International Journal of Anatomical Science 2012, 3(1): 12-13

Case Report

Agenesis of Isthmus of Thyroid Gland – A Cadaveric Study

Radhika Krishnan J, Priya G, Sundarapandian S.

Department  of  Anatomy,  SRM  Medical  College  and  Research  Centre,  SRM  University, Kattankulathur 603 203, Tamil Nadu, India.

Key Words: isthmus, agenesis of thyroid isthmus, congenital absence of isthmus

Abstract: The thyroid gland is an endocrine gland, situated in the lower part of the front and sides of the neck. The gland consists of right and left lobes that are joined to each other by Isthmus lying anterior to the second, third and fourth tracheal rings. A number  of  morphological  variations  and  developmental  anomalies  of  the  thyroid gland have been reported (Dixit D et al., 2009). In this study, an absence of the Isthmus of thyroid gland is observed. The incidences of agenesis of Isthmus, along with the developmental and clinical significance are discussed.

The thyroid gland, brownish red and highly  vascular,  placed  anteriorly  in  the lower neck  consist  of right  and  left  lobes connected by a narrow median Isthmus. The Isthmus of thyroid gland measures 1.25 cm transversely and vertically (Standring et al.,2005). Absence is quiet rare in humans. The incidence of agenesis of the thyroid isthmus has been reported to vary from 5 – 10 % by Pastor et al., (2006) and from 8-10% by Marshall (1895). The knowledge of various developmental anomalies of the gland and variation in vascular relations will help the surgeon in better planning of a safe surgery.

 Case Report

 During the routine dissection at SRM medical college, absence of isthmus of thyroid gland was noticed. The thyroid isthmus connects the lower parts of the lobes anterior to the tracheal cartilages. In the present study the isthmus is absent, only tracheal rings observed and along the lateral side conical shaped thyroid lobes observed. The  superior  thyroid  artery  branch  of  the external carotid artery runs downwards and reached the upper pole of the lobe and it divided into anterior and posterior branches. The anterior branch descends on the anterior border of the lobe and anastomosed with a branch  from  the  inferior  thyroid   artery. There is no anastomosis of anterior branch of superior thyroid artery with its fellow of the opposite side. The posterior branch of superior thyroid artery anastomosed with the ascending branch of the inferior thyroid artery. The inferior thyroid vein lies on the tracheal   rings   in   the   place   of   thyroid isthmus; they formed a plexus and drained into  the  left  brachiocephalic  vein. Knowledge of this type of variations gives a surgeon to avoid major complications.

 Discussion

 Agenesis  of  the  thyroid  isthmus  is the complete and congenital absence of the thyroid isthmus. During development, the thyroid gland appears as an epithelial proliferation in the floor of the pharynx between   the   tuberculum   impar   and   the copula and a point later indicated by the foramen caecum. The thyroid descends in front of the pharyngeal gut as a bilobed diverticulum. During this migration, the thyroid remains connected to the tongue by a narrow canal, the thyroglossal duct, which disappears later, then descends in front of the hyoid bone and laryngeal cartilages and reaches its final position in front of the trachea in the seventh week. By then, it has acquired  a small  median isthmus  and  two lateral lobes (Sadler, 2010). Sometimes during this development only two lateral lobes are formed without median isthmus which results in agenesis of thyroid isthmus. The   absence   of   the   isthmus   can   be associated with dysorganogenesis (Schanarder and de oliveire, 2008). This agenesis may be because of high division of the thyroglossal duct giving rise to two independent thyroid lobes with absence of isthmus (Schanarder and de oliveire, 2008). Clinically the diagnosis of agenesis of the isthmus can be done with scintigraphy. The diagnosis can also be done with ultrasonography,  computerized  tomography(CT),  Magnetic  resonance  imaging  (MRI)

Fig. 1 Photograph showing absence of isthmus

 Dixit D,  Shilpa MB, Harsh MP, Ravishankar MV (2009) Agenesis of isthmus of thyroid gland in adult human cadavers: A case series. Cases J,  2:6640.Duh QY, Ciulla TA, Clark OH (1994) Primary parathyroid hyperplasia associated with thyroid hemiagenesis and agenesis of the isthmus. Surgery, 115: 257-263.

Marshall CF (1895) Variations in the  form of the thyroid gland in man. J Anat Physiol, 29: 234-239.

Pastor VJF, Gil VJA, De Paz Fernandez FJ, Cachorro MB (2006) Agenesis of the thyroid isthmus. Eur J Anat, 10: 83-84.

Sadler TW (2010) Langman’s Medical Embryology,11th    edition.  Lippincott  Williams  &  Wilkins, pp278- 279.

Schanarder A, de oliveire PJ (2008) Thyroid isthmus agenesis associated with solitary nodule: A case report. Cases J, 1:211.

Standring  S,   Herold  E,   Healy  JC,   Jhonson  D,(Sadler, 2010).