PTH acts on the osteoblast and regulates bone resorption and formation. While a continuous increase in circulating PTH results in bone loss due to predominantly enhanced bone resorption, intermittent PTH treatment strongly stimulates bone formation and thereby increase bone mass. PTH has divergent effects on the osteoblast, regulating its proliferation, differentiation and survival. Although some evidence suggest an involvement of transcription factors such as fos family and local factors including IGF-1, mechanisms by which PTH stimulates bone formation remain largely unknown.