Hypocalcemia and hyperphosphatemia in Chronic Kidney Disease - USMLE Forums
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Old 08-06-2016
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Default Hypocalcemia and hyperphosphatemia in Chronic Kidney Disease

From what I understand, there will be decrease in calcium reabsorption in the distal tubule and decrease reabsorption of phosphate in proximal tubule and hence decrease in plasma calcium but how does it lead to hyperphosphatemia? Is it plasma phosphate or am I missing something?
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Old 08-06-2016
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CA is dec because V-D is not activated by alpha hydroxylase and PTH has resistance.
Phosphate is elevated because reduced phosphate excretion due to dec GFR.
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Old 08-07-2016
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Agreed. But what about the Ca that is not being reabsorbed? Activation of Ca by Vit D is after its reabsorbed in the distal tubule. Wont there be a rise in Ca that is already in the plasma along with the phosphate? If phosphate filtration decreases, won't Ca also decrease leading to hypercalcemia?
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Old 08-08-2016
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Hi there,

CKD: Reduced number of nephrons, reduced GFR -> 1. reduced production of vit. D (calcitriol), 2. reduced excretion of P.
1. leads to reduced intestinal Ca absorption + upregulation of PTH, due to lack of inhibitory effect of vit. D.
2. leads to hyperphosphatemia, which in turn worsens hypocalcemia, as phosphate binds calcium and leads to secondary hyperparathyroidism (which could correct Ca, P in the long run).

Hope this helps.
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Old 08-08-2016
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Kidney is damages ... Tubules dont work that well so phosphate isn't excreted well .... No alpha hydroxylation so no vit D to absorb calcium therefore hypocalcemia
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