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 demonstrated in in vivo and in vitro studies to link the parathyroid hormone (PTH) secretion rate and calcium concentration changes. In uraemic patients different patterns of calcium-mediated PTH secretion were reported in different types of renal bone diseases and a shift to the right and a steeper slope has been observed in secondary hyperparathyroidism. To gain more information that could predict indexes for successful medical therapy we investigated the calcium-PTH sigmoidal relationship in 42 hyperparathyroid patients with different degrees of secondary hyperparathyroidism; we classified as moderate those patients presenting basal PTH (PTHbas) < 600 pg/ml and bone alkaline phosphatase (AP) < 500 U/l, and severe those with a PTHbas > or = 600 pg/ml and bone AP > or = 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 hyperparathyroidism patients had PTHmax, PTHmin and slope, calculated in absolute values and relative values, lower than severe hyperparathyroidism patients but they did not differ in the minimal to maximal PTH ratio. In the moderate group the PTHbas correlated with all the curve parameters except PTHmin, calculated both in absolute and percentage values, while in the severe group PTHmin was the only parameter correlating to the PTHbas. In conclusion, by performing the dynamic test, we found that some glands were not suppressible among moderate hyperparathyroidism patients.