A drop in blood calcium levels stimulates the secretion of __________.

The Parathyroid Hormone (PTH) can be defined as a small peptide hormone that is secreted by the Parathyroid Glands.
The Parathyroid glands can be defined as small and round structures (usually four in number) which remain interspersed in the posterior region of the thyroid gland, but remain separated from the thyroid gland by a layer of connective tissue.
The parathyroid gland is composed of specialised epithelial cells known as Chief Cells which are responsible for the secretion of the Parathyroid Hormome (PTH) in response to low level of calcium in the blood.
Functions of the PTH hormone are:

The PTH is responsible for activating the osteoclast cells and inhibiting the osteoblast cells in the bone.
The osteoclast cells are responsible for secreting enzymes which stimulate the degradation of bone cells and promotes the release of calcium from the bones into the tissue fluid.
The osteoblast cells are responsible for storing calcium in the bones by promoting the deposition of calcium in the bone cells.
The PTH is also responsible for promoting calcium re-absorption from the urinary filtrate in the tubules of the kidney.
It also induces the production of calcitriol (activated form of Vitamin D3), which is a steroid hormone, in the kidneys.
Calcitriol is responsible for inducing the absorption of the calcium, taken in the diet, through the intestinal lining.