Abstract
Outer ring deiodination (ORD) of T4 in peripheral tissues produces T3, the predominant bioactive form of thyroid hormone. In contrast, the inactive metabolite rT3 is formed if T4 undergoes inner ring deiodination (IRD). Three enzymes are known to catalyze these reactions (1). The type I deiodinase of liver is probably identical to the kidney enzyme and converts T4 to either T3 or rT3, while it also catalyzes the further IRD of T3 and ORD of rT3 with, in both cases, 3,3’-T2 as the product (1). The type II deiodinase, which has been detected in the brain, pituitary, and brown adipose tissue, is a true ORDase that converts T4 only to T3 and rT3 to 3,3’-T2 (1). The brain contains an additional, type III, deiodinase which specifically catalyzes the IRD of T4 to rT3 and of T3 to 3,3’-T2 (1).
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