Free T3Free triiodothyronine · FT3
The unbound portion of the more active thyroid hormone.
Triiodothyronine (T3) is the more biologically active thyroid hormone. The thyroid makes some directly, but most is produced by converting T4 in other tissues. Free T3 measures the unbound, available fraction.
It matters as a picture of what tissues actually receive, since conversion from T4 can change independently of thyroid output.
In practice free T3 falls during serious illness, starvation and prolonged energy restriction as a normal adaptation rather than thyroid disease, which is why it is not usually interpreted alone or measured routinely.