Subjects will have had a bone biopsy to definitively determine low bone turnover. Subjects and the treating doctor decide that treatment with rhPTH (recombinant human parathyroid hormone) is optimal before admission into the study. During the study, subjects will have several tests that will clarify how rhPTH is affecting calcium metabolism and bone properties. All of the studies will be performed in the University of Chicago General Clinical Research Center (GCRC).