Selenium dosage and evidence
Written by Aaron CuhaReviewed Sep 2026
The short answer
SELECT randomised 35,533 men to selenium, vitamin E, both, or placebo and did not prevent prostate cancer. That is the honesty bar. Thyroid conversations exist and are clinician-owned. Do not stack selenium with iodine on a Hashimoto forum protocol.
- People given it
- 35,533
- Human studies
- 1
- Found nothing
- 0
Dose
Approved medicine. No human dose established for unlabelled uses
Labeled amounts sit on the prescribing information. This page records what trials measured, not a personal dose.
Sold as a dietary supplement. Not approved to prevent prostate cancer or to treat thyroid disease.
No first-hand reports gathered yet. That is a gap in what we have collected, not a finding that nothing has been reported.
What it has been measured to do
Measured in people
Goals Selenium has been measured for in a human being, most people first. Counts come from the study lines on this page and deliberately undercount.
- Cancer and tumours35,533 people · 1 study
35 of the 36 indexed goals have no study of any kind behind Selenium
No trial, no animal work, no cell work on this page reports a result for these. That is an absence of evidence rather than evidence of absence, but it does mean anything you have been told about Selenium and one of these did not come from a study cited here.
When this earns a spot
It matters when a clinician is managing a documented low selenium or a thyroid conversation they already own. It does not matter as a cancer-prevention default. SELECT tested selenium and vitamin E in 35,533 men and did not prevent prostate cancer. Hashimoto's plus iodine plus selenium is not a stack you build from a page.
Bloodwork that belongs beside the bottle
TSH and free T4 first. Serum selenium if the clinician wants a status number.
Default retest window: 90 days. If the named marker did not move, stop.
- Do not add iodine at the same time on your own.
- Hair selenium is not the decision test.
Reading the number
SELECT is the honesty bar: a prevention story that did not hold. Thyroid antibodies, if measured, stay with the clinician.
Marker glossary: TSH and free T4.
What people typically order
Ask for TSH and free T4. Ask whether serum selenium adds anything. Do not mail-order a thyroid protocol.
Show full evidence
Overview
Selenium is an essential trace element in selenoproteins, including glutathione peroxidases and deiodinases that handle thyroid hormone. That last fact is why it shows up next to TSH on this site.
The prevention story that sold a generation of bottles was cancer, especially prostate cancer. SELECT tested it properly, at scale, with vitamin E, and did not prevent prostate cancer. A page that leads with a thyroid anecdote and hides SELECT is marketing.
This stub will not write a Hashimoto protocol. Antibodies, iodine, and replacement thyroxine belong to a clinician. Excess selenium is toxic. More is not a longer life.
Mechanism of action
Selenocysteine is inserted into selenoproteins. Glutathione peroxidase uses it to reduce peroxides. Deiodinases use it in thyroid-hormone conversion. Those are mechanisms. SELECT is the prevention result.
Human evidence
The largest prevention trial in this lane is a null. Thyroid uses, if any, are not that trial and are not written as a protocol here.
- SELECT: no prostate-cancer prevention from selenium or vitamin E.
- Selenoproteins are real. Selling them as cancer prevention was the overreach.
What this does not tell you: SELECT was men, and it was a cancer-prevention design. It does not say selenium has no biological role. It says the famous prevention claim failed. Hashimoto's care is outside this stub.
The evidence, charted
Fig. 1a · evidence scale
35,533people, across 1 human study cited here
- 2009 · JAMA35,533100%
The whole total is one study of 35,533. Counts are the enrollment each human citation on this page states, which includes observational cohorts where nobody was given the compound. Studies still recruiting are excluded, and only studies cited on this page are counted.
Fig. 1b · evidence mix
All 1 citation here is human work.
- Human · given to people1100%
Counted from the citation list on this page, typed as this page types it. It understates any literature larger than the sources we cite, and a review counts once however many studies it covers.
Fig. 2 · evidence over time
Every citation here was published in 2009.
Too few distinct publication years on this page to plot as a timeline. The newest citation on file is from 2009, more than five years ago; the published record 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 jurisdictions shown. A jurisdiction's classification is a regulatory fact, not a verdict on the science; see Legal status below for the full text and any notes.
Fig. 4 · dose response
No human dose response curve exists
We draw this figure where the data supports it. For this compound in humans it does not, so the panel stays empty rather than borrowing an animal curve and implying it transfers.
Fig. 5 · molecular identity
- Modality
- Supplement
- Molecular weight
- Not on file
- Half-life
- None on file
- Sequence length
- None on file
No amino acid sequence is on file for Selenium, which is expected: a supplement is not built from residues.
Key studies & citations
- Human2009Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT).
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.
Frequently asked questions
Has Selenium been tested in people?
SELECT randomised 35,533 men to selenium, vitamin E, both, or placebo and did not prevent prostate cancer. That is the honesty bar. Thyroid conversations exist and are clinician-owned. Do not stack selenium with iodine on a Hashimoto forum protocol. 1 completed trial, 35,533 people given it. 35,533 people given it across 1 completed trial. 0 found nothing.
What is the Selenium dosage?
Approved medicine. No human dose established for unlabelled uses. Labeled amounts sit on the prescribing information. This page records what trials measured, not a personal dose. Sold as a dietary supplement. Not approved to prevent prostate cancer or to treat thyroid disease.
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. High TSH is a clinician conversation with free T4, and sometimes antibodies. This page will not pair selenium and iodine for you.
Did selenium prevent cancer?
Not in SELECT, which is the trial that was big enough to answer the claim people were making.