PTH and parathyroid gland adenoma: As shown in Figs. 2 and 3, a positive correlation between preoperative PTH levels and adenoma volume (r = 0.461, p < 0.0001) was seen, while no significant correlation was found between the volume of parathyroid adenoma and the intraoperative drop in PTH levels (r = 0.182, p = 0.067).