Prolactin alone determines how SENSITIVE the HPTA is.
The HIGHER your PROLACTIN, the more SENSITIVE the HPTA! If you have a SENSITIVE HPTA, it will DETECT ANY amount of exogenous steroids and will respond by SHUTTING DOWN.
On the contrary, if you have SUBNORMAL prolactin(very low), the HPTA is so DESENSITIZED, it CAN'T EVEN DETECT THE EXOGENOUS STEROIDS IN THE BODY!
Decreasing Proalctin while on cycle is the wise decision.