Objective: To investigate the alterations in thyroid function in different aged patients after unilateral thyroid lobectomy for benign thyroid tumor and the necessity of postoperative hormone replacement therapy (HRT). Methods: One-hundred and twenty patients with benign thyroid tumor undergoing unilateral thyroid lobectomy were divided according to their ages, into young group (less than or equal to 30 years, 23 cases), middle-aged group (greater than 30 to less than 60 years, 69 cases) and elderly group (greater than 60 years, 28 cases). The levels of free triiodothyronine (FT3), free thyroxine (FT4) and thyroid-stimulating hormone (TSH) in each group of patients were measured before operation and at 1 week, 1 month, 3 and 6 months, 1 year and 2 years after operation, respectively. Results: In young group, FT3 level was significantly decreased at postoperative week 1 and month 1 (P<0.05), but showed no statistical difference (P>0.05) at all later time points compared with preoperative level; FT4 level had no statistical difference at any postoperative time point compared with preoperative level (P>0.05); TSH level was significantly increased at postoperative week 1, month 1 and 3 (P<0.05) but showed no statistical difference at all later time points compared with preoperative level (P>0.05). In middle-aged group, either level of FT3, FT4 or TSH had no statistical difference at any postoperative time point compared with their preoperative levels (P>0.05). In elderly group, FT3 level was significantly decreased at postoperative week 1, month 1 and 3 (P<0.05), but showed no statistical difference (P>0.05) at all later time points compared with preoperative level; FT4 level was significantly decreased at postoperative month 1, 3 and 6 (P<0.05), but showed no statistical difference (P>0.05) at all other time points compared with preoperative level; TSH level was significantly increased at postoperative week 1, month 1, 3 and 6 (P<0.05) but showed no statistical difference at the later time points compared with preoperative level (P>0.05). No clinical hypothyroidism symptoms were noted in any of the 3 groups of patients. Conclusion: The recovery process of thyroid function varies among patients of different ages after hemithyroidectomy for benign thyroid tumor. HRT is unnecessary for middle-aged patients due to their strong compensatory ability, while a 6-month HRT for young patients due to a relatively large demand for thyroid hormones, and a 1-year HRT for elderly patients due to poor compensatory ability, are justified after operation.