BACKGROUND Thyroid hormones play an important role in body weight regulation. In this study, we investigated which body composition parameters cause a change in body weight after total thyroidectomy. METHODS We performed a retrospective cohort study of a prospectively maintained database of patients who underwent total thyroidectomy. Demographics, thyroid function tests, indications for surgery, final pathology, and postoperative thyroid status were collected. Body composition analyses measured by the bioelectrical impedance analysis method were recorded at two-time points, 12 months apart. RESULTS Forty-four patients were included in the study with a mean age of 51.6 years. There were statistically significant increases in weight (p=0.049), body mass index (p=0.021), and fat mass (p=0.001) over time. While 12 patients (27.3%) lost or maintained weight, 32 patients (72.7%) gained weight. There was no significant difference in age, sex, preoperative thyroid function tests, postoperative thyroid status, or pathology between those who gained weight and those who did not. Although changes in all body composition parameters were higher in males than in females, these differences were not significant overall. Multivariable regression analysis revealed a significant positive relationship between increase in fat mass and baseline free-T3 (p=0.041) and found that lower baseline percent body fat was a significant factor for greater fat mass gain (p=0.016). However, no predictors of change in weight were identified. CONCLUSIONS We conclude that total thyroidectomy results in a significant change in body weight and fat mass. Higher free-T3 and lower percent body fat at baseline were significant factors of fat mass gain.