Hypercalcaemia due to parathyroid hormone‐related protein: long‐term circulating levels may not reflect tumour activity

Parathyroid hormone-related protein is responsible for the hypercalcaemia caused by many tumours. Measurement of parathyroid hormone-related protein is becoming more accessible with the introduction of commercial assays. We report a case of hypercalcaemia of malignancy secondary to parathyroid hormone-related protein in a woman with renal carcinoma. The parathyroid hormone-related protein was assayed using a new immunoradiometric assay. We demonstrated an initial fall in parathyroid hormone-related protein and calcium levels after surgery and a rise in both before clinical relapse. However, the clinical relapse was itself associated with a fall in serum parathyroid hormone-related protein, nephrogenous cAMP and calcium, suggesting that the tumour had stopped producing parathyroid hormone-related protein or perhaps that post-translational processing had occurred as the tumour advanced. The tumour was investigated for parathyroid hormone-related protein mRNA content using reverse transcriptase polymerase chain reaction, both at diagnosis in surgically removed material, and using post-mortem specimens. The level of parathyroid hormone-related protein mRNA, while present, was much reduced in the recurrent tumour suggesting that active parathyroid hormone-related protein production fell substantially as the tumour advanced. This case suggests that, although demonstration of parathyroid hormone-related protein in hypercalcaemia is useful for diagnosis, tumoral secretion of this product may alter.