Circulating levels of midregional parathyroid hormone‐related protein in hypercalcaemia of malignancy

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Abstract
OBJECTIVE We have developed and evaluated a sensitive radioimmunoassay directed against the midregional part of parathyroid hormone‐related protein (PTHrP), which is involved in the syndrome of humoral hypercalcaemia of malignancy. PATIENTS Midregional PTHrP levels were studied in 41 consecutive inpatients with malignancy and hypercalcaemia, 32 normocalcaemic patients with malignancy, 21 patients with primary hyperparathyroidism, 34 patients with renal failure, and 87 normals. MEASUREMENTS The assay used an antiserum against the midregional amino acid residues 53–84 of PTHrP and PTHrP(1–86) as label and standard. Midregional PTHrP was stable in serum and plasma and could be measured directly without sample extraction. RESULTS Normal plasma concentrations ranged from undetectable ( 850 μmol/l. In hypercalcaemia caused by solid tumours, midregional PTHrP was elevated in 81% (22 of 27) of patients, ranging from undetectable to 203 pmol/l (median: 40 pmol/l). In these patients serum calcium correlated positively with PTHrP (P<0.01). Mean PTHrP levels were indistinguishable in subgroups with and without metastatic skeletal disease. The mechanism of hypercalcaemia in 14 patients with haematological malignancy was apparently different, since all but one had normal or only marginally elevated PTHrP levels. In 21 patients with primary hyperparathyroidism midregional PTHrP was normal in 20. The assay was therefore especially useful in distinguishing the latter condition from humoral hypercalcaemia of malignancy as the second major cause of hypercalcaemia. PTHrP was normal in all 32 patients with normocalcaemic malignancy. CONCLUSION This radioimmunoassay of midregional PTHrP provides high diagnostic sensitivity in the identification of humoral hypercalcaemia of malignancy caused by solid tumours. The assay should therefore be useful in the differential diagnosis of hypercalcaemia.