SUN-772 - Familial Hypocalciuric Hypercalcemia Diagnosed after Parathyroidectomy: A Case Report

Background:
Familial hypocalciuric hypercalcemia (FHH) is a rare, typically benign disorder caused by altered calcium-sensing receptor function, often misdiagnosed as primary hyperparathyroidism. Failure to identify FHH can lead to unnecessary parathyroid surgery as well as ineffective medical management.

Case Presentation:
A 35-year-old man presented to our endocrinology service in 2021 for evaluation of persistent hypercalcemia. He had a history of partial parathyroidectomy performed in 2006 for presumed hyperparathyroidism, without normalization of serum calcium. Past medical history was notable for chronic pancreatitis and alcohol use disorder. He also reported a history of prior transverse fracture of the third distal phalanx in 2017, but this was attributed to trauma. Additionally, he reported mother, brother, sister, son and daughter all having elevated calcium levels.

Despite parathyroid surgery in 2006, the patient continued to demonstrate elevated serum calcium with normal parathyroid hormone (PTH) levels. Initial laboratory results revealed persistent hypercalcemia (calcium 11.8 mg/dL) without recent calcium supplementation, markedly low vitamin D (12.8 ng/mL), and a fractional excretion of calcium (FECa) of 0.002. Given that FECa < 0.01 strongly favored FHH, a diagnosis of familial hypocalciuric hypercalcemia was made.

Management and Outcome:
Subsequent genetic testing in May 2025, confirmed FHH type 3 based on a pathogenic heterogenous variant of AP2S1: c44.G>T (p.Arg15Leu). The patient continues to suffer repeat bouts of pancreatitis. There continues to be debate on the contribution of prior alcohol use as opposed to chronic hypercalcemia. To further investigate, cinacalcet was initiated. Unfortunately, despite its use, the patient continued to have persistent hypercalcemia and readmissions for pancreatitis.

Conclusion:
This case highlights the importance of detailed evaluation of hypercalcemia before interventions such as parathyroid surgery. Evaluating urinary calcium excretion in patients with persistent hypercalcemia, particularly those with a family history should be considered as an initial step. Early recognition of FHH and its subtypes through the use of genetic testing can additionally aid in tailoring treatment and preventing unnecessary operative interventions and aid in guiding appropriate long-term management.

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