PTH and vitamin D deficiency: Importantly, in a rat model of hyperphosphatemic CKD, when intraperitoneal doses of 25(OH)D that correct vitamin D deficiency but are insufficient to reduce serum PTH, are combined with a paricalcitol dose, also insufficient per se to suppress serum PTH or parathyroid cell growth, the 25(OH)D/paricalcitol combination effectively inhibited parathyroid ADAM17 resulting in a 50% PTH reduction (38), despite no changes in serum calcium (38).