@dezzie
You may be interested in this clinical study:
https://onlinelibrary.wiley.com/doi/10.1359/jbmr.2001.16.5.925
52 postmenopausal women with osteoporosis on established HRT (longer than 2yrs) were randomized to add PTH(1-34) or to continue HRT alone for 3 years. BMD and bone turnover markers were monitored every 6mo, and vertebral fracture was evaluated as well. Subjects were reassessed 1yr after PTH discontinuation while HRT was continued.

This is a small study, though, and it does not address whether PTH1-34 + HRT is superior to PTH1-34 alone.