I kind of hate this one. Two things happen when testosterone output falls. The first is that an MD prescribles testosterone gel. The second is that after a protracted period of time you will need a series of shots of Beta Chorionic Gonadotropin to tell your balls to get their act together.
The main problem is that in most cases testosterone replacement is not done correctly. The maintenance of sexual function etc. is not testosterone based but is in fact DHT based. The body is very sensitive and will quickly down regulate with conventional testosterone. It takes a far higher dosage of DHT because the DHT is a byproduct of a reduction by the enzyme 5-alpha reductase. Unless the dosage is very high the body does not recognize DHT supplementation. There is a DHT gel, but, based on old information most MD's won't use it. You may need the gonadotropin shots weekly for about 4 months. In the future, the best rule if they are going to do this is to teach you to give yourself injections. Depo Testosterone (Testosterone Cypionate) is a long lasting ester and is usually taken either once or twice per week. You would then spend three weeks on testosterone and the fourth on Gonadotropin.
That stops the testicular atrophy. Again 95% of the MD's doing male HRT do not do this one well. This is what the best of them in fact do. . . .