Why should I measure ionized calcium and parathyroid hormone in cases of chronic renal failure?
In chronic renal failure serum total calcium may be within the reference range. However, this does not ensure that alterations in calcium metabolism are not present. Commonly in renal failure due to loss of functional renal tissue, calcitriol is inadequately produced. Calcitriol is important in the regulation of ionized calcium concentration. This deficient calcitriol production allows the serum ionized calcium concentration to fall, which stimulates the production of parathyroid hormone (PTH). PTH will try to raise ionized calcium back into the normal reference range by activating osteoclasts with resorption of bone. To maintain the ionized calcium concentration, PTH continues to elevate, causing secondary hyperparathyroidism. PTH itself is a uremic toxin, and some of the signs of chronic renal failure can be attributed to excess PTH. If secondary hyperparathyroidism is noted, then treatment can be initiated to decrease the PTH concentration (typically with calcitriol therapy).
