Why is osteoporosis a common complication of CKD?

Kidneys are unable to convert vitamin D to its active form - due to reduced vitamin D - extra parathyroid hormone is released in the blood to move calcium from inside the bones into the blood When vitamin D level is low, the absorption of calcium in the intestines becomes less, which then causes the level of calcium in the blood to go down. As a consequence the parathyroid glands become more active and produce more PTH that causes calcium to come out of the bones, therefore weakening the bones.