Free thyroxine (T4) is a measure of the amount of T4 that is not protein bound. Most T4 is protein bound, mainly to thyroxine-binding globulin. The unbound portion is the active part.
The normal range for free T4 is 9-25 pmol per litre. Its measurement is useful when a false high or false low T4 or T3 is suspected.
Note that reference ranges may vary between laboratories.
Last reviewed 01/2018