|Year : 2013 | Volume
| Issue : 2 | Page : 80-82
Absent thyroid isthmus: Embryological and clinical implications of a rare variation of thyroid gland revisited
Har Simarjit Kaur1, Upasna Kumar1, Sukhminder Jit Singh Bajwa2, Gurdeep Singh Kalyan1
1 Department of Anatomy, Government Medical College, Patiala, Punjab, India
2 Department of Anaesthesiology and Intensive Care, Gian Sagar Medical College and Hospital, Ram Nagar, Banur, Punjab, India
|Date of Web Publication||16-Apr-2013|
Sukhminder Jit Singh Bajwa
House No-27-A, Ratan Nagar, Tripuri, Patiala- 147 001, Punjab
Source of Support: None, Conflict of Interest: None
The thyroid gland is the most common endocrine organ which gets operated throughout the globe. It is a highly vascular endocrine gland that is composed of two lateral lobes connected by a narrow median isthmus having an "H" shaped appearance. A wide range of morphological variations and developmental anomalies of the thyroid gland have been reported in the literature which can have numerous clinical and surgical implications. In the present case, absence of the isthmus was observed during dissection on a female cadaver. The respective lateral lobes were positioned independently on either side of the trachea. The present case report aims at discussing the incidence of agenesis of isthmus and its developmental and clinical significance.
Keywords: Agenesis, isthmus, thyroid gland
|How to cite this article:|
Kaur HS, Kumar U, Bajwa SJ, Kalyan GS. Absent thyroid isthmus: Embryological and clinical implications of a rare variation of thyroid gland revisited. Thyroid Res Pract 2013;10:80-2
|How to cite this URL:|
Kaur HS, Kumar U, Bajwa SJ, Kalyan GS. Absent thyroid isthmus: Embryological and clinical implications of a rare variation of thyroid gland revisited. Thyroid Res Pract [serial online] 2013 [cited 2019 Jun 15];10:80-2. Available from: http://www.thetrp.net/text.asp?2013/10/2/80/110595
| Introduction|| |
The patho-physiological aspects of various endocrine disorders have been extensively studied. , However, the anatomical aspects always take a back seat whenever such patients present to the hospital for symptomatic treatment of their disturbed endocrinological milieu. , Thyroid gland is associated with diverse clinical presentations and patho-physiologies but only few studies have gone into depth of the anatomic anomalies and their clinical implications. 
The thyroid gland, brownish-red and highly vascular, is a palpable endocrine gland placed anteriorly in the neck and its position extends from the fifth cervical to the first thoracic vertebrae. It is ensheathed by the pre-tracheal layer of deep cervical fascia. The two lobes are connected by a narrow median isthmus. The normal size of each lobe of the thyroid gland has been described to be 5 cm long, its greatest transverse and antero-posterior extent being 3 cm and 2 cm respectively. The isthmus measures about 1.25 cm transversely as well as vertically and is usually placed anterior to the second and third tracheal cartilages.  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.  Besides, such anomalies can pose diagnostic and surgical challenges in addition to non-invasive and invasive airway management during emergency and surgical interventions.  Incidence of agenesis of the thyroid isthmus has been reported to vary from 5% to 10%.  The knowledge of various morphological and developmental anomalies of the gland will help the surgeons in better planning of a safe and effective surgery.  Various thyroid anomalies have been reported in the past such as persistence of pyramidal lobe, thyroglossal duct cyst and many more. However, the rarest anomalies include, agenesis or hemi-agenesis of thyroid gland, agenesis of isthmus alone or aberrant thyroid glands, which have been reported rarely as well.  The present case report highlights a rare case of agenesis of isthmus of thyroid gland and its developmental and clinical significance.
| Case Report|| |
During routine deep dissection of the neck on a female cadaver in the Department of Anatomy at our institute, it was observed that the thyroid gland had two lateral lobes but a strikingly absent isthmus part of the gland [Figure 1]. On examination, it the location of the thyroid gland was found to be normal. Each lobe was pyramidal in shape with apices directed laterally. The length of the right lobe was 5.5 cm and that of left lobe was 5.8 cm. The breadth was 3.8 and 3.6 cm right and left lobes respectively.
