Parathyroid hormone responsiveness in hypoparathyroidism with hypomagnesemia.

The failure to respond normally to parathyroid hormone (PTH) administration has been reported in patients with severe hypomagnesemia. A patient with hypoparathyroidism and a markedly decreased serum concentration of magnesium (0.7 mEq/liter), but a normal red blood cell magnesium level, is described who increased serum calcium concentration and decreased per cent renal tubular reabsorption of phosphate when parathyroid extract was given. It is suggested that PTH responsiveness in hypomagnesemic patients may, at least in part, be dependent upon the adequacy of intracellular magnesium stores. This interpretation is supported by the normal cellular (red blood cell) magnesium concentrations observed in this patient and in comparable studies in which PTH responsiveness in the presence of hypomagnesemia was demonstrated. In addition, a failure of optimal renal conservation of magnesium was noted to occur in this patient since, despite hypomagnesemia, urinary magnesium excretion was greater than the 1 mEq/day loss that is seen when magnesium conservation is induced by means such as dietary restriction.