Chemistry
Thyroid related
T3 or T4 uptake


Topic Completed: 1 March 2011

Revised: 31 December 2018, last major update March 2011

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

PubMed Search: T3 uptake [title]

Nat Pernick, M.D.
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Cite this page: Pernick N. T3 or T4 uptake. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/chemistryT3uptake.html. Accessed September 23rd, 2019.
Definition / general
  • Also called T3 or T4 resin uptake or Thyroid Hormone Binding Ratio (THBR)
  • Measures proteins, not thyroid hormones
  • Relative measurement of unoccupied thyroid hormone binding sites on circulating proteins, including thyroid binding globulin (TBG)
  • Should be reported as a ratio to normal serum
Laboratory
Indications
  • Previously used to calculate free thyroxine index (total T4 x T3 uptake), an estimate of free T4 (now free T4 is ordered directly)
  • Attempts to correct for changes in thyroid binding globulin due to liver disease, protein losing states, pregnancy or various drugs

Method
  • Add labeled T3 (or T4) plus resin (a secondary binder) to serum
  • Amount of labeled T3 bound to resin is measured by chemiluminescence
  • More labeled T3 is bound to resin if more thyroid binding globulins are occupied by thyroid hormone
  • Recommended to be based on the ratio between absorbent counts divided by the total minus absorbent counts, rather than the ratio between absorbent counts and total counts
Interpretation
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