Thyroid hemiagenesis: description of two clinical cases, anatomical review, and critical synthesis of the literature

Introduction: Thyroid hemiagenesis (THA) is a rare congenital thyroid condition in which one of the lobes of the gland fails to develop. The etiology remains uncertain, with the main hypotheses suggesting genetic alterations in thyroid embryogenesis associated with failure of the gland to descend to its final position. Additionally, the condition may be present in both asymptomatic and euthyroid patients, as well as in those with thyroid diseases such as hyperthyroidism, nodular goiter, and neoplasia. Objective: To critically review the literature on the subject, detailing known aspects of anatomy in THA, and the demographic findings of patients with this condition, while also adding two case reports to the literature. Methods: An extensive, systematized review of the MEDLINE database was carried out using the descriptor “thyroid hemiagenesis”. A total of 214 related articles up to October 2022 were retrieved. Out of these, 154 described new cases in the literature and contained demographic and epidemiological data of the reported patients. Studies without new case reports, systematic reviews, and those lacking described or available demographic data—rendered inadmissible for this review—were excluded from the analysis. Results: Our review identified the following prevalence and characteristics: 520 cases of THA were reported, and in 50 of them, absence of the isthmus also coexisted. Several thyroid conditions and statuses have been associated with hemiagenesis of the gland. Conclusion: THA is more prevalent in female, young, and euthyroid patients. Although rare, it is vital for head and neck surgeons to be aware of this possibility, thereby avoiding unnecessary manipulation of the absent thyroid bed and, consequently, preventing complications.