@s2208 Correct. Your body's hormones are carefully regulated in what is called the HPTA. (hypothalamic–pituitary-testicular axis)

There are feedback loops. If you add more testosterone, then that feeds back to the hypothalamus which then shuts down the body's natural testosterone production. (I am simplifying.)

Very problematic is that once this is shutdown, getting it kickstarted again is a bitch. Typically, body builders, who have only used testosterone on a 6 or 12 week cycle, will do what is called PCT (post cycle therapy) using a variety of drugs that are usually used to treat breast cancer to try to get the normal production started again.

But generally speaking, once someone is using exogenous testosterone, it should be considered a life-long necessary intervention to preserve ongoing quality of life.

Also, while using testosterone, one can find some unexpected problems. Pretend, for example, that one is a little overweight. Fat cells produce aromatase, which very helpfully converts testosterone into estrogen. This might give some very unexpected results from testosterone supplementation (eg getting fatter and growing mammary glands), so people taking testosterone often also employ what are called aromatase inhibitors.

This gets messy.

In general, if someone is considering exogenous testosterone, I would instead recommend losing weight, lifting progressively heavier things and getting plenty of sleep. In this way, the HPT axis up-regulates naturally to accommodate the body's demands.

It is sometimes that case that even if not overweight, aromatase is converting testosterone to estrogen. A good intervention for this is a natural supplement such as DIM (made from broccoli) that reduces this phenomenon.