Hypercalcemia With Non Suppressed Parathyroid Hormone in a Young Female- A Rare Case of Calcium Sensing Receptor Gene Related Familial Hypocalciuric Hypercalcemia-1

Background: Familial Hypocalciuric Hypercalcemia (FHH) is a rare disorder, associated with hypercalcemia and hypocalciuria and inherited in an autosomal dominant manner. Its true prevalence is unknown, and is estimated to be around 1 in 78,000 as compared to primary hyperparathyroidism (PHPT) which is around 1 in 1000. There are 3 mutations known to cause FHH. FHH-1 is caused by inactivating mutation in the calcium sensing receptor (CaSR) gene. Mutations associated with FHH-2 and FHH-3 are GNA11 and AP2S1 respectively. Case Presentation: 38-year-old female presented to endocrinology, for evaluation of hypercalcemia (10.8 mg/dL, with normal albumin). She was not on any calcium supplements or any other medications which can cause hypercalcemia. She did not have any prior fracture or nephrolithiasis or renal insufficiency. Her father reported to have hypercalcemia; no further evaluations of her father were available. Examination revealed an obese female with no skeletal or dental or other physical abnormalities. Laboratory evaluations: PTH= 37 pg/ml (15–65), 25 OH vitamin=D-30 pg/ml, Mg=2 mg/dL (1.5–2.6), phosphorus=3.1 mg/dl (2.5–4.8), 24 hour urine calcium=0.151 g/24 hours-with 24 hour urine calcium clearance of 0.008. Therefore, evaluations were highly suggestive of FHH. Subsequently, she had genetic evaluation which confirmed heterozygous CaSR mutation, confirming FHH-1. Her mother had genetic testing and was not found to have the mutation. So, it was concluded that the patient likely inherited the gene from her father, who also had hypercalcemia. She is being monitored clinically and with serial laboratory evaluations to monitor her calcium levels. Discussion: CaSR is expressed in parathyroid glands and kidneys which plays a key role in calcium regulation. CaSR inactivating mutation (seen in FHH-1) leads to hypocalciuria and hypercalcemia. 24 hour urine calcium clearance (urine Ca x serum Cr/ serum Ca x urine Cr) of <0.01 is highly suggestive of FHH. Furthermore, higher concentration of calcium is required to suppress PTH release leading to high or nonsuppressed PTH. This finding can mislead towards the diagnosis of PHPT and unnecessary parathyroid surgery, if the diagnosis of FHH is missed. FHH is usually a benign disorder; subtotal parathyroidectomy does not cure the disease. FHH, rarely can cause atypical complications such as pancreatitis and total parathyroidectomy may be indicated to prevent further episodes of pancreatitis. Conclusions: This is a rare case presentation of hypercalcemia due to CaSR inactivating mutation related FHH. FHH and PHPT are competing diagnoses, when a patient presents with hypercalcemia and has nonsuppressed PTH. FHH is rare, however needs to be suspected in a young patient with family history of hypercalcemia, to avoid misdiagnosis of PHPT and unnecessary surgical intervention.