Since the kidneys actually make the 1,25 D3 from D2, when you dont have active kidney tissue, you are completely dependent on exogenous D3. To make matters even worse the typical hypocalcemia of renal failure cranks up the parathyroids to keep calcium up to normal levels, by mobilizing bone calcium and dissolving the skeleton. The resulting hyperparathyroidism raises the calcium levels and tends to deposits calcium in other organs, calcinosis. Adding D3 to this can make it worse. Its a real rock and a hard place scenario!
The kidney also make erythropoietin (EPO) needed to to generate red cells from marrow, so it also has to be provided in renal failure to treat the chronic anemia.
Its a very tricky business taking over the endocrine functions of the kidney with loss of the normal regulatory feedback mechanism.
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