[Thyroid hemiagenesis: report of five cases and literature review].

Thyroid hemiagenesis is a highly uncommon congenital abnormality, as attested by the fact that only approximately 300 cases had been reported up to 2010. Thyroid hemiagenesis is defined as the lack of development of one thyroid lobe or one thyroid lobe and thyroid isthmus, and was first reported by Handsfield-Jones in 1866. Its prevalence in the general population, estimated from screening ultrasound studies in different countries, is approximately 0.06% (ranging from 0.05% to 0.2%), with left thyroid lobe agenesis being more common.