Incidental hypercalcemia in a symptomatic adult: Diagnostic clue to familial hypocalciuric hypercalcemia

ABSTRACT Background: Familial hypocalciuric hypercalcemia (FHH) is a rare, benign autosomal dominant disorder characterized by mild hypercalcemia, hypocalciuria, and normal or mildly elevated parathyroid hormone (PTH) levels. It is frequently misdiagnosed as primary hyperparathyroidism, potentially leading to unnecessary surgical interventions. Case Presentation: We report the case of a 25-year-old male who presented with vague musculoskeletal discomfort and gastrointestinal symptoms, including bloating, abdominal cramps, and alternating constipation and diarrhea, consistent with a prior diagnosis of irritable bowel syndrome. Routine laboratory work-up revealed persistent mild hypercalcemia (10.3–10.4 mg/dL) and inappropriately elevated PTH levels (135–150 pg/mL), with sufficient vitamin D levels (36 ng/mL). Neck ultrasound and 99mTc-sestamibi parathyroid scan were unremarkable. Dual-energy X-ray absorptiometry showed normal bone density. A 24 h urinary calcium excretion revealed hypocalciuria, with a calcium-to-creatinine clearance ratio of 0.007. Based on this biochemical profile, genetic testing was performed and identified a heterozygous missense mutation (c.622T>G; p.Trp208Gly) in the CASR gene, confirming the diagnosis of FHH type 1. The patient was counseled on the benign nature of the disorder and was initiated on cinacalcet as an off-label, symptom-targeted therapy. Genetic counseling and screening were recommended for first-degree relatives. Conclusion: FHH should be considered in cases of mild hypercalcemia with elevated or inappropriately normal PTH and low urinary calcium. Early identification using biochemical markers and confirmatory genetic testing is essential to avoid misdiagnosis and unnecessary surgery and to guide appropriate family screening.