The body converts the thyroid hormone T4 into its active form, T3, using enzymes that require selenium as a component.
Evidence from Studies
Supporting (2)
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When people didn’t have enough selenium, their bodies struggled to turn the thyroid hormone T4 into the active form T3. When they took selenium supplements, their bodies got better at making T3, proving selenium is needed for this job.
Thyroid function in patients with selenium deficiency exhibits high free T4 to T3 ratio
When people don't have enough selenium, their body can't turn the thyroid hormone T4 into the active form T3. When they take selenium supplements, their body starts making T3 again — proving selenium is needed for this process.
Contradicting (2)
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The study gave kids with thyroid problems extra selenium to see if it helped turn T4 into T3, but it didn’t work — T3 levels didn’t go up. So even though selenium is needed for the job, having more of it didn’t help in this case, which means the claim might be too simple.
This study found that even when a key selenium part of an enzyme was changed, the enzyme still worked — which means selenium might not be as necessary as we thought for turning T4 into T3. That goes against the common belief.
Gold Standard Evidence Needed
According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.