ESRD and Phosphate Levels???

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Why does phosphate increase in end stage renal disease?

I get that vitamin D is gonna be low, and since the function of vitamin D is to increase absorption of calcium and phosphate, how come only calcium is high in the serum?????

Phosphate isn't be excreted and therefore it stays in the serum, binding free calcium. This lowers calcium.

PTH is secreted because of low calcium, and that releases phosphate as well, but phosphate isn't being excreted and so it binds to calcium too. Low calcium again. (secondary hyper-pth)

Metabolic acidosis is taking place too, so bone releases calcium to try and buffer (osteroporosis)

And you mentioned vitD deficiency as well (osteomalacia).
And anemia cause of no epo.

Actually calcium is low in the serum. phosphate cannot be excreted due to ESRD so it stays in the serum and complexes with calcium so therefore the calcium serum is low and you have secondary hyperparathyroidism.

Renal failure --> no more active vitamin D production --> low Ca++ in blood --> parathyroids sense low Ca++ and PTH released --> Ca++ is released from bone in an attempt to compensate (but it never can keep up with the degree of Ca++ wasting by the nonfunctional thick ascending limbs of the failing kidneys).

The kidneys normally excrete phosphate in the presence of PTH, but the kidneys aren't working. Phosphate doesn't get excreted and instead it builds up in the blood.

I'm not sure if you mis-typed in your original question, but to clarify: in renal failure, Ca++ is low and phosphate is high.