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Abstract 1029 Purpose of study: Transfusion dependent thalassemia (TM) patients are routinely supplemented with vitamin D due to their increased risk of developing osteoporosis. Recent studies from North America have found that these patients have high rates of vitamin D “deficiency” and “insufficiency,” despite supplementation. The amount of vitamin D supplementation required to raise serum 25 hydroxy-vitamin D (25-OHD) to optimal levels is not known in these patients. Recent studies have linked 25-OHD levels to hypercalciuria and nephrolithiasis in patients with TM. The purpose of this study is to determine the effect of various doses of vitamin D supplementation on vitamin D stores and calcium excretion in TM patients. Description of project: Prospective, single-blind, placebo-controlled study of TM patients followed in the transfusion center at Weill Cornell/New York Presbyterian Hospital. Patients with 25-OHD concentrations between 15–29 ng/mL were eligible for this 3-month study. Subjects were assigned in a block type of enrollment to the “high dose” equivalent of 2,000 IU of vitamin D per day versus placebo. Results: 14 subjects were enrolled, with 8 assigned to the “high dose” group and 6 assigned to the placebo group. The “high dose” group consisted of 6 females, aged 15.2–45.5 years with an average baseline 25-OHD level of 22.4 ng/mL (15–26). The “placebo” group consisted of 4 females, aged 22.5–45.7 years with an average baseline 25-OHD level of 19.8 ng/mL (16–24). After the 3 month study period, hypercalciuria developed more frequently in those treated in the “high dose” group. In the placebo group, hypercalciuria was noted in 1/6 (16.7%) spot urine calcium/creatinine tests and 0/3 (0%) 24 hour urine calcium estimations. In the “high dose” group, the corresponding number of patients based on the same methods of testing were 5/8 (62.5%) and 2/5 (40%). No episodes of hypocalcemia, hypercalcemia or nephrolithiasis occurred in either group. Conclusion: Our findings suggest that “high dose” vitamin D supplementation results in higher rates of hypercalciuria in TM patients. Further studies are necessary to determine the optimal dose of vitamin D supplementation to minimize the risk of osteoporosis while preventing nephrolithiasis in TM patients. Disclosures: No relevant conflicts of interest to declare.