"The majority of
low testosterone patients are secondary, not primary, which means low LH and FSH production are what's most common.
The most common reasons of low
testosterone other than age:
1. Prior
anabolic steroid use
2. Prior or existing rec drug use
3. Prior or existing pain med use
4. Prior or existing antidepressant use
5. Prior or existing statin or BP med use
6. Heavy and long term exposure to corticosteroids
7. Heavy/excessive alcohol consumption
8. Long terms consumption of petrochemicals
9. Long term use of prescription sleep meds
10. Poor/unhealthy lifestyle - lack of sleep, bad diet, infrequent eating, bad food choices, sedentary, etc.
Any of these things can cause a drop in testosterone, 1-9 severely reducing the production of LH and FSH. When you say yes to any two of those you're going to be in bad shape but often it only takes one, especially 1-4.
And of course there are young (under 30) guys with genetic conditions that we all hear about but that's a different story. However, some of them fall in the same categories as over 30 due to the above list. In all cases as this, there isn't some marker that we can look at like a tumor and say "that's what's causing your low testosterone without a doubt." But we do know those items have that affect and that treatment corrects it (as long as you're on treatment. Which begs another point, the old phrase "treatment is for life" implying you cannot stop. This is a bad phrase and simply isn't true. If you start TRT it should be because you need it. If you need it and stop, your natural production will start back up again but it will only take you to where you were before. You'll still have low levels but there isn't anything beyond that that happens should you have to stop."
Good luck with what you choose.