We used Equation (4) to examine TRCa/Ccr as a function of [PTH] in seven patients with primary hyperparathyroidism (PHPT), 29 patients with CKD (mean MDRD estimated GFR of 29.5 mL/min/1.73 m2, range 14–49), and 28 controls with normocalcemia and estimated GFR >60 mL/min/1.73 m2 [16]. Here, PTH is linked to primary hyperparathyroidism.