The clandestine key: 1,25-dihydroxyvitamin D3 Thomas Hiemstra, Department of Renal Medicine, Cambridge University Hospitals 3 August 2007 Surprisingly Lyell et al attribute hypercalcaemia in a patient with ATLL solely to a modest elevation in PTHrP. The features of this case suggest the paraneoplastic production of the active vitamin D hormone calcitriol. Lymphocytes and macrophages express the enzyme vitamin D 1-α hydroxylase  and the production of calcitriol by lymphomas is well described .First, the authors report a normal serum phosphate although a value is not provided. PTHrP induced hypercalcaemia is often accompanied by hypophosphataemia due to its phosphaturic effects. Second, PTH was markedly suppressed in keeping with calcitriol toxicity, although hypercalcaemia would contribute.In addition the authors describe increased expression of RANKL in ATLL patients, promoting osteoclastogenesis. RANKL expression is upregulated by calcitriol . The measurement of 1,25-dihydroxyvitamin D may have illuminated pathophysiology here.References1. Zehnder D, Bland R, Williams MC, McNinch RW, Howie AJ, Stewart PM, Hewison M: Extrarenal expression of 25-hydroxyvitamin d(3)-1 alpha-hydroxylase. J Clin Endocrinol Metab 2001, 86(2):888-894.2. Seymour JF, Gagel RF: Calcitriol: the major humoral mediator of hypercalcemia in Hodgkin's disease and non-Hodgkin's lymphomas. Blood 1993, 82(5):1383-1394.3. Kitazawa S, Kajimoto K, Kondo T, Kitazawa R: Vitamin D3 supports osteoclastogenesis via functional vitamin D response element of human RANKL gene promoter. J Cell Biochem 2003, 89(4):771-777. Competing interests I declare that I have no competing interests.