THU236 Mild Asymptomatic Hypercalcemia Secondary To A Novel Calcium-sensing Receptor Mutation

Disclosure: J. Tarkoff: None. Background: Familial hypocalciuric hypercalcemia (FHH) is a rare cause of mild hypercalcemia that is usually secondary to heterozygous mutations of the calcium-sensing receptor (CaSR) with autosomal dominant inheritance. The distinction between this entity and primary hyperparathyroidism can be difficult. Clinical Case: A 15-year-old female presented to the endocrinology clinic for an incidentally found elevated total calcium of 10.9 mg/dl (8.9-10.4) and elevated ionized calcium 5.5 mg/dl (4.8-5.3 mg/dl). Her labs at that time also showed a PTH of 29 pg/mL (12-71) and phosphorus 3.5 mg/dl (2.5-4.5 mg/dl). She denied any symptoms of hypercalcemia and there is no reported family history of mineral disorders or fractures. She does not take any medications. Her physical exam was unremarkable. Our evaluation showed calcium 10.5 mg/dl, phosphorus 4 mg/dl, magnesium 2.2 mg/dl (1.5-2.5 mg/dl) 25-hydroxyvitamin D 32 ng/ml (30-100 ng/ml), fractional excretion of calcium (FeCa) based upon spot urine sample = 0.0104, TSH 1.29 mIU/L (0.5-4.3 mIU/L), alkaline phosphatase 79 U/L (45-150 U/L), and normal renal function. Given her spot FeCa being <0.02, sequencing of the CASR gene was offered to the family because of potential overlap of her FeCa with primary hyperparathyroidism for which the treatment would be surgical (1). Commercial genetic testing was notable for a previously unreported heterozygous nonsense mutation, c.3001C>T (p.Gln1001X). It is expected to disrupt the final 78 amino acids of the protein in its intracellular domain and may play a role in preventing downstream signaling. At follow-up without treatment, her calcium has ranged between 10-10.9 mg/dl and she remains asymptomatic. Clinical lesson/conclusion: Hypocalciuria in the setting of hypercalcemia should raise the suspicion of FHH even without family history. Genetic testing of the CaSR is a crucial step in facilitating the diagnosis of FHH, especially given the importance of distinguishing it from primary hyperparathyroidism. Reference: (1) Christensen, S.E., Nissen, P.H., Vestergaard, P., Heickendorff, L., Brixen, K. and Mosekilde, L. (2008), Discriminative power of three indices of renal calcium excretion for the distinction between familial hypocalciuric hypercalcemia and primary hyperparathyroidism: a follow-up study on methods. Clinical Endocrinology, 69: 713-720. Presentation: Thursday, June 15, 2023