The purpose of this protocol is to add on additional exploratory studies to investigate changes in bone quality parameters with PTH(1-84) treatment of hypoparathyroidism. In addition to the biochemical hallmarks of hypoPT, it has been found that the microscopic structure of the bone, as well as the bone remodeling system, are markedly abnormal in this disease. How these abnormalities may be corrected with PTH(1-84) administration are not fully understood. The studies outlined in this add-on protocol are designed to shed light on the mechanistic ways that PTH(1-84) replacement may restore normal bone metabolism. These mechanistic studies are beyond the scope of the parent NPS study, which was designed to assess the safety and efficacy of PTH(1-84) in hypoPT treatment. Subjects who are participating in the NPS' REPLACE, RELAY, and RACE Studies and the HEXT Study at Columbia University will be invited to participate in this add-on protocol, which will involve a separate IRB-approved informed consent. Study procedures: 1. High Resolution Peripheral Quantitative Computed Tomography (HRpQCT; XtremeCT, Scanco): Done at the same visit as DXA. In the REPLACE study twice, in RELAY once (or not at all if done within the last 6 months), in RACE twice, and in HEXT three times. 2. Osteolineage: At Baseline/Randomization/Visit One and at 4, 8, 12, 24, and 52 weeks of treatment in the REPLACE, RELAY, or RACE Study, if applicable, or at baseline and each 6-months visit in the HEXT Study, blood test for circulating osteogenic cells (10 cc) will be performed 3. Sclerostin: At Baseline/Randomization/Visit One and at 4, 8, 12, 24, and 52 weeks of treatment in the REPLACE, RELAY, or RACE Study, if applicable, or at baseline and each 6-months visit in the HEXT Study, blood test for sclerostin (5cc) will be performed Funding Source - FDA OOPD