Intermittent parathyroid hormone (iPTH) is the only FDA-approved therapy for bone loss due to conditions such as osteoporosis that increases bone formation by osteoblasts; all other therapies approved for osteoporosis block bone resorption by osteoclasts. The anabolic effects of iPTH are likely due to a combination of multiple mechanisms, including induction of immediate-early genes, increased expression and/or activity of essential osteoblast transcription factors, and downregulation of anti-osteogenic proteins, such as sclerostin. In contrast, continuous administration of PTH induces bone loss primarily due to up-regulation of RANKL expression and inhibition of osteoprotegerin expression.