Morphological Variations of the Thyroid Gland among the People of Upper Assam Region of Northeast India: A Cadaveric Study

The thyroid gland is a highly vascular gland placed anteriorly in the lower part of the neck consisting of two symmetrical lateral lobes connected by a midline isthmus [1]. The size of the thyroid gland varies considerably with age, sex, physiological state, race and geographical location and it is observed that the size is larger in female than in male [2]. The right lobe is slightly larger than the left one [3]. The thyroid gland appears as an epithelial proliferation in the floor of the pharynx between the tubercular impar and the copula in the form of bi-lobed diverticulum, called the thyroglossal duct. The lower part of the duct develops in to median isthmus and two lateral lobes. The upper connection of the duct with the floor of pharynx later disappears [4]. Agenesis of the thyroid isthmus is defined as the complete and congenital absence of the isthmus [5]. A high division of the thyroglossal duct generate two independent thyroid lobes with the absence of isthmus [6]. The embryological remnant of the caudal end of the thyroglossal duct commonly known as the pyramidal lobe often ascends from the isthmus or adjacent part of either lobe (more often from the left side) towards the hyoid bone [7]. The upper end of the pyramidal lobe continues as a fibro-muscular strand, called the Levator Glandulae Thyroideae (LGT), attached to the hyoid bone. When the pyramidal lobe is absent, LGT may attach to the upper part of the isthmus [8]. Thyroid diseases are among the most common endocrine disorders in India [9]. Most of the diseases, affecting the thyroid gland require medical and surgical intervention. The surgeons planning a total thyroidectomy operation must be aware about the morphological variations of the gland. Therefore, it is important to know the details regarding surgical anatomy of the thyroid gland. The North-eastern region of India is famous for its diverse ecology and distinct culture and lifestyle specific to different ethnic groups. Different ethnic groups existing in Assam are Ahoms, Bodos, Koch, Matak, Chutiya and different branches of Kacharis such as Dimasa, Sonowal and Thengal.

The prevalence of goitre remained high (42.2%) in Dibrugarh district [10]. Results of the morphological study of the thyroid gland in the people of this region are most likely to differ from those with adequate iodine intake, as iodine is very essential for the biological activity of the thyroid gland. The present study is an approach to popularise the information pool on the morphology of the thyroid gland and help the clinicians in their practice. Moreover, limited information is available regarding the morphological variations of the thyroid gland from the Upper Assam region of Northeast India. The present study is an approach to popularise the information pool on the morphology of the thyroid gland and help the clinicians in their practice.

Materials and Methods

A cross-sectional descriptive study was done in the Department of Anatomy, Assam Medical College and hospital, Dibrugarh for a period of one year from August 2014 to July 2015. A total of 80 thyroid glands (30 adult and 50 perinatal cadavers) were dissected and out of which 49 were male and 31 were female. Adult specimens (30) were collected from the cadavers provided for the dissection and perinatal specimens (50) were obtained from Department of Obstetrics & Gynaecology, Assam Medical College & Hospital.

After taking the approval from the institutional ethical committee, the cadavers were received in the Department of Anatomy. The particulars of the specimens were collected from the cadaver register. Cases with known thyroid diseases or crushing injury to the neck and those foetuses with gross congenital malformation were excluded from the study.

A vertical incision was given on the skin from the chin to the sternum in the mid line. The subcutaneous fat and deep fascia was exposed; the infrahyoid muscles were identified and reflected laterally. The pretracheal fascia was removed and the right and the left lobe of the gland were identified. The thyroid gland was examined carefully for the presence of isthmus, pyramidal lobe and LGT. When the pyramidal lobe was present, its position was observed. LGT if present, its extension and its relation with the pyramidal lobe were also observed.

Statistical Analysis

Statistical analysis was done using chi-square test by SPSS 21.0.

Results

A total 80 number of thyroid glands were collected from 49 male and 31 female cadavers. Presence of pyramidal lobe, levator glandulae thyroideae and absence of isthmus were the only morphological variations observed in the gland. Forty eight out of the total glands dissected had morphological variations the others had normal anatomy [Table/Fig-1]. Morphological variations were more common in males but the difference was not statistically significant (chi-square value=0.43, df=1; p-value = 0.67). All the major morphological variations observed in the study are summarized in [Table/Fig-2].

[Table/Fig-1]:

Morphological variations in either sex.

Sex	Morphological variations	Normal anatomy
Male	28	21
Female	20	11
Total	48	32

[Table/Fig-2]:

Different types of morphological variations and sex distribution of the cadavers dissected.

Morphological variations	Male	Female	Total	Percentage
Pyramidal lobe (PL)	18	13	31	38.75%
Absence of isthmus (ISTH)	11	6	17	21.25%
Presence of Levator Glandulae Thyroideae (LGT)	9	6	15	18.75%

Note: More than one variations were seen in few specimens

The isthmus was absent in 17 (21.25%) cases; of which 11 were male cadavers [Table/Fig-3]. The pyramidal lobe was observed in 31(38.75%) cases. Among them, pyramidal lobe was found more frequently in males (18). In 74.2% cadavers, the pyramidal lobe was attached to the left side of the isthmus [Table/Fig-4] and to the middle in 25.8% [Table/Fig-5]. According to the origin and location of pyramidal lobe, Milojevic B et al., classified five types of pyramidal lobe [11]. On the basis of that classification, we have found two types-Type III (74.2%) and Type I (25.8%) with the left sided being predominant [Table/Fig-6]. The LGT was present in 15(18.75%) cadavers [Table/Fig-5] and in all specimens LGT was extending from the apex of the pyramidal lobe to the hyoid bone.

