The page below is a sample from the LabCE course Laboratory Assessment of Thyroid Function. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Assembly of the T3 Molecule

The hormone triiodothyronine (T3) is primarily derived from T4 through the removal of an iodine residue by special enzymes called deiodinases. This T4 to T3 conversion process occurs in the:
  • Liver (~60%)
  • Intestines (~20%)
  • Circulation (~20%)*
While T4 is more abundant than T3 in an approximate ratio of 9:1, T3 is far more metabolically active than T4.
* The T3 produced in circulation is influenced by the adrenal hormones (eg, cortisol) and forms an inactive configuration of T3 called reverse T3, in which one of the iodine residues is in a mirrored position on the T3 molecule. This will be discussed further in greater detail in an upcoming page on reverse T3 analysis.