PTH and secondary hyperparathyroidism: Since the eGFR values of the present CKD patients were distributed between 5.0 and 58.8 mL/min/1.73 m2, it is reasonable to speculate that 25(OH)D metabolism is heavily affected by secondary hyperparathyroidism, based on our results indicating that serum PTH starts to increase at a level below 50 mL/min/1.73 m2 and then in an eGFR-dependent manner thereafter26.