Free T3
The unbound, active fraction of the faster-acting thyroid hormone, most of which your body makes by converting T4.
The unbound, active fraction of the faster-acting thyroid hormone, most of which your body makes by converting T4.
It shows whether that conversion is happening, which TSH and free T4 together cannot tell you.
It is the last of the three thyroid numbers to move and the least stable, so it is a supporting value rather than a headline. Any acute illness pushes it down without the thyroid being involved, and high-dose biotin distorts the assay.
Worth discussing as part of the thyroid panel rather than on its own.
On a report in another language.
A lab report names this analyte in its own language. Vitals Vault recognises each of these and files them all under Free T3.
Catalog entry last updated 2026-08-27. Browse all 165 metrics.