A) Serum B-CTX, a marker of bone resorption. B) Serum P1NP, a marker of bone formation. C) Serum calcium. D) Serum phosphorus. Denosumab was initiated at Day 0 and discontinued at day 210, indicated by the arrow. After discontinuation of denosumab there was a marked increase in BTMs, especially the resorption marker CTX that was associated with marked hypercalcemia in the presence of a suppressed serum PTH, and 1,25 (OH)2 vitamin D. Normal ranges for calcium and phosphorus are indicated by boxes. Normal ranges for CTX and P1NP in children are not well defined.