Three general approaches have been used to develop comparable FT4 and FT3 immunoassay methods ... in the case of highly bound hormones, such as thyroxine, the Law of Mass Action can be used ...

[154] "Classic" uptake assays used T3 tracer because the lower T3-TBG binding affinity relative to T4-TBG resulted in a ... Traditionally, the free thyroxine index, sometimes called a "T7" is ...