THU406 24-hydroxylase Deficiency, A Rare Cause Of Hypercalcemia

Disclosure: J. Hodge: None. M. Gortakowski: None. N. Barua: None. Background: 24-hydroxylase is encoded by CYP24A1 and functions to convert 1,25 dihydroxyvitamin D and 25-hydroxyvitamin D to their inactive metabolites([1]). 24-hydroxylase deficiency due to CYP24A1 variants is rare and clinical presentation is characterized by hypercalcemia, nephrolithiasis and nephrocalcinosis. Treatment may include thiazide diuretics, sodium cellulose phosphate and ketoconazole.([1]) We present a case of hypercalcemia due to 24-hydroxylase deficiency. Clinical Case: A 56-year-old male with chronic kidney disease initially presented with hypercalcemia at age 48 to another institution. He was initially diagnosed with primary hyperparathyroidism in the setting of an inappropriately normal parathyroid hormone (PTH) with hypercalcemia. He subsequently had one parathyroid gland removed a year after diagnosis, then another two years later due to continued hypercalcemia. However, he continued to have persistent hypercalcemia, 10.9-12.7 mg/dL but now in the setting of low PTH. In addition, his 1,25 dihydroxyvitamin D was high normal and 25-hydroxyvitamin D was high off exogenous vitamin D supplementation, which raised the suspicion for 24-hydroxylase deficiency. He presented to our institution eight years later. Initial evaluation revealed, calcium 12.4 (8.6-10.5 mg/dL), phosphorous 2.9 (2.5-4.5 mg/dL), normal albumin, PTH 13 (15-65 pg/mL), 1,25 dihydroxyvitamin D 87.2 (19-79.3 pg/mL) and 25-hydroxyvitamin D 38.3 (20-50 ng/mL), 24hr urine calcium 0.23 (0.05-0.30 g/24hr), fractional excretion calcium 0.02. His 25-hydroxyvitamin D:24,25 dihydroxy vitamin D ratio was unable to be calculated, since 24,25 dihydroxy vitamin D was below the limit of quantification. He also had evidence of nephrocalcinosis on renal ultrasound. Evaluation of other causes of non-PTH mediated hypercalcemia included normal angio converting enzyme, PTHrP level, vitamin A level, SPEP and UPEP. Chest X-ray did not demonstrate any evidence of granulomatous disease. He has been managed conservatively for suspected 24-hydroxylase deficiency. He has been advised to limit calcium and vitamin D intake, as well as sun exposure, with close monitoring of his calcium levels. His calcium levels have ranged from 10.7-12.4 mg/dL and he has had one hospitalization in the setting of heart failure where his calcium levels improved with furosemide to 8.7-9.9 mg/dL. Conclusion: In patients with persistent non-PTH dependent hypercalcemia, 24-hydroxylase deficiency should be a differential diagnosis when other common causes are excluded. References: 1. Nesterova G, Malicdan MC, Yasuda K, et al. 1,25-(OH)(2)D-24 hydroxylase (CYP24A1) deficiency as a cause of nephrolithiasis. Clinical Journal of the American Society of Nephrology. 2013;8(4):649-657. doi:10.2215/cjn.05360512 Presentation: Thursday, June 15, 2023