
- 35,533
- Human studies
- 1
- Found nothing
- 0
Dose
No human dose established
Amounts that were given in studies sit in Human evidence on this page. None of them is a personal dose.
1 completed trial, 35,533 people given it
1 effects reported, 1 unwanted, by people using it, not a trial
Goal pages: Ageing and longevity, Immune system and inflammation
In plain English
In plain English, from Aaron
This is how I would explain it to a friend. The evidence, with the numbers, is further down the page.
Selenium is a trace mineral your body needs in small amounts. It helps some proteins fight damage, and it helps switch thyroid hormone on. For years it was sold to prevent cancer. A huge trial tested that, and it did not work. Too much selenium is toxic.
What people take it for
- Preventing cancer, especially prostate cancer.
- Thyroid health, including Hashimoto's.
- General antioxidant protection.
What studies in people showed
SELECT gave 35,533 men selenium, vitamin E, both, or a dummy pill. None of it lowered prostate cancer. That was the claim that sold the bottles, and it failed at scale. It was a trial in men, built to test cancer. It does not say selenium has no job in the body.
What people report
Reports, not study results
The good
- We have not collected user reports on this one yet.
The bad
- The risk this page flags: too much selenium is toxic, and more does not mean a longer life.
Where these come from: We have no forum or clinic reports for this one. The risk listed comes from the warnings on this page.
My bottom line
Selenium did not prevent cancer in the one trial big enough to know. If your thyroid test is off, see your doctor and get the full panel. Do not build your own selenium and iodine plan off a forum.
An opinion, not a finding. I am a coach, not a doctor.
What it has been measured to do
Measured in people
Goals Selenium has been tested for in people, biggest studies first. We count only what the studies on this page say, so the real numbers may be higher.
- Cancer and tumours35,533 people · 1 study
35 of the 36 indexed goals have no study of any kind behind Selenium
No study in people, animals or cells on this page reports a result for these. That does not prove Selenium does nothing for them. It does mean anything you have heard about Selenium and one of these did not come from a study cited here.
When this earns a spot
It matters when a doctor is treating a confirmed low selenium, or a thyroid problem they already manage. It is not a default for preventing cancer. The SELECT study gave selenium and vitamin E to 35,533 men, and it did not prevent prostate cancer. Hashimoto's plus iodine plus selenium is not a stack you build from a web page.
Bloodwork that belongs beside the bottle
TSH and free T4 first. Blood selenium if your doctor wants a number.
Test again after about 90 days. If that blood number did not move, stop.
- Do not add iodine at the same time on your own.
- A hair selenium test is not the one to decide on.
Reading the number
SELECT is the reality check: a prevention story that did not hold up. Thyroid antibodies, if measured, are for your doctor to read.
What these tests mean: TSH and free T4.
What people typically order
Ask for TSH and free T4. Ask whether blood selenium adds anything. Do not mail-order a thyroid plan.
Show full evidence
What is Selenium?
SELECT, a trial of 35,533 men, tested selenium, vitamin E, both, or a dummy pill. Selenium did not prevent prostate cancer. That is the honesty bar. Thyroid questions exist, and they belong with your doctor. Do not pair selenium with iodine because a Hashimoto's forum said so.
Selenium is a trace mineral your body needs. It is built into proteins called selenoproteins. Some of them break down harmful peroxides. Others help turn thyroid hormone into its active form. That last fact is why selenium shows up next to TSH, a thyroid test, on this site.
The story that sold a generation of bottles was cancer prevention, especially prostate cancer. SELECT tested that properly, in 35,533 men, alongside vitamin E. Selenium did not prevent prostate cancer. A page that leads with a thyroid story and hides SELECT is marketing.
This page will not write a Hashimoto's plan. Antibodies, iodine and thyroid hormone pills belong with your doctor. Too much selenium is toxic. More does not mean a longer life.
How does Selenium work?
Selenium is built into selenoproteins as a special amino acid. An enzyme called glutathione peroxidase uses it to break down harmful peroxides. Other enzymes use it to switch thyroid hormone on. That is how it works. SELECT is what happened when people took it to prevent cancer.
What have studies in people found about Selenium?
The biggest prevention trial here found no benefit. Any thyroid uses are a separate question, and this page does not turn them into a plan.
- SELECT: neither selenium nor vitamin E prevented prostate cancer.
- Selenoproteins are real. Selling them as cancer prevention went too far.
What this does not tell you: SELECT studied men, and it was built to test cancer prevention. It does not say selenium has no role in the body. It says the famous prevention claim failed. Care for Hashimoto's is outside this page.
The evidence, in charts
Fig. 1a · how many people
35,533people, across 1 study in people cited here
| Study | People | Share |
|---|---|---|
| SELECT, JAMA 2009 | 35,533 | 100% |
The whole total is one study of 35,533. Counts are the number of people each study on this page says it signed up. That includes studies that only watched people and gave nobody the compound. Studies still signing people up are left out, and only studies cited on this page are counted.
Fig. 1b · evidence mix
All 1 source here is a study in people.
- Study in people · done in people1100%
Counted from the list of sources on this page. It counts only what we cite, so there may be more studies out there. A review counts once, however many studies it sums up.
Fig. 2 · evidence over time
Every source here was published in 2009.
Too few different years on this page to draw a timeline. The newest source on file is from 2009, more than five years ago. Research on it may have gone quiet.
Fig. 3 · legal status at a glance
US
Not approved
UK
Not approved
AU
Not approved
CA
Not approved
Not approved in any of the four countries shown. This is what the law says, not a verdict on the science. See Legal status below for the full text and any notes.
Fig. 4 · how much does what
Nobody has tested different amounts in people
This chart would show how the effect changes as the amount goes up. For this compound, nobody has measured that in people. So we leave it empty. We will not borrow an animal chart and pretend it works the same in people.
Fig. 5 · what the molecule is
- Kind of compound
- Supplement
- Molecule weight
- Not on file
- Half-life
- None on file
- Chain length
- None on file
No amino acid sequence is on file for Selenium, which is expected: a supplement is not built from amino acids.
The studies we cite
- Study in people2009Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT).
Selenium did not prevent prostate cancer. A huge trial gave men selenium, vitamin E, both, or a dummy pill. None of it lowered prostate cancer. The claim that sold the bottles failed when it was tested at scale.
SELECT. 35,533 men. Selenium, vitamin E, both, or placebo. No reduction in prostate cancer. The prevention claim that sold the bottles failed at scale.
Common questions
Has Selenium been tested in people?
Yes. 35,533 people given it across 1 completed trial cited here, 0 of which found nothing. SELECT, a trial of 35,533 men, tested selenium, vitamin E, both, or a dummy pill. Selenium did not prevent prostate cancer. That is the honesty bar. Thyroid questions exist, and they belong with your doctor. Do not pair selenium with iodine because a Hashimoto's forum said so.
What is the Selenium dosage?
No human dose established. Amounts that were given in studies sit in Human evidence on this page. None of them is a personal dose. 1 completed trial, 35,533 people given it
Is Selenium FDA-approved?
Sold as a dietary supplement. Not approved to prevent prostate cancer or to treat thyroid disease.
Should I take selenium because my TSH is high?
No. A high TSH is a talk with your doctor, with a free T4 test and sometimes an antibody test. This page will not pair selenium and iodine for you.
Did selenium prevent cancer?
Not in SELECT, the trial big enough to answer the claim people were making.