HYPERCALCEMIA DUE TO PARATHYROID HORMONE-RELATED PROTEIN - LONG-TERM CIRCULATING LEVELS MAY NOT REFLECT TUMOR-ACTIVITY

Parathyroid hormone related protein is responsible for the hypercalcae mia caused by many tumours. Measurement of parathyroid hormone-related protein is becoming more accessible with the introduction of commerci al 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 immu noradiometric assay. We demonstrated an initial fall in parathyroid ho rmone-related protein and calcium levels after surgery and a rise in b oth before clinical relapse. However, the clinical relapse was itself associated with a fall in serum parathyroid hormone-related protein, n ephrogenous cAMP and calcium, suggesting that the tumour had stopped p roducing parathyroid hormone-related protein or perhaps that post-tran slational processing had occurred as the tumour advanced. The tumour w as investigated for parathyroid hormone-related protein mRNA content u sing reverse transcriptase polymerase chain reaction, both at diagnosi s 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 parathyro id hormone-related protein production fell substantially as the tumour advanced. This case suggests that, although demonstration of parathyr oid hormone-related protein in hypercalcaemia is useful for diagnosis, tumoral secretion of this product may alter.