The Effect of Thyroid Lobe Volume on the Common Carotid Artery Blood Flow in Thyroidectomy Position

<b>Background/Objectives</b>: This study investigates the effect of thyroid lobe size on common carotid artery hemodynamics during thyroidectomy. While prior research has reported reduced carotid blood flow during the procedure, the impact of thyroid size remains unclear. We hypothesized that larger thyroid lobes may influence carotid flow dynamics via external compression. <b>Methods</b>: Adult patients undergoing elective thyroidectomy were prospectively included. Doppler ultrasonography measured carotid artery diameters and flow characteristics at three time points: before anesthesia induction, after induction, and after surgical positioning. Regional cerebral oximetry was recorded. Each carotid artery was analyzed separately. <b>Results</b>: Data from 202 carotid arteries (132 patients) were analyzed. Baseline carotid diameters and flow velocities were similar between patients with normal and large thyroid lobes. Anesthesia induction reduced flow velocities in all patients. After surgical positioning, patients with large thyroid lobes had significantly increased peak systolic velocity, leading to an overestimation of carotid blood flow, when using formula-based calculations. Manually traced Velocity Time Integral confirmed the increase in peak systolic velocity and a shortened systolic/diastolic ratio in these patients. Receiver operating characteristic analysis identified a thyroid lobe volume cutoff of 19.7 mL (AUC: 0.93, Sensitivity: 85%, Specificity: 98%). Regional cerebral oxygen saturation remained unchanged (<i>p</i> > 0.05). <b>Conclusions</b>: Larger thyroid lobes are associated with altered carotid flow dynamics during thyroidectomy, emphasizing diastolic flow. While these findings provide insight into thyroid-related hemodynamic changes, their applicability to patients with pre-existing carotid stenosis or peripheral artery disease remains uncertain, as our study population did not include such cases.