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T4 to T3

Meaning

T4 to T3 refers to the critical peripheral conversion process in which the prohormone thyroxine (T4), the primary secretory product of the thyroid gland, is metabolically transformed into the biologically active hormone triiodothyronine (T3). T4 is considered the reservoir, while T3 is the three to five times more potent hormone that acts on nuclear receptors to regulate cellular metabolism across virtually all body tissues. This conversion is a fundamental step in thyroid hormone action, ensuring that tissues receive the necessary active hormone to regulate energy expenditure, body temperature, and protein synthesis.