What will happen if there is high concentration of both calcitonin and PTH in blood?
Short Answer: quite surprisingly, both will occur.

Long Answer: as is quite well known, calcitonin lowers blood Ca2+ levels. It performs this task by two methods:


it inhibits the activity of osteoclasts in the bones (Custoff, 2008)
it inhibits reabsorption of Ca2+ in renal tubular cells, allowing it to be excreted in urine (Potts et al, 2008)


However, the second mechanism is only minor. On the other hand, parathyroid hormone (PTH) performs just its opposite: it increases Ca2+ levels in the blood.

So, lets come to the main point: since calcitonin and PTH have opposing functions, what would happen if both were acting simultaneously?

It seems, funnily though, that both the functions will occur simultaneously. Wong et al, 1975, in their research, showed that the target cells of calcitonin and PTH are different i.e. calcitonin and PTH act on different osteoclasts. So, to put it simply, while activity of some osteoclasts would be decreased, that of some others would be increased. Overall, Ca2+ levels in the blood would remain pretty much constant.