The effects of intravenous and oral cimetidine in primary hyperparathyroidism

In vitro studies suggest that cimetidine affects parathyroid hormone secretion. It has been reported that cimetidine (Ci) reduces serum immunoreactive parathyroid hormone (iPTH) in hyperparathyroid subjects (HPTH) and chronic renal failure patients. Studies were performed to determine the effects of intravenous and 8 days of oral Ci on serum iPTH, total calcium (TCAL), ionized calcium (ICAL), phosphorus (PHO), and magnesium (MAG), as well as on the urinary excretion of calcium, PHO, and cAMP. Ci, 300 mg, was injected intravenously into 11 HPTH and 9 of these received normal saline as a control. Eight normal subjects (N) were similarly studied. Results analyzed by analysis of variance indicated that although IV Ci and placebo both caused a significant change with time in iPTH, in HPTH and N, the fall was greater with Ci (P less than 0.005). Intravenous Ci decreased iPTH in HPTH approximately 36% (P less than 0.001) and N approximately 25% (P less than 0.025). In HPTH and N, ICAL was affected by neither IV cimetidine nor placebo. IV Ci decreased TCAL and MAG in a similar fashion when compared to placebo, though change in each variable was statistically insignificant. In 6 HPTH an 8 day period of oral Ci, 300 mg every 8 hours, produced no consistent changes in the measured serum variables. In summary, though the acute effect of IV Ci was to reduce iPTH, it appears to have no independent effect on ICAL or TCAL. Oral Ci during an 8 day period of administration had no consistent effect on mineral metabolism. We conclude that the therapeutic efficacy of oral Ci in HPTH has yet to be demonstrated.