FGF23 and polycystic kidney disease: In addition, homozygous Pkd1Col1a1(3.6)-cKO mice had significantly higher levels of serum BUN, PTH, and FGF23, but lower levels of phosphorus, and no changes in calcium levels at 6 weeks of age (Table 2), consistent with the development of renal impairment caused by polycystic kidney disease and secondary induction of compensatory hormonal changes (Table 2).