Familial hypocalciuric hypercalcaemia type 1 due to a rare <i>CASR</i> missense mutation: a delayed diagnosis in a man with chronic hypercalcaemia

Familial hypocalciuric hypercalcaemia (FHH) is a rare disorder that represents a minute but important part of the differential diagnosis of hypercalcaemia. We describe a man in his 60s who was re-referred to endocrinology because of hypercalcaemia thought to be due to primary hyperparathyroidism (PHPT) that had not been followed up for 13 years. In his early 50s, the hypercalcaemia was accompanied by normal serum parathyroid hormone (PTH) levels, normal 24-hour urinary calcium excretion and normal bone density and kidney imaging, and no parathyroid adenoma was demonstrated on neck imaging. He declined further tests and follow-up. Repeat PTH and 24-hour urinary calcium excretion were still normal, but a low calcium-creatinine clearance ratio prompted genetic testing for FHH. This was positive for a rare mutation in the calcium-sensing receptor gene, and a diagnosis of FHH type 1 was made. This case highlights the differential diagnosis of hypercalcaemia and the challenges in differentiating FHH and PHPT.