Why is the serum calcium usually low in renal failure?

The kidney is the primary organ involved in calcium homeostasis. It plays two main roles:

• It is the final processing site of vitamin D into its active metabolite. (Technically, the reaction taking place is the 1α-hydroxylation of 25-hydroxycholecalciferol to get 1,25-hydroxycholecalciferol - if you really need to know.) This active form of the vitamin then acts to increase calcium (and phosphate) absorption from the gut.

• The second aspect of renal calcium control is that it is the site of action of parathyroid hormone (PTH), which increases calcium reabsorption from the kidneys.

In renal failure, there is usually retention of phosphate. This raised phosphate level inhibits the kidney's activation of vitamin D, probably in an effort to decrease its own level (since vitamin D increases gut phosphate absorption). Unfortunately this has the side-effect of also lowering calcium levels.

Furthermore, in chronic renal failure, there is permanent loss of the kidney tissue, meaning that neither of the kidneys' two roles in calcium homeostasis can be adequately performed - there is simply not enough kidney.

Soon, we'll explain all the common biochemical findings in renal failure... when I have time!