Chemistry
Thyroid related
T3 or T4 uptake

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: T3 uptake [title]
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