Estrogen plays a central role in control of bone strength. In a healthy individual, bone mass is maintained by balanced activity of bone forming osteoblasts and bone resorbing osteoclasts. Both these cell types are reported to respond to estrogen. Estrogen promotes the activity of osteoblast and induces osteoclast apoptosis. After menopause the level of this hormone declines. Hence, in an ageing female osteoporosis occurs. Parathormone promotes mobilisation of calcium from bone into blood. Excessive activity of parathormone causes demineralisation of bone leading to osteoporosis.