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We hypothesized that impaired peripheral sensitivity to parathyroid hormone (PTH) may play a role in reelevation of PTH after successful operation for primary hyperparathyroidism (pHPT).Factors affecting reelevation of PTH were determined in 90 patients who underwent parathyroidectomy for pHPT. PTH/nephrogenous cyclic adenosine monophosphate ratio, as an index of renal resistance to PTH, was examined in relation to factors shown to influence reelevation of PTH.Serum PTH levels were elevated above the upper limit of normal in 23 patients (26%) at 1 week and in 39 patients (43%) at 1 month after parathyroidectomy. These 39 normocalcemic patients with elevated serum PTH at 1 month after parathyroidectomy had a higher preoperative serum level of PTH and lower serum phosphate and 25-hydroxyvitamin D (25OHD) concentrations than those with normal PTH (n = 59). Elevated PTH and low 25OHD were shown by multivariate analysis to be significant predictors of reelevation of PTH. Renal resistance to PTH was higher in patients with vitamin D deficiency or renal insufficiency than in patients with normal serum vitamin D concentrations or normal renal function, and it increased according to increases in levels of PTH.The mechanism of PTH reelevation in patients with pHPT after successful parathyroidectomy appears to be renal resistance to PTH.