Chemistry
Thyroid related
Triiodothyronine (total T3)

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: Triiodothyronine [title]
Cite this page: Triiodothyronine (total T3). PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/chemistryT3.html. Accessed December 6th, 2016.
Pathophysiology
  • Usual daily production is 50 nmol/day
  • 80% from deiodination of T4 in nonthyroid tissue, 20% arises in thyroid gland from coupling of mono-iodotyrosine plus di-iodotyrosine (i.e. tyrosine plus 3 iodides) and deiodination of T4
  • This conversion from T4 is blocked by glucocorticoids, amiodarone and propranolol
  • Metabolized by conjugation with sulfate and conversion to triiodothyroacetic acid (triac)
  • More T3 is free than T4 (.3% vs. .03%)
    • Free T3 is active, protein bound form is inactive and does not enter cells
  • Values usually parallel T4, but not are usually measured
  • With nonthyroidal illness (myocardial infarction), T3 levels may be low, although TSH normal
  • Low T3 syndrome is a strong predictor of death in cardiac patients (Circulation 2003;107:708)
Laboratory
Indications
  • To diagnose T3 thyrotoxicosis (5% of hyperthyroidism, more common with iodine deficiency), particularly if T4 is normal or mildly elevated

Methodology
  • Competitive immunoassay using enzymes, fluorescence or chemiluminescence

Reference range
  • Serum: 60 - 160 μg/dL (0.9 to 2.5 nmol/L), higher upper limit in pregnancy

Limitations
  • Difficult to interpret if receiving thyroid preparations containing T3

Conversion factor
Interpretation
  • May be normal in hyperthyroidism if nonthyroidal illness or taking drugs (amiodarone or propranolol) that reduce T4 to T3 conversion
  • T3 gives clinicians a sense of the severity and recovery from hyperthyroidism, because it increases more and decreases faster than T4
  • Note: T3 normal in 15-30% of hypothyroid cases, so not diagnostic
    • Depressed only if severely hypothyroid (T4 < 2 μg/dL, 32 nmol/L)