I am on HRT and that is not my experience at all. There really isn't any supplementation. Exogenous testosterone supresses the HPTA so the pituitary stops producing LH and FSH which are the hormones that tell the testes to produce testosterone and a few other hormones. HCG, which I also take, is an analog for LH and is used to keep the boys from shrinking and to encourage them to produce whatever they are capable of.
There is primary (the boys aren't producing in the presence of adequate signaling hormones) and secondary (the signals aren't being produced) hypogonadism. We oldsters tend to be secondary, but not always.
When I was on a low dose, 50 mg/week, of testosterone cypionate my total and bioavailable testosterone levels dropped below the previous baseline due to the HTPA supression. 50 mg wasn't enough to get me back to the starting point. As we get older, we require more testosterone to reach the same levels as a younger man. I just turned 60 last month and I inject 100 mg every 3.5 days. Dividing the dose like this keeps me from needing an aromatase inhibitor.
Maintaining a level of bioavailable testosterone in the upper quartile of the normal range promotes longevity and general good health.