Thyroid hemiagenesis is a rare form of thyroid dysgenesis, in which one thyroid lobe fails to develop. The true prevalence of this rare abnormality is about 0.05-0.2% in normal population. We aimed to determine prevalence of thyroid hemiagenesis in patients with various thyroid disorders and a normal population in a mild to moderate iodine-deficient area. The clinical and thyroid ultrasonography records of 4,833 patients who presented with various thyroid disorders were reviewed. In addition, ultrasonographic data of two large surveys carried out for the community screening of iodine status of children (n = 4,772) and thyroid disorders of adult subjects (n = 2,935) were analyzed. In patients with thyroid disorders, we found 12 cases with thyroid hemiagenesis (0.25%). Thyroid hemiagenesis was due to the agenesis of the left lobe in all cases. The underlying thyroid diseases were Hashimoto's thyroiditis (n = 4), euthyroid multinodular goiter (n = 4), and toxic adenoma (n = 1). Three subjects have no underlying thyroid disease. In ultrasonography screening of normal population, altogether, the absence of the left lobe was detected in only two cases, indicating a true prevalence of thyroid hemiagenesis of 0.025%. None of the reviewed patients had thyroid dysfunction. Our community-based data is in accordance with previous studies in terms of prevalence and male-to-female ratio.