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Total and Free Forms of T3 and T4

More than 99% of T4 and T3 in circulation is 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.
It is for this reason that most current analytical assays for thyroid hormones are designed to measure the free forms, free T4 (FT4) and free T3 (FT3).