[Table/Fig-3]:

Thyroid gland with absence of isthmus in an adult cadaver.

Note: LL - Left lobe; RL - Right lobe

[Table/Fig-4]:

Pyramidal lobe with levator glandulae thyroideae arising from the left half of isthmus.

Note: LL - Left lobe; RL - Right lobe; LGT - Levator Glandulae Thyroideae; PL - Pyramidal lobe

[Table/Fig-5]:

Pyramidal lobe with levator glandulae thyroideae arising from the middle of isthmus.

Note: LL - Left lobe; RL - Right lobe; LGT - Levator Glandulae Thyroideae; PL - Pyramidal lobe; ISTH - Isthmus

[Table/Fig-6]:

Classification of the pyramidal lobe on the basis of its attachment to the thyroid gland.

Sex	Centralpart of theisthmus(Type I)	Junctionof the rightlobe withthe isthmus(Type II)	Junctionof theleft lobe withthe isthmus(Type III)	Left lobe(Type IV)	Right Lobe(Type V)	Total
Male	5(16.1%)	0	13 (41.9%)	0	0	18
Female	3 (9.6%)	0	10 (32.25%)	0	0	13
Total	8 (25.8%)	0	23 (74.2%)	0	0	31

Discussion

The morphological variations of the thyroid gland found in present study were compared with other studies [Table/Fig-7] [12-18].

[Table/Fig-7]:

Morphological variations of the thyroid gland compared with other studies [12-18].

Author	Sample Size	Year	Absent Isthmus	PyramidalLobe	LevatorGlandulaethyroideae
Ranade AV et al., [12]	105	2008	33%	58%	49.5%
Nurunnabi ASM et al., [13]	60	2008	-----	41%	20%
Begum M et al., [14]	60	2009	------	26.7%	15%
Joshi S D et al., [15]	90	2010	16.7%	37.8%	30%
Kulkarni V et al., [16]	20	2012	10%	------	30%
Veerahanumaiah S et al., [17]	89	2014	9%	46%	41%
Rathod S Mansingh et al., [18]	76	2015	13%	58%	-----
Present Study	80	2015	21.25%	38.75%	18.75%

Incidence of absence of isthmus varies from 5% to 10% [5]. The absence of isthmus in the present study closely matched with the study by Joshi et al. (15 of 90) [15]. The differences between various studies were probably due to difference in sample selection or due to differences in the population studied.

The pyramidal lobe observed in the present study was comparable with that of (41%) [13] and (37.8%) [15]. In the present study, the pyramidal lobe was situated more on the left side of the gland which was also observed in various studies [11,14,15]. In studies based on the surgically removed thyroid gland after total thyroidectomy operation in Turkey and Serbia, 65.7% [19] and 61% [20] of thyroid glands respectively had pyramidal lobes. The conclusion of the above discussion is that presence of pyramidal lobe is quite common in all over the world.

The levator glandulae thyroidae observed in the present study was similar to observations of 20% [13] and 15% [14].

The morphological variation in the thyroid gland does not affect the function of the gland but presence of these variations may simulate some thyroid pathologies. Agenesis of isthmus is a differential pathological diagnosis for autonomous thyroid nodule, thyroiditis, primary carcinoma, neoplastic metastases and infiltrative diseases such as amyloidosis [21]. Agenesis of isthmus can be diagnosed via scintigraphy, ultrasonography, computed tomography and magnetic resonance imaging. Total or near total thyroidectomy is recommended for patients with ongoing thyroid cancer, those who refuse radio-ablation as a therapeutic procedure, or have a life threatening reaction to antithyroid drugs such as vasculitis, agranulocytosis and liver failure and is the operation of choice for patients undergoing surgical treatment for Grave’s disease [22]. Incomplete excision of thyroid gland in patients with auto-immune disorders may cause recurrence of the diseases. Anatomical variations of thyroid gland may cause many difficulties during complete removal of the gland. Removal of the pyramidal lobe and other accessory thyroid tissue has considerable beneficial effect on the total thyroidectomy operation [19].

In the present study, more than two-third of the thyroid glands dissected had one or more morphological variations hence presence of pyramidal lobe or other morphological variations should be looked for during total thyroidectomy operation to ensure complete removal of the gland tissue.

Limitation

This type of study can be done in all age groups; but present study focused only on adult and perinatal cadavers. We have only used dissection as a tool for the study but radiological techniques like MRI, CT scan and Ultrasonography can also be applied.

Conclusion

Precise and accurate knowledge of variations associated with the thyroid gland would be helpful for surgeons in performing total, subtotal & partial thyroidectomy and in the evaluation of Scintigraphic findings.

Note: More than one variations were seen in few specimens

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