How is free T4 measured, and how is the free T4 index (FTI) calculated?

Updated: Mar 03, 2021
  • Author: Philip R Orlander, MD, FACP; Chief Editor: George T Griffing, MD  more...
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Free T4 can be directly measured via equilibrium dialysis. Results are independent of binding protein concentrations. However, this test is more costly and generally takes longer to return. Free thyroid hormone levels can be estimated by calculating the percentage of available thyroid hormone-binding sites (triiodothyronine [T3] resin uptake, or thyroid hormone binding ratio [THBR]) or by measuring the TBG concentration. A free T4 index (FTI) serves as a surrogate of the free hormone level. The FTI is the product of T3 resin uptake and total T4 levels.

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