Chemistry
Thyroid related
Free thyroxine index

Author: Nat Pernick, M.D. (see Authors page)

Revised: 23 February 2016, last major update March 2011

Copyright: (c) 2002-2016, PathologyOutlines.com, Inc.

PubMed Search: Free thyroxine index [title]
Cite this page: Free thyroxine index. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/chemistryFTI.html. Accessed December 2nd, 2016.
Definition / General
  • Also called T7 index (since measurement involves T3 and T4)
Laboratory
Indications
  • Quantitate pituitary resistance to thyroid hormone (J Clin Endocrinol Metab 2000;85:2299)
  • Previously the primary method to estimate free T4 (now measure free T4 directly) or screen for thyroid dysfunction (now use TSH)

Calculation
  • FTI = total T4 x T3 uptake / mean T3 uptake of reference laboratory, or total T4 x serum binding correction factor
Interpretation
  • Usually increased during hyperthyroidism, decreased during hypothyroidism
  • Attempts to correct for changes in total thyroxine caused by changes in thyroid binding globulin
  • Limitations:
    • Nonspecific, low results also associated with non-thyroidal illness
    • Free T4 is more accurate in women taking oral contraceptives (Clin Nucl Med 1981;6:168) and in general