SERT is significantly decreased during chronic SSRI treatment (on the order of 80-90%). This is what most doctors won't tell you. YES, it's a Selective Serotonin Re-uptake Inhibitor, but that's not the full story becuase it is also decreasing SERT instead of just blocking its action. This is from Progesterone - indirectly. Progesterone blocks Estradiol's protective effects on SERT in the POA. This area is critical for sexual function. This is why Estradiol treatments work for PSSD for a while, and how high T boosts also help because of aromatization. This is well known stuff. Seen in sex reassignments for decades.
Effects of chronic antidepressant treatments on serotonin transporter function, density, and mRNA level. - PubMed - NCBI
It also has nothing to do with mRNA expression of SERT (From SSRI Treatment), but rather appears to be internalization of the receptors, or a block from them making it to the synapse. This coincides with the info above. Estradiol PROTECTS SERT.
The Serotonin Hypothesis for Anxiety and Depression is woefully outdated, and some doctors are even starting to admit that.
I'm quite confident that Serotonin remains high in PSSD cases. This leads to ROS formation and scarring in the penis. It also explains why most people don't return to the level of anxiety or depression that they had before the drug. If Serotonin was the cause of the mental illness, having it not work after the SSRI would make the mental illness worse. That's not what we see at all. Additionally, Serotonin inhibits Dopaminergic transmission. With an abundance of 5HT, dopamine is inhibited. This is why Dopamine agonists work temporarily for PSSD - especially in cases of low libido/anhedonia.
The Mechanisms of Post-SSRI Sexual Dysfunction (PSSD) (Ghost 2016)