Severe Hypocalcemia During JAK1/2 Inhibitor Therapy for Myelofibrosis in a Patient with Liver Cirrhosis.

We herein report a 56-year-old man with severe hypocalcemia during ruxolitinib therapy for myelofibrosis transitioning from Janus kinase 2 mutation-positive polycythemia vera. Blood transfusions were administered every one to two weeks for ruxolitinib-induced anemia. Blood tests revealed hypocalcemia with low tartrate-resistant acid phosphatase-5b, 25-hydroxyvitamin D (25 (OH) D), and 1,25-dihydroxyvitamin D3 (1,25 (OH) 2 D3) levels within the lower reference range. Intact-parathyroid hormone levels were relatively low compared to calcium levels. Severe hypocalcemia with ruxolitinib is rare and may be caused by a combination of factors, impaired vitamin D activation due to liver or renal insufficiency, accumulation of calcium-chelating agents from blood transfusions, and inadequate compensatory response to PTH.