Dissociation between changes in immunoreactive parathormone and its biological indices induced by cimetidine in...

In three out of four patients with primary hyperparathyroidism, 2 000 mg of cimetidine daily caused a reduction of immunoreactive parathormone (iPTH) when measured at 8.30 and 11.30 on days 16 and 17 on treatment. Serum Ca, PO4 and maximal tubular reabsorption of PO4 remained unchanged. Excretion of cAMP/100 ml GFR remained elevated to at least the same extent as before treatment. Two patients, in whom cimetidine treatment was continued for an additional 4 weeks, did not show further hormonal or biochemical changes compared with the evaluation on days 16 and 17. We conclude that reduction of iPTH is not accompanied by any change in biological activity of this hormone. The reason for this discrepancy remains unclear.