kittygalore, most bone loss is caused by hormone loss. Our endocrine system consists of the glands that produce hormones. Some people have bone loss from autoimmune disease. Rheumatology involves the study of autoimmunity. You can find endocrinologists and rheumatologists who have a specialty in bone loss. Both specialties have other complex diseases to handle. Some endocrinologist are focused on diabetes and thyroid disease. Some rheumatologist are focused SLE, Mast Cell disease, MS, etc.
You are more likely to find an endocrinologist with an osteoporosis focus. If you find an endocrinologist with an osteoporosis only practice, you'd probably receive the most experienced and up to date treatment.