After seeing three periodontists, and being honest to them, they do not know the cause of my bone loss. at 39 y/o, how do i find the root cause?

Treat perio disease: Many variables, what was your dental status prior to going to three periodontists? Did you see your dentist on a prescribed basis? Were you told that periodontal disease is evident? Were you referred to a periodontist? Do you floss and brush properly? Are you diabetic, a smoker or suffer from any physical ailments? These and more are questions to be pondered- more important treat the problem now.