Circulating vitamin D metabolites and hypercalcaemia of malignancy.

Plasma concentrations of 1.25 dihydroxycholecalciferol were measured in 44 patients with malignancy associated hypercalcaemia and related to other hormonal regulators of calcium metabolism. Immunoreactive PTH concentrations were suppressed in all but 2 patients and, as a group, patients with hypercalcaemia of malignancy had lower 1.25 dihydroxycholecalciferol concentrations than normocalcaemic cancer patients. 1.25 dihydroxycholecalciferol concentrations were clearly detectable in a significant proportion (43%) of hypercalcaemia cases however, suggesting that in these patients the active vitamin D metabolite may contribute to the pathogenesis and maintenance of the hypercalcaemia by stimulating bone resorption, and/or by increasing absorption of calcium from the intestine. Measurement of plasma 1.25 dihydroxycholecalciferol concentration does not provide a wholly reliable method for distinguishing the hypercalcaemia of malignancy from primary hyperparathyroidism.