Familial hypocalciuric hypercalcemia (FHH) is a rare disease (ORPHA#405, www.orpha.net) and most likely underdiagnosed, that clinicians should be aware of in the differential diagnosis of a hypercalcemia. Appropriate identification of the FHH has implications in treatment and also for the family, since it is an automosal-dominant disease, due to mostly a heterozygous loss-of-function mutation of the CASR (calcium-sensing receptor) gene, but also much less freqüent mutations of another two genes (AP2S1 and GNA11). In case of clinical and biochemical suspicion of FHH, a genetic evaluation is mandatory. Nevertheless, an important number of patients, the genetic study is negative. This observational study is intended to perform a descriptive review of cases with clinical and biochemical suspicion of FHH who underwent a genetic study in the usual clinical practice. Clinical, biochemical and radiological characteristics, treatment, follow-up and comorbidities of genotype-negative participants will be compared with genotype-positive cases.

Inclusion Criteria:

• Patients with clinical and biochemical suspicion of FHH who, at the discretion of the physician in routine clinical practice, were asked to perform a genetic evaluation of FHH and whose genetic results are available.

Exclusion Criteria:

• Genetic study of FHH is not available or was not performed despite clinical and biochemical suspicion of FHH.

Participants with biochemical and clinical suspicion of FHH, namely hypercalcemia, hypophospatemia, normal or elevated parathormone levels and very low urine calcium excretion defined as a calcium creatinine clearance ratio <0.01, and who were asked at discretion of the clinician for a genetic evaluation of any of the FHH causing genes.