I just take DHEA pills. A decent brand helps me - but I do fluctuate a lot. Even on my E-patch, I still can vary.
my testosterone was low several years ago--i got it up with a T-cream---but for some reason my body can't maintain it, and it can't maintain the DHEA levels, either
I had AI and I too suffered from very low Vit D and iron.
Also you say you suspected you always had low testosterone. Please do not confuse (as many do) low testosterone with low libido. They are not the same.
I had low D and low iron that I remember.