RT3 can be considered the “brake” of the system. This is pushed when T4 or T3 is high.  It is also raised by cortisol, and therefore can be a subjective measure of stress physiology. There are other factors that also influence RT3, such as bile acids which act as signal molecules (hormones) regarding the peripheral conversion of T4 to T3.

RT3 can be considered the “brake” of the system. This is decreased when T4 or T3 is low.  It is also lowered by less stress and better sleep, and therefore can be a subjective measure of stress physiology. There are other factors that also influence RT3, such as bile acids which act as signal molecules (hormones) regarding the peripheral conversion of T4 to T3.