I'm not sure it matters all that much. A number of years ago, my gynecologist referred me to an endocrinologist as around the time my osteopenia was approaching osteoporosis. This particular endocrinologist heads up the Osteoporosis Clinic at a major hospital in my city. I am also aware of rheumatologists who also take a strong interest in and include treating patients with this problem as part of their practice. Years ago, I volunteered at the Osteoporosis Society and those who came to lecture were either rheumatologists or endocrinologists.
I still see that same endocrinologist and finally started on Prolia after refusing to do anything for over four years (while my bones got progressively worse). My fourth injection is in November. My bone density has definitely improved.