Some patients experience long-term voice change after thyroidectomy. One of the most common symptoms of voice change is pitch lowering, which is closely related to unfavorable voice quality. Here we observed voice outcomes for 6 months of follow-up after thyroidectomy to identify factors closely related to low-pitched voice. We retrospectively reviewed the records of 2,297 patients who underwent thyroidectomy between January 2014 and December 2017. All the patients had their vocal status examined using videostroboscopy, acoustic voice analyses, aerodynamic study, and Thyroidectomy-Related Voice Questionnaire scores. We stratified patients into 2 groups (low-pitched voice versus favorable voice) according to pitch lowering (reduction in speaking fundamental frequency ≥12 Hz 1 month after thyroidectomy compared to the preoperative value). We compared preoperative data with postoperative data collected 1, 3, and 6 months after thyroidectomy to identify factors contributing to low-pitched voice. Univariate logistic regression analyses showed that factors related to low-pitched voice were female sex, older age, low body weight, short stature, and a high positive lymph node ratio. Multivariate analyses showed that female sex and older age were significantly associated with a negative prognosis for low-pitched voice 1 month after thyroidectomy (odds ratios 0.41 and 1.04, respectively; P < .001). Receiver operating characteristic curves for predicting sustained low-pitched voice during 6 months showed that speaking fundamental frequency ≥12.48 Hz 1 month after thyroidectomy was the optimal cutoff value, with 87.9% sensitivity and 95.8% specificity (P < .001). Female sex and older age are strongly associated with increased risk for low-pitched voice after thyroidectomy. Speaking fundamental frequency ≥12.48 Hz 1 month after thyroidectomy can be used to predict sustained low-pitched voice after thyroidectomy.