... (TSH), which is released from the pituitary gland. This pathway is regulated via a closed-loop feedback process: ... -life of T3, which is only 2.5 days. This compares with the half-life of T4, which ... is about 6.5 days. Application: It affects almost every physiological process in ... individual. T3 acts on the majority of tissues within the body, with a few exceptions including the ...