Also termed tetraiodothyronine, nicknamed is thyroxine. Secreted by the thyroid along with T3 (triiodothyronine...also called thyroxine), this thyroxine is mostly conjugated in the blood by TBG (thyroxine-binding globulin), whereas the more active T3 tends to float free. T4 is broken down to T3 and forms a stable feeder reserve, preventing rapid shifts in its more labile relative
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