The purpose of this research study is to evaluate antiresorptive therapy with denosumab (Prolia) for prevention of bone loss after stopping teriparatide (TPTD) in premenopausal women with idiopathic osteoporosis. Premenopausal women who have received TPTD in the FDA Orphan Diseases Program-funded trial, "A Phase 2 Study of Teriparatide for the Treatment of Idiopathic Osteoporosis in Premenopausal Women" (NCT01440803) may be eligible to participate in the current study, a 36-month open-label pilot study of denosumab (Prolia®, 60mg subcutaneous (SC) every 6 months). The goals of the study are to estimate the effects of denosumab on central and peripheral, as well as trabecular and cortical, bone mass and microstructure and to obtain preliminary data to inform the design of a future randomized study. This study presents the first opportunity to study the effects of denosumab after TPTD in this unique and severely affected group of young women. Funding Source: FDA Office of Orphan Products Development (OOPD).