Agenesis of Isthmus of the Thyroid Gland in a Patient with Graves-Basedow Disease and a Solitary Nodule

The thyroid is a vascular endocrine gland with two lateral lobes connected by a narrow, median isthmus. Although a wide range of congenital anomalies of the thyroid gland has been reported in the literature, agenesis of the thyroid isthmus is a very rare congenital anomaly. Thyroid isthmus agenesis does not manifest clinical symptoms, and it can be confused with other thyroid pathologies. We describe a patient with no isthmus of the thyroid, associated with Graves-Basedow disease. Thyroid isthmus agenesis should be kept in mind in order for surgical procedures involving thyroid pathologies to be carried out safely.


Introduction
e thyroid gland consists of two lobes connected by an isthmus, which also includes thyroid tissue. ere are numerous studies investigating the length and depth of the thyroid isthmus by a means of age and evaluating the relationship between reference values and thyroid disorders [1]. e incidence of isthmus agenesis, referring the total absence of the isthmus of the thyroid gland, is not fully known, and the condition is reported through case reports and cadaver studies.
e most common cause of hyperthyroidism is Graves-Basedow disease. Hyperthyroidism usually accompanies with diffuse goiter, ophthalmopathy, and skin changes [2]. ere are reports of thyroid developmental anomalies such as single lobe hemiagenesis accompanying Graves' disease [3]. We present a case of absence of the thyroid isthmus associating with Graves-Basedow disease.

Discussion
e isthmus of the thyroid gland contains normal thyroid tissues, with a dimension of 1.25 cm transversely and vertically [1,4]. Isthmus agenesis of the thyroid gland is an uncommon developmental anomaly. According to a cadaveric study, isthmus agenesis is seen 5 fold more in males. [4]. Although the incidence varies from 3% to 10% in different studies, the true �gure is uncertain [5][6][7], possibly because of nosymptom population. is is diagnosed only when there is a symptom, such as nodular goiter, thyroiditis, or primary carcinoma [8].
e underlying etiology of isthmus agenesis has not been well de�ned. Genetic factors and defects in embryological development seem to play an important role in thyroid isthmus agenesis, mutations in the genes responsible for the development of the thyroid may be associated with isthmus agenesis, especially TITF1-2 genes [9] and chromosome 22 [10]. In embryogenesis, the thyroid gland begins to develop in the early days of gestation. e thyroglossal duct grows from endodermal thickening in the �oor of the pharynx at the level of second and third pharyngeal pouches. It descends to its �nal location anterior to the trachea and bifurcates to form the two thyroid lobes connected by a median isthmus [5][6][7][8].
As described by Pastor Vázquez et al. [8], a fusion anomaly of the thyroglossal duct in the midline leads to two independent lobes with no isthmus of the thyroid gland.
yroid gland may have various developmental anomalies. To the best of our knowledge, this is the �rst case report of isthmus agenesis in the English literature, associated with Graves-Basedow disease with a solitary nodule. Due to its rare nature, isthmus agenesis should be kept in mind for safe surgery to avoid complications during neck operations, and rare but possible coincidence with Graves' disease that also has a thyroid nodule. �on�ic� of �n�eres�s is paper has been read and approved by all authors. e authors of this paper report no con�ict of interests.