hypercalcemia doesnt cause osteopenia and pathological fractures. it causes confusion/dehydration/mental status change and, in rare cases, pancreatitis. plus, it is hardly possible to drink enough milk to give yourself hypercalcemia. now perhaps if you take too many vitamin D pills (like waaaay too many) or too much tums (like waaaaaay too many) you can get hypercalcemia.
about the osteopenia and the resulting pathological fractures: this is due as mentioned above to hyperparathyrodism (most likely 2ndary...but eh up for debate). your body senses low [Ca] and as a result increases production of PTH which activates osteoclasts (actually works thru osteoblasts or something like that to stimulate osteoclasts via RANK and RANK-L receptors) which increase bone resorption ie take the Ca and P from your bones to increase the serum Ca. And what happens when you don't have as much Ca and P in your bones? Osteopenia or literally weak bones.
about the pathological fractures. actually, these hardly have to do with hyperparathyroidism unless it's in association with squamous cell carcinoma of the lungs as a paraneoplastic phenom secreting PTH-rp. Rather, it's due to mets to the bone (exclude the paraneoplastic syndromes) that cause weakening of bone to the point that just walking down stairs causes fractures. As expected, it's associated with hypercalcemia because to weaken bone you have to release Ca and P into serum.
