CASR and hyperparathyroidism: 1,25-dihydroxyvitamin D [1,25(OH)2D], the bioactive metabolite converted from circulating 25-hydroxyvitamin D (25OHD), is thought to prevent chronic PTH hypersecretion and the eventual development of hyperparathyroidism by stimulating CASR gene transcription, inhibiting PTH gene transcription, and blocking parathyroid cell proliferation (8).