Background and Objectives: Many patients experience hypothyroidism after lobectomy. The purpose of this study was to investigate the effects of the extent of surgery (isthmus-saving lobectomy vs. isthmus-removing lobectomy) on postoperative hypothyroidism after lobectomy. Materials and Methods: In total, 144 consecutive benign or low-risk papillary thyroid cancer patients with euthyroid status were retrospectively enrolled from January 2016 to June 2018; 100 patients underwent isthmus-saving lobectomy (IS group), and 44 patients underwent isthmus-removing lobectomy (IR group). Thyroid function tests (TSH) were performed at 3, 6, 12, and 24 months after surgery. Levothyroxine was administered to patients with overt hypothyroidism and those with subclinical hypothyroidism who complained of obvious symptoms of hypothyroidism. Results: The TSH level was significantly lower in the IS group at 3 and 6 months after surgery (p=0.049 and 0.026, respectively). Among patients with TSH ＜2 μIU/mL preoperatively, 17 patients (17/66=25.8%) in the IS group had significantly maintained TSH levels at ＜2 μIU/mL compared with 2 patients (2/21=9.5%) in the IR group even after surgery (p=0.039). Postoperative overt hypothyroidism was found in 5 patients (5.0%) in the IS group and 6 patients (13.6%) in the IR group (p=0.048), and the numbers of patients who took medication after the surgery, 10 (10%) and 14 (31.8%), were significantly lower in the IS group (p=0.001). Conclusion Isthmus-saving lobectomy in patients with euthyroid status reduced the incidence of postoperative hypothyroidism and thyroid hormone replacement and helped in maintaining a low TSH level when compared with isthmus-removing lobectomy.