|Figure 1: Encircled part shows lobes of thyroid gland. (∙) represents absent thyroid isthmus|
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Apices pointed toward the oblique line of thyroid cartilage and were related to superior thyroid arteries and external laryngeal nerves. The bases were corresponding to the 3 rd tracheal ring and related to inferior thyroid arteries and recurrent laryngeal nerves. There was no pyramidal lobe. The nerve supply of both the lobes was normal. On careful examination, no accessory thyroid tissue was found.
| Discussion|| |
Agenesis of the thyroid isthmus can also be defined as the complete and congenital absence of the thyroid isthmus.  In one of the studies done on Caucasian cadaver, agenesis of isthmus of thyroid gland with enlarged lobes was reported. Though the incidence of agenesis varies from 5 to 10%, the incidence in north-west Indians is reported to be 7.9 in gross specimens. 
Normally the two lobes of thyroid gland are joined together by an isthmus in the upper part of trachea. Absence of isthmus is indeed a quite rarity in humans.  The basic mechanism of agenesis of isthmus can be attributed to anomalous embryological development. Accessible literature suggests that chromosome 22 plays a major role in the thyroid development. 
The thyroid gland appears as an epithelial proliferation in the floor of the pharynx between tuberculum impar and the copula at a point later indicated by foremen cecum. Subsequently, thyroid gland decends in front of the pharyngeal gut as bilobed diverticulum. During this migration, the thyroid remains connected to the tongue by a narrow canal, the thyroglossal duct, this duct normally disappears.  Rarely, a high separation of thyroglossal duct can engender two independent thyroid lobes and pyramidal lobes with the absence of isthmus.  Further development is dynamic and progressive as the thyroid gland descends in front of the hyoid bone and the laryngeal cartilages. It reaches its final position in front of the trachea by the seventh week. By then, it has acquired a small median isthmus and two lateral lobes.
In present case, the agenesis of isthmus of thyroid gland was also due to a high separation of lobes with the absence of isthmus. Most importantly, it was not associated with other anomalies [Figure 1].
The isthmus may be missing in amphibians, birds and among mammals-monotremes, certain marsupials, cetaceans, carnivores, and rodents. In rhesus monkey (macacus rhesus), the thyroid glands are normal in position but there is no isthmus.  Literary evidence has established the absence of isthmus in 2.0-4.0% of cases. It has also been observed that in such cases a band of connective tissue named levator glandule thyroidae extends from the apex of right or left lobe or isthmus of the thyroid gland to the hyoid bone.  This type of variations should be kept in mind during trans-thyroid tracheostomy procedures. 
The morphological difference in the evolutional origin does not result in any changes in thyroid function. Usually agenesis of isthmus is difficult to determine unless the patients present for other thyroid pathologies. Agenesis of isthmus can be diagnosed via scintigraphy, ultrasonography, computed tomography, and magnetic resonance imaging. When absence of isthmus is suspected, the individual may be directed for a differential pathological diagnosis such as autonomous thyroid nodule, thyroiditis, primary carcinoma, neoplastic metastases and infiltrative diseases such as amyloidosis.  Understanding of thyroid anatomy and associated anatomical variations are very important so that these variations are not over looked in the differential diagnosis.
Agenesis of isthmus can be associated with, absence of a lobe or the presence of ectopic thyroid tissue and hence in clinical practice when such a condition is diagnosed, it is necessary to perform a differential diagnosis against other pathologies such as autonomous thyroid nodule, thyroiditis, and so on. While planning for thyroidectomy one should be prepared to find variations like ectopic thyroid nodules around the normally-located thyroid gland and also has to be precise in dissection as important nerves and vessels lies in the vicinity of thyroid gland. Tracheo-stomy can be potentially dangerous in such cases if a pre-procedure examination is not carried out as in securing invasive airway during emergencies, injuries or during unanticipated difficult to impossible intubation. 
Therefore, a thorough knowledge of the thyroid anatomy and its associated anatomical variations is very important for the clinicians so as to avoid undue complications pre-operatively and while securing difficult invasive airway.
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