ABSTRACT
The biologic activity of thyroid hormones is dependent on location of the iodine atoms (Ch. 39). Target tissue deiodination of the T4 outer ring (5’-deiodination) is a “step up,” producing T3 (Part E), which is 3-8 times more potent in target cells than T4. Deiodination of the T4 inner ring is a “step down,” producing rT3, which is metabolically inert. Further deiodination of T3 and/or rT3 results in iodothyronine derivatives (i.e., T2 & T1) with no metabolic activity, then ultimately thyronine (T0). Types, specificities and locations of the various deiodinases are discussed in Ch. 39.
