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Familial hypocalciuric hypercalcemia (FHH) is a cause of hypercalcemia with autosomal dominant pattern of inheritance and high penetrance. In most of the cases it can be shown to be due to an inactivating mutation on the gene coding for the calcium-sensing receptor (CaSR). Heterozygous cases usually do not present symptoms and they are diagnosed as an incidental finding. We report three affected children with an inactivating heterozygous mutation, p.Phe789del, in exon 7 of the calcium-sensing receptor gene (CASR gene), situated in chromosome 3q21 (Ensembl ENSG00000036828), which results in elevated serum calcium, normal o high level of parathyroid hormone (PTH) and reduced urinary excretion with hypocalciuria. It is very important to determine the difference between FHH and primary hyperparathyroidism. Therefore, in a mild to moderate PTH-dependent hypercalcemia we must perform a family study and determine the urinary excretion of calcium. The presence of any other affected family member or reduced urinary calcium excretion is enough to suspect FHH, and this should be confirmed by the mutational analysis of the CASR gene, in order to establish the correct diagnosis, differentiated from primary hyperparathyroidism, to avoid unnecessary investigations or operations.

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