|Year : 2015 | Volume
| Issue : 2 | Page : 115-118
Thyroid anomalies with its embryological and clinical correlation
R Sharada1, Chandni Gupta2, Sneha Kalthur2, Shakuntala R Pai2, GB Rairam1
1 Department of Anatomy, S. Nijalingappa Medical College, Bagalkot, India
2 Department of Anatomy, Kasturba Medical College, Manipal University, Manipal, Karnataka, India
|Date of Web Publication||16-Mar-2015|
Department of Anatomy, Kasturba Medical College, Manipal - 576 104, Karnataka
Source of Support: None, Conflict of Interest: None
Objective: Thyroid gland is the first endocrine gland to start developing in the embryo. It is well-known for its developmental anomalies such as persistence of pyramidal lobe, thyroglossal duct, agenesis of isthmus, agenesis or hemiagenesis of thyroid gland, or aberrant thyroid glands. These anomalies may cause clinical functional disorders and should be kept in mind while doing surgery on the gland. Materials and Methods: A study was conducted on 37 cadavers to look for the anomalies of thyroid gland. Results: We observed six cases of agenesis of the isthmus of the thyroid gland. Four cases were of agenesis of isthmus alone, fifth case had isthmus agenesis with duplication of the thyroglossal duct remnant and a pyramidal lobe, and sixth one had isthmus agenesis with a thyroglossal duct which divides before entering the two lateral lobes. Conclusion: Such knowledge is very essential for the surgeon while performing thyroid surgeries. Understanding the surgical anatomy of the thyroid gland and its possible variations is essential for safe and effective surgery.
Keywords: Agenesis, isthmus, thyroid gland, thyroglossal duct
|How to cite this article:|
Sharada R, Gupta C, Kalthur S, Pai SR, Rairam G B. Thyroid anomalies with its embryological and clinical correlation. CHRISMED J Health Res 2015;2:115-8
|How to cite this URL:|
Sharada R, Gupta C, Kalthur S, Pai SR, Rairam G B. Thyroid anomalies with its embryological and clinical correlation. CHRISMED J Health Res [serial online] 2015 [cited 2019 May 19];2:115-8. Available from: http://www.cjhr.org/text.asp?2015/2/2/115/153254
| Introduction|| |
The thyroid gland is an extremely vascular endocrine gland. It has two lateral lobes which are joined by an isthmus which gives it an 'H'-shaped appearance. The thyroid gland is placed anteriorly in the neck, extending from the fifth cervical to the first thoracic vertebrae. It is covered by the pretracheal layer of deep cervical fascia. The normal size of each lobe of the thyroid gland is approximately 5 cm long, its transverse and anteroposterior diameter being 3 and 2 cm, respectively. The isthmus measures about 1.25 cm in its both transverse as well as vertical extent and is usually placed anterior to the second and third tracheal rings. 
Thyroid gland appears in embryos of about 17-18 pairs of somites as an evagination of a specially differentiated part of the pharyngeal wall into the underlying mesenchyme which is connected by a connecting stalk. The fourth pouch produces the parathyroid IV and an ultimobranchial body. The dorsal and ventral portions of the fourth pouch together with the lower ultimobranchial body are collectively called as the caudal pharyngeal complex.  In embryos of 7-10 mm, the lower part of the thyroglossal duct differentiates into the glandular tissue.
Development of thyroid gland is a complex relationship of the thyrogossal duct to the hyoid bone. The contribution of the thyroglossal duct in the formation of two lateral lobes and involvement of the ultimobranchial body to give rise to the whole gland properly have enough room for a lot of anomalies to develop. Knowing the variations in the gland will prepare the surgeons to improve the safety while doing surgery on the gland.
The anomalies of the development of the thyroid gland distort the morphology of the gland, and may cause clinical functional disorders and various thyroid illnesses. The knowledge of various developmental anomalies of the gland and variation in neurovascular relations will benefit the surgeon in better planning of a safe and effective surgery. Aim of the present study was to look for various anomalies of the thyroid gland.
| Materials and Methods|| |
Study was conducted on 37 formalin-fixed adult human cadavers. Ethical clearance was taken by the local ethical committee. Midline dissection of the neck was done to look for the thyroid gland and its possible anomalies like thyroglossal duct remnants, levator glandulae thyroideae, and other neurovascular abnormalities. During the dissection, variations which were present were noted and photographed.
| Results|| |
Agenesis of isthmus was found in 6 cases out of 37 cases, that is, in 16.2% of cases.
Four cases were of agenesis of isthmus alone, that is, in 10.8% of cases.
In fifth case, in addition to isthmus agenesis we also found the thyroglossal duct remnant which was extending from the inferior border of the hyoid bone to the lateral lobes of the thyroid gland. It was present to the left of midline and split into two before entering into the lateral lobes. The remnant was fibrous and it looks like a collapsed duct [Figure 1].
|Figure 1: Isthmus agenesis with thyroglossal duct remnant which was extending from inferior border of hyoid bone to lateral lobes of thyroid gland and before entering into two lateral lobes it splits into two|
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In sixth case there was isthmus agenesis with a remnant of thyroglossal duct and in addition to these we also found a fibrous band which finally ends in a pyramidal lobe towards the left lobe of the gland. In this case both the middle thyroid veins fuse to form a single vein and finally they drain into the left brachiocephalic vein. The thyroglossal duct remnant in this case was found to be towards the right of midline [Figure 2].
|Figure 2: Isthmus agenesis with a remnant of thyroglossal duct and also a fi brous band which ended in a pyramidal lobe towards left lobe of the gland. Both the middle thyroid veins also fuses to form a single vein and they drain into the left|
Click here to view
| Discussion|| |
Agenesis of isthmus of thyroid was reported by Gruber and Marshall in the 19 th century with an incidence of 5 and 10%, respectively. 
