Thyroid Research and Practice

ORIGINAL ARTICLE
Year
: 2019  |  Volume : 16  |  Issue : 3  |  Page : 100--104

Morphological variations of the thyroid gland: An insight on embryological and clinicoanatomical considerations


Honnegowda Thittamaranahalli Muguregowda1, G Krishna2, KG Prakash3,  
1 Department of Anatomy, Kannur Medical College, Kannur, Kerala, India
2 Department of Anatomy, RR Medical College, Bengaluru, Karnataka, India
3 Department of Anatomy, Azeezia Medical College, Meyyanoor, Kollam, Kerala, India

Correspondence Address:
Dr. Honnegowda Thittamaranahalli Muguregowda
Department of Anatomy, Kannur Medical College, Kannur, Kerala
India

Abstract

Background: The thyroid gland, a highly vascular endocrine gland, is composed of two lateral lobes connected by isthmus. A wide range of morphological alterations of the thyroid gland such as hypoplasia, ectopy to hemiagenesis, and genesis are common because of disturbed embryogenesis. The significance of diagnosing such extensions of the thyroid gland is crucial. It may alter the normal anatomical relationship of the thyroid gland with other adjacent cervical structures. Further, it may be confused with a tumor mass on magnetic resonance imaging and scintillation scans. Methods: The primary aim is to study the thyroid gland for gross anatomical variations (weight, height, and breadth) and secondarily to document the morphometric and congenital/developmental variation gross of the thyroid gland in 65 cadavers from the coastal belt of South India. The glands were according to the various age groups of the cadavers. Results: The mean thyroid weight was 26.01 ± 7.14 g. In males, it was 26.59 ± 6.96 g, whereas in females, it was 20.93 ± 8.98 g. The pyramidal lobe was present in 35 (53.8%), and the presence of the levator glandulae thyroideae was noted in 25 (38.48%) of male cadavers. Agenesis of the isthmus was found in 20 (30.76%). However, the accessory thyroid tissue was found in only one female cadaver. Conclusions: Our morphometric study on cadaveric thyroid glands highlights individual and ethnic variations. Knowledge of various developmental anomalies and unusual variations of the thyroid gland is of paramount importance to differentiate it from other pathologies such as autonomous thyroid nodule and thyroiditis.



How to cite this article:
Muguregowda HT, Krishna G, Prakash K G. Morphological variations of the thyroid gland: An insight on embryological and clinicoanatomical considerations.Thyroid Res Pract 2019;16:100-104


How to cite this URL:
Muguregowda HT, Krishna G, Prakash K G. Morphological variations of the thyroid gland: An insight on embryological and clinicoanatomical considerations. Thyroid Res Pract [serial online] 2019 [cited 2019 Dec 12 ];16:100-104
Available from: http://www.thetrp.net/text.asp?2019/16/3/100/271150


Full Text



 Introduction



The thyroid gland lies deep to the infrahyoid muscles, located anteriorly in the neck at the level of the C5–T1 vertebrae.[1] The size and shape of the thyroid gland may alter remarkably with age, gender, physiological condition, race, and geographical location. It is reported that it may be larger and heavier in females than in males, and thyroid gland increases in size during menstruation and pregancy.[2] Harjeet et al.[3] measured the thyroid of Indian individuals and found that the mean weight of the thyroid glands in adults was smaller than in Western Caucasian and Japanese individuals. It is reported that the weight of the thyroid gland is less in Jamaicans and Japanese compared to Americans and Europeans.[4] Hegedüs et al.[5] have demonstrated the relationship between thyroid volume, as estimated by ultrasound, and body weight, age, and sex in normal individuals. The weight of the gland varies based on physiological status and in females enlarging during menstruation and pregnancy. Ueda[6] correlated thyroid gland volume (estimated by ultrasound) with height, weight, body surface, and age in children of 6 months–15 years and did not find a significant difference in thyroid volume between males and females over this age range. 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.[7],[8] Accessory nodules/ectopic thyroid tissue of the thyroid is usually found at the embryonic origin of the thyroid gland, at the foramen cecum, or at the thyroglossal tract within the tongue.[9] These ectopic thyroid tissues may later undergo malignant changes.[10] Incidence of absence of left lobe of thyroid gland were 80% and absence are of absence of isthmus.[11] Approximately the 50% of thyroid gland have a pyramidal lobe, which varies in size and extends from isthmus to hyoid bone.[12] The upper end of the pyramidal lobe continues as a fibromuscular strand, called 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.[13],[14] The present study is an approach to highlight various developmental and morphological variations 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 South India population.

