Early in the course of chronic kidney disease, you have the upregulation of FGF23 in the kidney in an attempt to excrete more phosphate. As the kidney start to progress in its failure, FGF23 production goes down--> Less phosphate excreted. And you have a lower GFR, so less phosphate being filtered out. Damaged kidney is not going to respond properly to PTH (which is why it isn't sufficient to increase calcium levels to normal)
