When serum calcium level is low, PTH is up regulated.

当血清钙水平较低时，PTH升高。

Bone remodelling as a whole is highly dependent on serum calcium levels, which, in turn, are kept in the normal range by a balance between parathyroid hormone, or PTH, calcitonin and vitamin D.

整个骨重建进程高度依赖于血清中钙的含量，而将血清中钙的含量维持在正常值范围内是由甲状旁腺激素、降血钙素与维生素D间的制约平衡所控制的。

Hypocalcemia refers to abnormally low levels of calcium in the blood and is generally defined as serum calcium level lower than 2.1 millimoles per liter.

低钙血症指血液中钙含量异常低，通常定义为血清钙含量低于2。1mmol/L。