Taking testosterone switches off your own sperm production
Testosterone from outside suppresses the pituitary hormones that drive the testis, so it stops making its own. Guidelines advise against it for men planning to conceive.
What the study found
Testosterone from outside shuts down the testicle's own supply
Testosterone given from outside suppresses the pituitary hormones driving the testis, so the testis stops making its own. Sperm production needs an internal concentration far above anything in the blood, and that collapses. Guidelines advise against it in men planning to conceive.
The caveat · Expert review, not a systematic one
The mechanism account is an expert review rather than a systematic synthesis, and it describes recovery as highly variable rather than putting numbers on it. Its suggestion that newer short-acting oral and nasal formulations partly spare sperm production is preliminary and rests on no fertility outcomes. The guideline is expert synthesis, and by its own grading many of its recommendations sit on low-quality evidence.