|Year : 2015 | Volume
| Issue : 4 | Page : 563-565
Thyroid isthmus agenesis
Samrat Sapkota, P Haresh Kumar, Rishi Pokhrel
Department of Anatomy, Armed Forces Medical College, Pune, Maharashtra
|Date of Web Publication||14-Jul-2015|
Department of Anatomy, Armed Forces Medical College, Pune, Maharashtra
Source of Support: None, Conflict of Interest: None
The thyroid gland is the largest of all endocrine glands and is placed anteriorly in the lower neck in level with fifth cervical and first thoracic vertebrae. It is "H" shaped with two large lateral lobes, connected by a narrow isthmus in the midline. The gland bears very important clinical significance; both physiologically and pathologically and; a wide range of variations and anomalies of the gland have been reported. We report a case of complete agenesis of thyroid isthmus found during routine cadaveric dissection for the purpose of teaching learning of medial undergraduates at our department. This anomaly is quite uncommon and bears important clinical importance in wide fields of surgical and medical specialties. The embryological basis of the anomaly is high separation of a thyroglossal duct, which can give rise to two independent thyroid lobes with no isthmus.
Keywords: Endocrine, thyroidectomy, variation
|How to cite this article:|
Sapkota S, Kumar P H, Pokhrel R. Thyroid isthmus agenesis. Med J DY Patil Univ 2015;8:563-5
| Introduction|| |
The thyroid gland, brownish-red and highly vascular, is placed anteriorly in the lower neck, level with the fifth cervical to the first thoracic vertebrae.  The gland is bilobed in shape, and two lobes are normally joined in front of the trachea by connection of thyroid tissue called as the isthmus of the gland. The gland tissue is about 25 g in weight and produces hormones triiodothyronine, tetraiodothyronine and calcitonin, which play important roles in normal metabolic activity of the human body.
Probably, the thyroid is singular among the endocrine glands in those normal physiological activities so readily crosses the borderline and became pathological. Thyroid gland is known to vary in both its structure and the levels of hormones it produces; during various physiological conditions, as well as pathologies.  Structural variations of the thyroid gland are, especially important due to the fact that the gland is the most common site of endocrine tumors with the exception of gonadal tumors alone.  Consequently it is also the most common endocrine gland that is surgically intervened.  We are report a case of complete agenesis of thyroid isthmus discovered during cadaveric dissection.
| Case Report|| |
Complete absence of thyroid isthmus was found in a 70-year-old male cadaver during routine dissection carried out in our department for the purpose of teaching medical undergraduates [Figure 1]. The gland and the surrounding structures were further dissected carefully, and photographs were taken.
Each lateral lobe was approximately 5 cm in length, and their location was usual. The texture of the gland was normal with no evidence of growth or nodules. Superior thyroid artery arose from external carotid artery [Figure 2] and inferior thyroid artery from thyrocervial trunk, which in turn was a branch of subclavian artery [Figure 3], which too is a usual finding. No thyroid tissue was found between two lateral lobes, anastomosis between superior thyroid arteries was absent and inferior thyroid veins were also absent. Levator glandulae thyroidae and thyroglossal duct were not found. External and recurrent laryngeal nerves were found in their usual location. No skin scar or the fibrosed tissue could be detected ruling out any possibility of thyroid surgery. No other gross variations were found in the same cadaver.
The cadaver used for dissection was obtained through body donation program of our department and informed written consent was obtained from the next of kin for use of cadaver for the purpose of medical teaching and research.
| Discussion|| |
Two kinds of endocrine cells are seen in adult thyroid gland, the follicular and the parafollicular cells, or "C" cells, which originate from two different embryological cell families. The follicular cells from the endodermic cells of the primitive pharynx and the parafollicular cells from the neural crest.  The primitive pharynx forms thyroglossal duct whose caudal end bifurcates and gives origin to the thyroid lobes and the isthmus. At the same time that its caudal growth is taking place, the cephalic end of the thyroglossal duct degenerates.  Thyroglossal duct dividing high up can give rise to two well separated thyroid lobes without any niche of thyroid tissue between them that is, isthmus and hence such absence of connection can be traced back into its early embryological development. 
True incidence of isolated thyroid isthmus agenesis is difficult to estimate as in the majority of cases it is asymptomatic, and subjects usually have normal levels of thyroid hormones.  Most of the descriptions in the literature about congenital thyroid abnormalities are related with hemiagenesis, which include one lobe and sometimes the isthmus. Among 71,500 patients who underwent thyroid investigation only ten of them had isthmus hemiagenesis.  Others surgical studies reported an incidence nearby one in 2000 on two reviews of available world literature found, respectively a total of 94 and 256 cases of thyroid hemiagenesis, but among these cases the isthmus was absent in only 50% of the patients where the isthmus was specifically mentioned. 
Thyroid isthmus agenesis, though itself being asymptomatic, can be associated with various other types of dysorganogenesis, such as the absence of a lobe or the presence of ectopic thyroid tissue or familial syndromes and chromosomal aberrations and hence in clinical practice when such a condition is diagnosed, it is necessary to perform a thorough workup to rule out other differential diagnosis. , Identification of such variation at the time of planning of surgery becomes vital. Clinically, the diagnosis of agenesis of the isthmus can be done with scintigraphy, which can also be performed with an overload of thyroid-stimulating hormone; in the event of there being nonfunctioning thyroid tissue in the isthmus. The diagnosis can also be done with the aid of ultrasonography, computerized tomography, magnetic resonance imaging or intraoperatively during a surgical procedure.  When the condition is suspected it is necessary to perform an in-depth interview addressing previous surgical procedures in the cervical region (isthmectomies due to neoplasms, decompressive techniques due to thyroiditis or due to transthyroid tracheotomies). 
Absence of isthmus can be associated with, agenesis of a lobe or the presence of thyroid tissue elsewhere  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 anticipate the variations like ectopic nodules around the normally-located thyroid gland and the dissection also has to be precise as important nerves and vessels lie in the vicinity of thyroid gland. 
| Conclusion|| |
An isolated isthmus agenesis is uncommon. It has an embryological basis, and its occurrence is significant both surgically and medically. The condition should be ruled out before any intervention in the region of the neck. If this variation is found its possible cause should be sought after in the form of dysorganogenesis syndromes, chromosomal aberrations or familial conditions.
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[Figure 1], [Figure 2], [Figure 3]