What is negative feedback interaction between calcitonin and parathyroid hormone?

The parathyroid gland is the major regulator of blood Ca levels. When the Ca levels in the blood decrease, the parathyroid gland releases more parathyroid hormone. This in turn increases osteoclasts which results in more bone breakdown. this releases more Ca into the blood stream. When Ca levels in the blood increase the thyroid gland releases Calcitonin. this decreases the activity of the osteoclasts which in turn decrease the levels of Ca in the blood.