Calcium plays a very important role in the formation of bones. Write about the role of endocrine glands and hormones responsible for maintaining calcium homeostasis.

$(i)$ Parathyroid gland secretes Parathyroid Hormone $(PTH)$,which maintains the balance of $Ca^{++}$ and $PO_4^{3-}$ in blood and other tissues. It induces the transport of calcium from bones into the blood and increases the reabsorption of $Ca^{++}$ by the kidneys and intestine.
(ii) Thyroid gland secretes Thyrocalcitonin $(TCT)$ hormone. It inhibits the release of $Ca^{++}$ from bone and lowers blood calcium levels; thus,it acts against $PTH$ and maintains the homeostasis of $Ca^{++}$.