More than 99% of T4 and T3 in circulation are bound to the proteins albumin and thyroxine-binding globulin (TBG). Only the free or unbound fraction of T4 and T3 are biologically active and are able to impart metabolic activity.
For this reason, most current analytical assays for thyroid hormones are designed to measure the free forms, free T4 (FT4) and free T3 (FT3).
