Which of the following cause osteoporosis?

PTH acts on bones and stimulates the process of bone resorption (dissolution / demineralisation). Hypersecretion of PTH thus causes demineralization of bone leading to osteoporosis. Oestrogens decrease the number and activity of osteoclasts. Oestrogen deficiency increases the number of osteoclasts and decreases the number of osteoblasts resulting in overall bone resorption. Oestrogen deficiency can lead to  excessive bone resorption accompanied by inadequate bone formation, as commonly observed in postmenopausal women.