<p>Hemithyroid agenesis is a rare congenital abnormality with an absence of one thyroid lobe with unknown etiology. The suggested mechanism is the bi-lobulation failure of the central anlage with the thyroid tissue migrating to one side of the midline. However, some studies reported thyroid nodules, autoimmune thyroid diseases, and thyroid neoplasms associated with hemithyroid agenesis. Hence, early and prompt diagnosis and follow-up are necessary. We report a case of a 34-year-old female patient who presented to our department with a left neck mass and an incidental finding of a left thyroid lobe agenesis. The patient was explained about her condition without further investigations for hemithyroid agenesis and kept under regular follow-up because the patient was euthyroid without any thyroid lesion. We also present a relevant review of the literature and discuss the management of this condition as an incidental finding.</p>