It is called PTH, or parathyroid hormone. It is part of the endocrine glands and to look this up i'd suggest a physiology book
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pth
Calcitonin
The parathyroid glands are responsible for monitoring the calcium level in the blood. When the calcium level is too low, the parathyroids produce parathyroid hormone (PTH) to increase the calcium...
In short, the endocrine system.