Free triiodothyronine (T3) is a measure of the amount of T3 that is not protein bound. Most T3 is protein bound, mainly to thyroxine-binding globulin. The unbound portion is the active part.
The normal range for free T3 is 3-9 pmol per litre. It's measurement may be useful if a false high or low T3 is suspected.
Note that reference ranges may vary between laboratories.
Last reviewed 01/2018