| Summary: | ABSTRACT: Objective: Hypercalcemia can be caused by a variety of pathologies and factors. Vitamin D plays a central role in calcium homeostasis, where tight control of its metabolism is necessary. Inadequate 25-hydroxyvitamin D-24-hydroxylase (CYP24A1) enzyme activity leads to failure of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D catabolism, resulting in hypercalcemia.Methods: This study consists of a case report and brief review of the literature. A young woman with persistent hypercalcemia of unknown origin was investigated by measuring phosphate, parathyroid hormone, and vitamin D metabolites, as well as by evaluating kidney and bone involvement.Results: The patient was asymptomatic and had a cystitis-like syndrome during childhood. The patient exhibited hypercalcemia and hypercalciuria with suppressed PTH. Liquid chromatography with tandem mass spectrometry analysis of vitamin D metabolites revealed elevated 1,25-dihydroxyvitamin D3 (301 pg/mL) and an elevated ratio of 25-hydroxyvitamin D3 to 24,25-dihydroxyvitamin D3 of 390. Screening for CYP24A1 gene mutations was performed. Bi-allelic mutations in the coding region of CYP24A1 were identified (Arg396Trp/Leu409S), confirming the diagnosis of hypercalcemia due to CYP24A1 mutation. The patient's family members were also studied.Conclusion: This particular case of hypercalcemia due to a mutation in CYP24A1 emphasizes 2 main concerns: vitamin D-related hypercalcemia should be considered at any age in patients with hypercalcemia, and asymptomatic patients with CYP24A1 mutations are at risk of developing hypercalcemia arising from vitamin D supplementation.Abbreviations: 1,25-(OH)2D3 1,25-dihydroxyvitamin D3 25-OH-D3 25-hydroxyvitamin D3 24,25-(OH)2D3 24,25-dihydroxyvitamin D3 CYP24A1 25-hydroxyvitamin D-24-hydroxylase FGF23 fibroblast growth factor 23 IIH idiopathic infantile hypercalcemia LC-MS/MS liquid chromatography with tandem mass spectrometry PTH parathyroid hormone
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