Relation between T4 and T3

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Chapter: Essential pharmacology : Thyroid Hormones And Thyroid Inhibitors

Thyroid secretes more T4 than T3, but in iodine deficient state this difference is reduced.


RELATION BETWEEN T4 AND T3

 

·    Thyroid secretes more T4 than T3, but in iodine deficient state this difference is reduced.

 

·       T4 is the major circulating hormone because it is 15 times more tightly bound to plasma proteins.

 

·       T3 is 5 times more potent than T4 and acts faster. Peak effect of T3 comes in 1–2 days while that of T4 takes 6–8 days.

 

·        T3 is more avidly bound to the nuclear receptor than T4 and the T4receptor complex is unable to activate/derepress gene transcription.

 

·        About 1/3 of T4 is converted to T3 in the thyroid cells, liver and kidney by D1 type of 5’deiodinase (5’DI) and released into circulation. In addition, T3 is generated within the target cells (skeletal muscle, heart, brain, pituitary) by another type (D2) of 5’DI.

 

Thus, it may be concluded that T3 is the active hormone, while T4 is mainly a transport form; functions as a prohormone of T3. However, it may directly produce some nongenomic actions.

 

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