Thyroid related
Free thyroxine index

Topic Completed: 1 March 2011

Revised: 31 December 2018, last major update March 2011

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

PubMed Search: Free thyroxine index [title]

Nat Pernick, M.D.
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Cite this page: Pernick N. Free thyroxine index. website. Accessed January 21st, 2020.
Definition / general
  • Also called T7 index (since measurement involves T3 and T4)
  • 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)

  • FTI = total T4 x T3 uptake / mean T3 uptake of reference laboratory, or total T4 x serum binding correction factor
  • 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
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