The loss of your gonadotropins is the main cause of the often dramatic loss of semen volume during TRT. Particularly FSH. Most of your semen in is produced in the seminal vesicles being 80% and a smaller amount is produced by the prostate. Essentially TRT interrupts the whole reproductive system in the male and that in itself can cause sexual dysfunctions for some men. I think when the stimulation from your gonadotropins ceases, it is possible that the seminal vesicles atrophy and it may take some time for them to return to normal.
It could also be possible that this interruption or atrophy of these glands if caused for long enough may alter the function of these glands permanently for some males. Shutting any gland down in the body for a lengthy period of time is not something to be taken lightly in medicine. Once we go through puberty all the glands and organs associated with sexual function are meant to work consistently until we die, not have periods of shut down and then restart. I do not think we fully understand what may happen to them during these unnatural periods we create.
It could also be that your FSH levels have not returned back to where they were before TRT.
I would also consult with your urologist as to if some form of retrograde ejaculation may be occurring due to issues caused from the shut down or from an entirely different cause.
As a side note, as a young man I had a bout of Non-specific urethritis (NSU). After this infection, my ejaculate levels were never quite the same and I experienced complete retrograde ejaculation at times. So I would also check for any form of low grade infection in the area.
