Now, when the parathyroid hormone actually reaches its target cell, because it is not soluble in lipids, that means it cannot actually pass across the membrane of the target cell. Instead it binds onto the membrane of the target cell, it binds to special protein receptors and initiates a secondary messenger system that basically creates some type of change inside the cell. Now the question is, what exactly is the function, what is the purpose of the parathyroid gland or the parathyroid hormone? Well, the function of the parathyroid hormone is basically the opposite of calcitonin. So previously we discussed a hormone that is produced by the thyroid gland known as calcitonin. And calcitonin is responsible for decreasing the concentration of calcium inside our blood.