I'm not misunderstandings, you are conflating several different things: TRT, testosterone as a contraceptive, and testosterone as a body building steroid. TRT

I'm not misunderstandings, you are conflating several different things: TRT, testosterone as a contraceptive, and testosterone as a body building steroid. TRT is a very low dose of testosterone, used to supplement the body when it doesn't produce enough. It is reversible as you say, and has a negative effect on fertility but not enough to be reliably effective as a contraceptive at TRT dosages. Testosterone as a contraceptive or steroid is a far higher dose. This is the one that has permanent effects.