A group of Italian researchers gave finasteride to rats and noticed that the number of androgen receptors in their brains went up. Moreover, the effects persisted long after the drug had been discontinued.…[T]hey then called in men with PFS, took skin from the penis and found that the density of androgen receptors in men with PFS was about twice that of those without. Now, remember the idea of the testosterone bell curve and damping effects (little testosterone, little growth, more testosterone, more growth, even more testosterone, reduced growth)? I think this is what we are seeing here. With a greater concentration of receptors, the organ becomes more sensitive to testosterone and at a certain point, paradoxically, that sensitivity may shut down.
—The Virility Paradox: The Vast Influence of Testosterone on Our Bodies, Minds, and the World We Live In: February 2018