Objective: To investigate the incidence, time to onset, and risk factors for the development of hypothyroidism after hemithyroidectomy. Background: The incidence and risk factors for hypothyroidism in patients undergoing partial thyroid surgery remains unclear. Hypothyroidism is an under–appreciated sequel of hemithyroidectomy. The early recognition of this postoperative complication m ay prevent symptom s of hypothyroidism and recurrent thyroid disease. Methods: We conducted a prospective observational study of patients undergoing hemithyroidectomy from July 2010 to July 2011 at department of General Surgery, CSM Medical University, Lucknow. Patients were analyzed for age, sex, family history of thyroid disease, pre and postoperative thyroid stimulating hormone (TSH), histological diagnosis, size of residual thyroid gland, histological evidence of thyroiditis, and time period to develop hypothyroidism. Results: Hypothyroidism was diagnosed in 22.7% of 26 patients. The mean time to diagnosis was within 4.5 months. Patients with postoperative hypothyroidism had a higher incidence of a high–norm al preoperative TSH level. Age, gender, family history of thyroid disease, thyroid pathology, and weight of resected tissue were not significant risk factors for hypothyroidism. Conclusions: An incidence of 22.7% is higher than anticipated and all patients should have postoperative thyroid function assessment. However, a high–norm al preoperative TSH levels is indications for close monitoring