Historical Estimation of the Free Thyroxine Index (FTI)

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The page below is a sample from the LabCE course Laboratory Assessment of Thyroid Function. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Historical Estimation of the Free Thyroxine Index (FTI)

The FTI is a normalized determination that remains relatively constant in healthy individuals and compensates for abnormal levels of binding proteins.
What did it measure?
  • The FTI is determined by the following calculation: FTI = Total T4 x (%T-Uptake/100)
Why was it ordered?
  • Rough estimate of the unbound fraction of T4
  • Hyperthyroidism causes increased FTI
  • Hypothyroidism causes decreased FTI
What has replaced it?
  • FTI has been replaced with FT4 assays