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I'm confused about the phosphate level in questions like these. Decreased calcitriol would decrease phosphate absorption while PTH decrease lowers phosphate excretion. I'm assuming that the PTH decrease has the greater effect with serum phosphate levels?
PTH = "Phosphate trashing hormone" if PTH is high Phosphate must be low - they are always opposite (unless d/t renal failure then Phosphate will be high - kidneys will be unable to get rid of phosphate)
So low Ca d/t low PTH does not effect 25 H. Vit D ... only 1,25 H Vit D (active Calcitriol)?
Clarification because I was confused: PTH stimulates kidney to produce 1,25-(OH)2 D3 (calcitriol) via 1α-hydroxylase in proximal convoluted tubule. Therefore, without parathyroid glands, low PTH, 25,D is not converted and therefore not down (normal or up). phosphate "trashed" by PTH as eloquently stated above.
Here the primary defect is high up from the parathyroid gland, there is decresed or no PTH which normally trashes phosphate but not in this case so serum PHOSPHATE INCREASES and the serum calcium is low because PTH should have prevented the urine calcium so there is calciuria and no resorption from bone-LOW CALCIUM, Vitamin-D is independent of PTH so stays NORMAL