PARTIAL ESCAPE OF MAGNESIUM FROM THE RENAL ACTION OF PARATHYROID HORMONE IN HYPERPARATHYROIDISM

The paradox of the potentially hypermagnesaemic actions of parathyroid hormone (PTH) and the occurrence of hypomagnesaemia in hyperparathyroidism prompted this study of renal handling of calcium and magnesium in hyperparathyroidism. In 15 patients the serum ultrafiltrable calcium (UFCa) and magnesium (UFMg), the urinary excretion of these ions and of sodium, and the clearance of creatinine (CCr), were determined. The fractional reabsorptions of calcium (TRCa%) and of magnesium (TRMg%) were calculated. At levels of CCr above 25 ml/min TRMg% was found to be inversely correlated to UFCa R = -0.73, p <0.01) and to urinary sodium excretion per unit of CCr (R = -0.87, p <0.001), while no correlation could be established between UFCa and sodium excretion. It is concluded that a high concentration of calcium in the ultrafiltrate and a high urinary sodium excretion both appear to depress TRMg%, enabling normomagnesaemia to be maintained or hypomagnesaemia to develop, despite the enhancing effect of PTH on the tubular reabsorption of magnesium (TRMg).