PTH and chronic kidney disease: Tertiary HPTH refers to the condition of a subset of patients with CKD who develop ionized hypercalcemia and excessive PTH secretion that is not inhibited by high serum iCa concentration.116–119 It is likely that such patients had high PTH concentrations in association with normal or low serum iCa (renal secondary HPTH) earlier in the clinical course of CKD.4 The set point for control of PTH secretion may be altered in CKD such that higher concentrations of iCa are necessary to inhibit PTH secretion.