Comparison of results of agenesis of isthmus and presence of pyramidal lobe in our study with other authors is shown in [Table 1]. Gangbo et al., suggests chromosome 22 may be involved in thyroid development and caused a syndrome (agenesis of isthmus with agenesis of gallbladder).  Mikosch et al., studied 71,500 patients who underwent thyroid investigation. Ten of them had isthmus hemiagenesis. 
The agenesis of isthmus can be described as an anomaly of embryological development. The thyroid gland starts developing as a median thickening of endoderm on the floor of the pharynx between the first and second pharyngeal pouches. This area later invaginates to form the median diverticulum. This thyroid diverticulum grows and become a solid cellular cord called the thyroglossal duct. The duct develops caudally and bifurcates to form the thyroid lobes and the isthmus. At the same time during its caudal growth, the cephalic end of the thyroglossal duct degenerates during the 5 th -10 th gestational week. A high division of the thyroglossal duct can form two independent thyroid lobes with the absence of isthmus. 
Sultana et al., studied the pyramidal lobe in 48 male and 12 female cadavers of Bangladesh; they found the incidence in male as 52.1% and in female it was 41.7% with the predominance being to the left side. They found that in 73.33% cases, pyramidal lobe was associated with the levator glandulae thyroideae with 26.66% being found independently.  We also got two cases (5.4%) of thyroglossal remnant with one case associated to pyramidal lobe. Ignjatovic has reported a case of double pyramidal lobe in 5,000 surgeries performed. The thyroglossal duct had branched beyond the hyoid bone. Hence, the two pyramidal processes joined each lateral lobe of the thyroid gland, but above it was a single process forming an inverted 'Y'-shaped thyroglossal duct.  Similar case was also found in our specimen with branching of thyroglossal duct ('Y'-shaped thyroglossal duct), but no pyramidal lobe.
A similar type of figure is shown by Mc Vay's, Lanz and Waschsmuth, and Marshall which shows bifurcation of the lower end of pyramidal process (thyroglossal duct), one part going to the lateral lobes each and above it is joined to the hyoid bone by a fibrous cord. In all these cases, the thyroid isthmus was absent. ,,
Braun et al., also reports the presence of pyramidal lobe. He found that the pyramidal lobe branched off more frequently from the left part of the isthmus in 16 cases than from the right (7 cases) or midline (9 cases).  In present study also we got one pyramidal lobe which was coming from left lobe of the gland. Won and Chung have also reported the pyramidal lobe. They found the frequency of the pyramidal lobe located in left (48.8%), right (26.4%), and middle of isthmus (21.7%). The pyramidal lobes located in both sides of the isthmus were 3.1% of the cases. 
Sgalitzer found that during 5 th week of development of the thyroglossal duct, the lingual duct will disintegrate as the hyoid bone body develops posterior (dorsal) to the thyroglossal duct. The epithelium of the thyroglossal duct is thyrogenic and as the lateral lobes develop, the thyroglossal duct degenerates.  If thyroglossal duct did not degenerate, we called it as persistence of thyroglossal duct; which we got in two of our cases.
We got two remnants of thyroglossal duct in sixth case, which no authors have mentioned till now and in that case we also found two middle thyroid veins which fused to form a single vein and drained into the left brachiocephalic vein.
Clinically, the diagnosis of agenesis of the isthmus can be done with scintigraphy, which can also be done by using an overload of thyroid-stimulating hormone (TSH). The diagnosis can also be done by using other radiological measures like ultrasonography, computed tomography (CT), magnetic resonance imaging (MRI), or while doing a surgical procedure. When an image of the absence of isthmus is detected, a differential diagnosis against autonomous thyroid nodule, thyroiditis, primary carcinoma, neoplastic metastasis, and infiltrative diseases such as amyloidosis should also be kept in mind. 
Incomplete atrophy of the thyroglossal tract, or retained epithelial cysts, forms the basis for the origin of thyroglossal duct carcinoma (TGDC). Approximately 7% of the adult population has thyroglossal remnants, with an equal sex distribution. Thyroglossal duct cysts are the second most common neck masses and most common congenital cervical abnormalities in children.
Mutation of genes has been involved in thyroid morphogenesis, but failure of the isthmus fusion in the midline may be the principal cause of an isolated isthmus agenesis. Ultrasonography is still the key investigation to diagnose isthmus agenesis, but the presence of other pathological conditions diverts the attention and misleads the diagnosis.
Proper identification of vessels is very important in order to avoid major complications. Hence, a thorough knowledge of the thyroid anatomy and its associated anatomical variations is very much essential, so that these anomalies are not overlooked in the differential diagnosis.
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[Figure 1], [Figure 2]