Ultrasonography is supposed to provide a reliable preoperative estimate of thyroid volume. This prospective study compares the estimated thyroid volume (EV) to real volume (MV), obtained by measuring the excised gland after surgery. One hundred one patients undergoing total thyroidectomy were selected for the study. Indications for surgery were: multinodular goiter, diffuse toxic goiter (DTG), uninodular disease. In all cases, ultrasound was repeated 1 month after surgery to verify complete thyroid removal. EV was underestimated in 89 cases; it perfectly matched the MV in 5 and was overestimated in 7. Mean EV was 28.3 mL (range, 7-50) and mean MV 36.2 mL (range, 7-76); this difference was statistically significant (p < 0.0001). Patients were then divided in groups according to EV (< or > than 25 mL) and thyroid morphology. Highly significant differences were found between all groups (p < 0.0001) except the DTG population, where the difference was less significant (p < 0.042). The study demonstrates that a correct preoperative measurement of the thyroid gland is not achievable because the volume estimated by ultrasound is largely underestimated in comparison to the real volume of the excised gland. Nevertheless ultrasound is more reliable in DTG than in other thyroid diseases. Increasing the number of cases may help to verify a new mathematical model.