About thyroxinethyroid hormone, replacement or supplemental therapy in hypothyroidism. mechanism of action of thyroxinethyroid hormones- thyroxine (t4) and triiodothyronine (t3) exert the therapeutic action by regulating protein synthesis. t4 and t3 enter the cells and combine with nuclear receptor. t3 receptor complex binds to specific dna sequence called "thyroid hormone response elements" present in the regulatory region of specific genes and causes derepression of gene transcription or direct activation of gene transcription. thus causes expression of predetermined genetically coded pattern of protein synthesis. it causes induction of adenylyl cyclase and proliferation of ?-adrenoceptors and a better coupling of these two. it regulate growth, development, improves carbohydrate utilization and bmr.