THE PTH-CALCIUM RELATIONSHIP CURVE IN SECONDARY HYPERPARATHYROIDISM, AN INDEX OF SENSITIVITY AND SUPPRESSIBILITY OF PARATHYROID-GLANDS

A sigmoidal relationship, fitting a four-parameter model, has been dem onstrated in in vivo and in vitro studies to link the parathyroid horm one (PTH) secretion rate and calcium concentration changes. In uraemic patients different patterns of calcium-mediated PTH secretion were re ported in different types of renal bone diseases and a shift to the ri ght and a steeper slope has been observed in secondary hyperparathyroi dism. To gain more information that could predict indexes for successf ul medical therapy we investigated the calcium-PTH sigmoidal relations hip in 42 hyperparathyroid patients with different degrees of secondar y hyperparathyroidism; we classified as moderate those patients presen ting basal PTH (PTHbas) <600 pg/ml and bone alkaline phosphatase (AP) <500 U/l, and severe those with a PTHbas greater than or equal to 600 pg/ml and bone AP greater than or equal to 500 U/l. Changes in ionized calcium (iCa) were induced by calcium-free dialysis on the first day, to induce hypocalcaemia up to serum iCa 3.5 mEq/l, and calcium 8 mEq/ l dialysis on the third day, to induce hypercalcaemia. The moderate hy perparathyroidism patients had PTHmax, PTHmin and slope, calculated in absolute values and relative values, lower than severe hyperparathyro idism patients but they did not differ in the minimal to maximal PTH r atio. In the moderate group the PTHbas correlated with all the curve p arameters except PTHmin, calculated both in absolute and percentage va lues, while in the severe group PTHmin was the only parameter correlat ing to the PTHbas. In conclusion, by performing the dynamic test, we f ound that some glands were not suppressible among moderate hyperparath yroidism patients.