Forearm Bone Mineral Density in Familial Hypocalciuric Hypercalcemia and Primary Hyperparathyroidism: A Comparative Study

Studies have shown that cancellous bone is relatively preserved in primary hyperparathyroidism (PHPT), whereas bone loss is seen in cortical bone. Familial hypocalciuric hypercalcemia (FHH) patients seem to preserve bone mineral in spite of hypercalcemia and often elevated plasma parathyroid hormone (PTH). The objective of this study was to compare total and regional forearm bone mineral density (BMD) in patients with PHPT and FHH and to examine if differences can be used to separate the two disorders. We included 63 FHH, and 121 PHPT patients in a cross-sectional study. We performed dual-energy X-ray absorptiometry scans of the forearm, hip and lumbar spine and measured a number of biochemical variables. PTH patients had significantly lower Z-scores in all parts of the forearm compared to FHH. This was also the case after adjustment for body mass index. When stratifying for age, gender and PTH, T-scores were still significantly lower in PHPT patients than in FHH patients at the total, the mid and the ultradistal forearm, but not at the proximal 1/3 forearm. In a multiple regression analysis BMD Z-score was lower in PHPT compared to FHH at the total forearm, the mid forearm and the ultradistal forearm but not the proximal forearm when adjusting for biochemical variables including PTH, 1,25(OH)₂D and Ca²⁺. These observations support that inactivating mutations in the CASR gene in bone cells in FHH may protect against forearm bone loss. Differences between the two groups in total or regional forearm BMD were inferior to the calcium/creatinine clearance ratio as a diagnostic tool to separate FHH from PHPT.