PTH and tumor of parathyroid gland: Because oxyphil-dominant parathyroid tumors appear to drive biochemically more severe disease presentation in PHPT (17–21) and oxyphilic hyperplasia is associated with the loss of calcimimetic responsiveness in SHPT (12), we hypothesized that chronic 25OHD deficiency could produce changes in parathyroid oxyphil content and gene expression that reduce calcium sensitivity, increase PTH hypersecretion, and promote adenoma development in PHPT.