Dysfunction of the parathyroid glands is an important cause of complications after thyroid surgery. Intraoperative monitoring of the function of the parathyroid glands can be performed using parathyroid hormone (PTH) kinetics. Unilateral thyroid surgery is associated with a decreased risk for postoperative hypocalcemia (POH) and permanent hypoparathyroidism (PEH). We focused on unilateral thyroid surgery by monitoring the functionality of the parathyroid glands and comparing the perioperative PTH kinetics of patients with and without POH. In a prospective study, 143 patients scheduled for unilateral thyroid surgery underwent monitoring of perioperative changes in serum PTH and serum calcium levels, and of clinical symptoms of hypocalcemia. The rates of POH and PEH were 18.2 and 0%, respectively. In patients without POH, PTH significantly increased from the time of skin incision to the end of the operation and after the operation (20.1 pg/ml, IQR 15.5-26.8 vs. 21.4 pg/ml, IQR 16.4-29.5; p=0.005), which was not the case in patients who developed POH. In a multivariate analysis of predictive factors for POH, two parameters became significant, namely female gender (odds ratio 6.87, 95% confidence interval 0.92-51.01) and lower initial serum calcium levels (odds ratio 3.54*e(-8), 95% confidence interval 3.63*e(-12); 0.00). The rate of POH was unexpectedly high. Rather than intraoperative PTH declines, an unstable balance of factors that influence calcium metabolism likely is the major contributor to POH after unilateral thyroid surgery. There was no case of PEH after unilateral, primary thyroid surgery, which underlines the need for an individualized approach to the extent of resection.