PTH and Osteopenia: Indeed, a spectrum of skeletal abnormalities seen in CRF are as follows: Osteitis fibrosa cystica, characterized by increased osteoclast and osteoblasts activity, peritrabecular fibrosis, and high serum level of PTH (usually higher than 350–500 pg/mL); adynamic bone disease, characterized by low or absent bone formation due to low osteoblast-bone formation and osteoclast-bone resorbing activities accompanied by low serum levels of PTH; and osteomalacia, osteopenia or osteoporosis, and mixed renal osteodystrophy.