The level of Calcium in the blood is very important. Too little Calcium and the bones don't form correctly; your nerves will also not function properly. Too much Calcium and the Calcium starts getting deposited on the walls of your veins and arteries leading to vascular calcification which can be fatal if not corrected in time.
The body realizes this and has a small gland called the Parathyroid gland (4 of them actually) to produce a hormone called the Parathyroid hormone whose sole purpose of existence is to regulate the amount of Calcium in the blood. When the Calcium level in the blood becomes too low, the glands produce more Parathyroid hormone that causes increased absorption of Calcium from the store house of Calcium in the body - the bones. When the Calcium becomes too high, the glands produce less Parathyroid hormone which reduces the amount of Calcium absorbed from the bones into the blood.
So, the amount of PTH being produced depends (inversely) on the amount of Calcium in the blood.
For people on dialysis, the amount of Calcium in the blood is related, among other things, to the amount of Calcium in the dialysate that is used during dialysis. This is in direct contact with the blood (through a membrane). So, too much Calcium in the dialysate causes Calcium coming into the blood from the dialysate. Too little Calcium in the dialysate causes Calcium to move out from the blood into the dialysate. All this affects the PTH directly.
So, when the PTH goes too high or too low, check the Calcium level in the dialysate and tweak that around.
Many people opt for cinacalcet, touted as a wonder drug for PTH. While it may be true that Cinacalcet is a wonder drug, it must be used only when the body's natural mechanism of producing PTH in relation to the Calcium level fails completely.