Purpose Following hemithyroidectomy, the remnant thyroid lobe typically compensates avoiding the need for lifelong thyroxine, as required in total thyroidectomy. However, previous literature has identified the incidence of hypothyroidism following hemithyroidectomy at 20-30% and risk factors of Hashimoto’s disease, side of resection, and demographic variables. This retrospective study investigates the incidence, risk factors and natural history of hypothyroidism following hemithyroidectomy in a sizeable single surgeon cohort. We aim to better characterise this complication to improve preoperative risk stratification and patient outcomes. Methodology The study cohort consists of 279 patients who underwent hemithyroidectomy between 2013-2023 under a single surgeon. Incidence was calculated at 1, 3, 6 months, and overall, after hemithyroidectomy. Univariate and multivariate logistic regression identified risk factors of hypothyroidism. Results Hypothyroidism incidence at 1-month post-operatively was 20%. Risk factors included preoperatively diagnosed Hashimoto’s disease, histological lymphocytic thyroiditis, and right lobe resection. This study is first to report smoking as a protective factor against postoperative hypothyroidism. Of those hypothyroid at 1-month post-hemithyroidectomy, 45% were observed for natural recovery while 55% were treated with thyroxine. 8% of patients developed delayed hypothyroidism after 1-month. Conclusion This study utilised a single surgeon cohort to investigate incidence, risk factors and natural history of hypothyroidism following hemithyroidectomy, minimising variability in surgical technique and follow-up practices. Findings support known risk factors like Hashimoto’s disease but identify smoking as a novel protective factor in hypothyroidism development. These insights may assist clinical decision making in order to improve outcomes after hemithyroidectomy.