The parathyroid gland exerts primary control over balancing calcium levels within the bloodstream, which influences bone density. The actual regulation process has a very delicate balance because blood calcium levels in the human body range from 9.0 to 10.1, only leaving room for error of 1.1. [An entire list of human glands]
The parathyroid gland is often confused with the thyroid gland; however, these two glands hold very different roles in the human bodies. The thyroid gland only regulates metabolism and not blood calcium levels, while the parathyroid gland regulates just the opposite (regulates blood calcium levels and not metabolism).