Solution:
The secretion of PTH is regulated by the circulating levels of calcium ions.
Low level of Ca2+ in the blood stimulates the parathyroid glands to release more PTH.
PTH increases blood Ca2+ levels (e.g., by increasing the activity of osteoclasts).
A higher-than-normal level of calcium ions Ca2+ in the blood stimulates parafollicular cells of the thyroid gland to release more calcitonin which inhibits the activity of osteoclasts, thereby decreasing the blood Ca2+ level.
