The blood calcium level is lowered by the deficiency of:

Parathormone ($\text{PTH}$), secreted by the parathyroid glands, is a hypercalcemic hormone—it increases blood ${\text{Ca}}^{2+}$ levels by stimulating bone resorption and reabsorption in the kidneys. Therefore, a deficiency in $\text{PTH}$ leads to low blood calcium (hypocalcemia). Calcitonin, conversely, lowers blood calcium.