Background. Weight gain is frequently reported after hemithyroidectomy but the significance is recently discussed. Therefore, the aim of the study was to examine changes in body weight of hemithyroidectomized patients and to evaluate if TSH increase within the reference range could be related to weight gain. Methods. In a controlled follow-up study, two years after hemithyroidectomy for benign euthyroid goiter, postoperative TSH and body weight of 28 patients were compared to preoperative values and further compared to the results in 47 matched control persons, after a comparable follow-up period. Results. Two years after hemithyroidectomy, median serum TSH was increased over preoperative levels (1.23 versus 2.08 mIU/L, P < 0.01) and patients had gained weight (75.0 versus 77.3 kg, P = 0.02). Matched healthy controls had unchanged median serum TSH (1.70 versus 1.60 mIU/L, P = 0.13) and weight (69.3 versus 69.3 kg, P = 0.71). Patients on thyroxin treatment did not gain weight. TSH increase was significantly correlated with weight gain (r = 0.43, P < 0.01). Conclusion. Two years after hemithyroidectomy for benign euthyroid goiter, thyroid function is lowered within the laboratory reference range. Weight gain of patients who are biochemically euthyroid after hemithyroidectomy may be a clinical manifestation of a permanently decreased metabolic rate.