Explanation

Parathormone is secreted from parathyroid gland. It maintains ${Ca}^{2 +}$ level in blood and lowers the serum phosphate. Parathormone caused the release of calcium from the bone and raises blood ${Ca}^{2 +}$ level, so parathormone is secreted during decreased blood ${Ca}^{2 +}$ level and maintains normal ${Ca}^{2 +}$ level.