Hi @gravity3 i transitioned to hrt without testosterone and admittedly I didn't do much reading on it. Here is a ytube video I think its pretty good by obgyn Dr. Hardwick-Smith. In this video, she explained the three main hormones in hrt - estrogen, progestin and testosterone regarding their forms, dosing, route of administration and blood levels to be monitored. In addition, she mentioned two other important hormones - thyroid hormone and cortisol; hope this is helpful:

There is another link I'll post later regarding the risks of hrt (mainly estrogen) in your other thread "is hrt good option after evenity", very informative.