Bones play an important role in regulating blood calcium levels, which must be maintained within narrow limits for functions such as muscle contraction and membrane potentials to occur normally. A deficiency of blood calcium is called hypocalcemia, and an excess of blood calcium is called hypercalcemia. Bone is the major storage site for calcium in the body, and movement of calcium into and out of bone helps to determine blood calcium levels. Calcium moves into bone as osteoblasts build new bone and out of bone as osteoclasts break down bone.

The hormones parathyroid hormone (PTH) and calcitonin help regulate blood calcium levels. Calcitonin, secreted by the thyroid gland inhibits osteoclasts and stimulates osteoblasts, thus decreasing blood calcium levels. Parathyroid hormone is secreted by the parathyroid glands, which are on the posterior side of the thyroid. Parathyroid hormone inhibits osteoblasts, stimulates osteoclasts, reduces the output of calcium by the kidneys and promotes the absorption of calcium by the small intestines, thus increasing blood calcium levels.