Hypocalciuria as a Key to Diagnosis: A Rare Case of GNA11-Related Familial Hypocalciuric Hypercalcemia (FHH) Type 2

Familial hypocalciuric hypercalcemia (FHH) is a rare autosomal dominant disorder of calcium homeostasis characterized by mild hypercalcemia, non-suppressed parathyroid hormone (PTH), and hypocalciuria. FHH type 2 (FHH2), caused by mutations in the GNA11 gene, is particularly rare. Differentiating FHH from primary hyperparathyroidism (PHPT) is crucial to avoid unnecessary surgical intervention. We report a 63-year-old male with persistent hypercalcemia and inappropriately normal PTH levels. Biochemical evaluation revealed marked hypocalciuria with a fractional excretion of calcium of 0.0039. Imaging studies were negative for parathyroid adenoma. Whole-exome sequencing identified a heterozygous missense mutation in GNA11 (c.161C>T; p.Thr54Met), confirming FHH2. The patient was treated with cinacalcet 30 mg daily due to significant hypercalcemia calcium levels > 1 mg/dL above the upper limit. At three months, serum calcium decreased, and PTH normalized. This case underscores the importance of urinary calcium assessment and genetic testing in PTH-dependent hypercalcemia and highlights a potential therapeutic role for cinacalcet in selected FHH2 patients.