Familial hypocalciuric hypercalcaemia (FHH) is a rare autosomal dominant disorder due to a loss-of-function mutation in the CASR gene encoding the calcium-sensing receptor. It can be challenging to differentiate FHH from primary hyperparathyroidism as both are characterised by PTH dependent hypercalcaemia and can be asymptomatic.
In this case report, we present a 64 year old gentleman referred to our clinic with persistent hypercalcaemia after originally being diagnosed with primary hyperparathyroidism and treated with parathyroidectomy. We outline our patient’s history and investigative findings which were consistent with FHH. We also present the results of his genetic testing where he was found to have a heterozygous unclassified variant (c.2540G>A;p.Gly847Asp) of the Calcium Sensing Receptor gene.
This CASR gene variant has not previously been reported to be associated with FHH. We discuss our suspicion this variant is pathogenic given its position, effect on polar charge and proximity to several known pathogenic CASR gene variants. Finally, we discuss the value of further genetic testing in our patient’s relatives to confirm segregation of this variant with the FHH phenotype with respect to guiding future management of our patient and others affected.