Thyroid hemiagenesis (TH) is a rare congenital abnormality of the thyroid gland, characterised by the absence of one lobe. The true prevalence of this congenital abnormality is not known because the absence of one thyroid lobe usually does not cause clinical symptoms by itself. Between 1970 and 2010, 329 cases of TH have been reported. It is interesting to note that most cases have an agenesis of the left lobe (80% of cases) followed by the isthmus (44-50% of cases). Although the female to male ratio was 1:1.4 in 24,032 unselected 11-to 14-yr-old schoolchildren from South-eastern Sicily, several other reports have documented a higher prevalence in women, which may indicate a possible gender association. Most cases of TH are diagnosed when patients present a lesion in the functioning lobe. The functioning lobe of the thyroid gland can be a site of pathological changes similar to a normally developed gland and may present a spectrum of diseases like multinodular goiter, colloid goiter, follicular adenoma, thyroiditis, hypothyroidism and hyperthyroidism. In three of our patients, TH was associated with Hashimoto thyroiditis (n = 1) and with subclinical hypothyroidism (n = 2). The frequency of thyroid abnormalities in patients with TH varies with age, due to the longer exposure of the hemi-agenetic gland to TSH overstimulation in older patients. This could explain the controversy about the benign character of this anomaly. Other extrathyroidal lesions, such as parathyroid adenoma or hyperplasia, cervical thymic cysts, ectopic sublingual thyroid gland and thyroglossal duct cyst have been reported with TH. Therefore, systematic follow-up of all identified cases is recommended.