Clinical Presentation and Management Approach in a Case of Familial Hypocalciuric Hypercalcemia Type 3 Due to APS21 Gene Mutation

Familial hypocalciuric hypercalcemia (FHH) is a genetic disorder caused by dysfunctional calcium homeostasis. Thus far, three types of FHH are known to be caused by mutations inCASR (FHH1), GNA11 (FHH2), and AP2S1 (FHH3). The patient in this case report is a 36-year old male that initially presented for a second opinion after being diagnosed with Primary Hyperparathyroidism(PHPT) with subsequent parathyroidectomy done at another institute, and developed recurrent symptomatic hypercalcemia. Prior to considering this patient for further surgical options, he underwent genetic testing, which revealed he had c.43C>T (p.Arg15Cys) mutation in the AP2S1 gene diagnostic of Familial Hypocalciuric Hypercalcemia Type 3 (FHH3). The patient’s father and sister also have hypercalcemia, and have been offered genetic testing. There have been cases reported of patients with FHH3 that have symptomatic hypercalcemia and that have associated cognitive issues. Many patients with FHH can be misdiagnosed and may undergo unnecessary parathyroidectomy. This case report further elucidates the need to raise awareness of FHH.