Relation between T4 and T3

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.