In my case it was caused by an adenoma on one of my parathyroid glands which increased blood calcium and caused a whole slew of problems. Have your bloodwork done and question your doctor about any levels that are above or below normal.
One treatment that I have found works great is PEMF and also works on many other problems My friend borrowed my home made unit and one treatment fixed his problem. If anyone wants a schematic of the unit I made I will post it free. There is published information about PEMF on NIH web site