Osteoporosis was not diagnosed in Terri's case. Lack of periods does not necessarily mean that you have this disease. Many can attest to this fact, including top female athletes. The stress from a bad marriage alone can cause havoc with your periods.
Terri must have been in excellent shape on the whole just to live through the last 13 years with no physical therapy and no treatment for infections (on Michael's orders). It takes a strong healthy body to fight off infections.