system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does testosterone production vary much between individuals?
gpt
It’s a hormone. Hormones are signaling mechanisms. Basically they are designed to vary.  

This is like asking why certain pianos emit different notes. It’s because the nature of a piano is that it has many notes. The whole function of it is to provide that variation.  

Like a piano with 88 keys, a hormone is a signaling mechanism with many varying levels of activation.

In this way, testosterone level is analogous to:  

* the speed of a car
* the height of the tides
* the brightness of a variable switch light
* the setting on a thermostat

And given that context, the reason testosterone levels vary between individuals is that those individuals have provided different amounts of inputs into the processes that determine testosterone levels.  

For example someone who has just done a bunch of sprinting will have higher testosterone than someone in the same overall health who’s been sitting still. Someone whose muscles are bigger will have higher testosterone than they would have if, all other things being equal, their muscles were smaller. Just in the same way that a car where the gas pedal has been floored for the last ten seconds will have a higher speed than a car where the brake has been pressed for the last ten seconds.