I thought it was very important to mention this study as all of us are on HRT and many of us also take Vitamin D3 due to low concentrations of 25-hydroxyvitamin D (hormonally inactive), the precursor to calcitriol, the hormonally active metabolite of Vitamin D.

https://www.ncbi.nlm.nih.gov/pubmed/9405729

"Serum concentrations of calcitriol (1, 25(OH)2D) increased, however, only in the HRT group (23.7%, P < 0. 05), and remained unchanged in other groups."

"Our results confirm the positive effect of 1 year of HRT on serum calcitriol. Vitamin D3 supplementation increased 25OHD concentrations, but did not affect calcitriol concentrations even though the initial levels were low. Interestingly, the combination of HRT and vitamin D3 did not increase serum calcitriol concentrations as much as HRT alone."

Summary: HRT alone increased the hormonally active metabolite of Vitamin D. HRT + Vitamin D3 supplementation did so, to a lesser extent and vitamin D3 supplementation alone had no effect.

Food for thought...

Sex hormones are also responsible for bone density so as long as you have sex hormones (either estrogen or testosterone) in your body, it is not a big concern about developing osteoporosis. It is important however, for those people who are only on anti-androgen therapy to consider taking VitD supplements because of the lack of sufficient amount of testosterone. (Also post-op)