PTH and cancer: We previously reported that vitamin D deficiency promotes the growth of human breast and prostate cancer cells in bone.5,8,10,13 This acceleration in cancer cell growth was, at least in part, attributable to the effects of vitamin D deficiency on the bone microenvironment, as hypovitaminosis D—via an increase in parathyroid hormone secretion—induces a significant increase in osteoclast-mediated bone resorption.5,8,10,13 In addition, treatment with 1,25(OH)2D3 inhibits breast and prostate cancer cell growth in vitro and in vivo.