As seen from the above chart that I lifted from Wikipedia, testosterone is released from Leydig cells in response to LH (luteinising hormone) which is in turn released from the pituitary in response to GnRH( gonadotropin releasing hormone)
You can also see that testosterone itself will inhibit release GnRH release to complete the feedback loop
Without the negative feedback the signal will get amplified endlessly till all men turn into the Hulk it get testosterone dependent cancers and heart disease.
Obviously, that doesn’t happen. And most of us manage to live pretty good lives till we reach of age.
This is because the GnRH release is pulsatile. It is not continuous. There’s no other way to run this system. A continuous release will shut the system down.
When one uses external sex hormones, testosterone for example, the system will shut down or atleast have to compensate. That’s not always a good thing.
In some cases, where the system is already defunct, due to disease or old age, the external testosterone supply if used sensibly, will help improving health and quality of life, simultaneously. This is only because the damage caused by a defunct HPG system exceeds what can be caused by a non pulsatile testosterone supply.
Synthetic testosterone will never be able to match the dynamic pulsatile nature of of natural endogenously produced testosterone.
Use of external testosterone, therefore, needs to be a last resort, in order to improve health and longevity. It shouldn’t be for frivolous reasons like aesthetics. Atleast, not without knowing the possible knock on effects.
The body is used to a pulsatile pattern of testosterone release. It’s tissues are used to that rise and fall. It is not used to a constant trickle, or worse a big flood.
ADDENDUM.
Testosterone levels falling precipitously with age, is not a natural phenomenon. Most aboriginal males that survive into old age have higher T levels than 20 year olds in modern society.
If you manage to live a life that doesn’t overwhelm the recovery capacity of your body.
Basically, clean , minimalist living, you might not need testosterone replacement.