This elevation is due to 2 phenomena: (a) inactivation of the CYP27B1 enzyme — thus, no conversion — and, more importantly, (b) secondary hyperparathyroidism that suppresses the activity and expression of the Cyp24a1 gene and CYP24A1 enzyme (9, 11). Here, CYP27B1 is linked to secondary hyperparathyroidism.