Modeling capacity limited PD response

Hi: I'm trying to develop a pop PK/PD model for a drug X that induces the secretion of the hormone Y, we are using IV infusion of X to reach the steady state, hormone Y secretion is stimulated initially by X infusion, however after 30 minutes no more Y hormone is secreted and Y levels start to decrease, which might be attributed to saturating the excretion of Y by the gland (No more hormone left in the gland). Can any one share with me a code or a modeling idea to model a scenario like that? or at least refer some article to read. Abuasal