 Methods



This prospective observational study was performed in the department of anatomy from March 2016 to September 2018 by dissecting the neck region of 65 cadavers of both genders (55 males and 10 females) of mean age 45.03 ± 19.4 years ranging between 19 and 75 years. The primary thyroid gland was studied for several parameters such as weight, height, and breadth. Furthermore, developmental anomalies were noted such as partial and total agenesis of the gland, presence of ectopic tissues or accessory thyroids, and permanent thyroglossal duct anomalies, such as cysts, fistulae, or sinuses, and pyramidal lobes or fibrous bands. With the informed consent of the relatives of the deceased, the cadavers were preserved in 10% formalin solution. Gross and fine dissections were carried out and then weighed on a digital balance (Sartorius® Analytical Balance 220 g × 0.1 mg). The measurements of the gland were taken with the help of digital Vernier calipers in the specimens.

Statistical analysis

Statistical analyses were performed using SPSS 15th version (SPSS Statistics for Windows, Version 24.0. Armonk, New York). Descriptive statistics were calculated as the mean ± standard deviation. For comparing quantitative data, parameters showing normal distributions were compared using one-way ANOVA test, and parameters which did not show normal distributions, groups were compared using the Kruskal–Wallis test. The Chi-square test was used for categorical data analysis. Statistical significance was defined as P < 0.05.

 Results



Our study included dissections of thyroid glands in 65 adult human cadavers. The mean age of the cadavers was 45.03 ± 19.4 years ranging between 19 and 75 years. Of 65 cadavers, 55 were male (84.6%) and 10 were female (15.3%). The mean thyroid weight was 26.01 ± 7.14 g. In males, it was 26.59 ± 6.96 g, whereas in females, it was 20.93 ± 8.98 g. In both males and females, there was no statistically significant difference in thyroid weight between the various age groups (P > 0.05). Although in the 18–30-year-old group and in the 51–80-year-old group, there was no significant difference between thyroid weight and gender (P > 0.05), in the 31–50-year-old group, the thyroid weight of the males was statistically significantly more than females (P< 0.05) [Table 1]. The length, width, and thickness of the left and the right lobes are shown in [Table 2]. As shown in [Table 3], in male and female cases, there was no statistically significant difference in thyroid length and thyroid width according to age groups (P > 0.05).{Table 1}{Table 2}{Table 3}

 Discussion



In the present study, the mean thyroid weight was 26.11 ± 8.14 g. In males, it was 26.93 ± 7.96 g, whereas in females, it was 21.93 ± 7.98 g. Snyder et al.[15] reported the weight of the gland to be 17.5 ± 6.8 g in males and 14.9 ± 6.7 g in females. In Japanese adults between the ages of 20 and 50 years, the gland weighed about 18 g in males and 1–2 g less in females.[16] In fact contrary to previous studies in our study, in 31–50-year-old group, the thyroid weight of the males was statistically significantly more than that of the females [Table 1].

In the present study, we have demonstrated that the mean length, width, and thickness of the left lobe are 4.76 ± 0.9, 1.89 ± 0.51, and 1.50 ± 0.45 cm, respectively, whereas the mean length, width, and thickness of the right lobe are 4.58 ± 0.66, 1.91 ± 0.67, and 1.54 ± 0.34, respectively. Joshi et al.[17] reported the average length of the right lobe as 4.32 cm and the left lobe as 4.22 cm. The thickness of the right lobe was reported as 1.13 cm and the left lobe as 1.18 cm. Further studies are needed to validate our results in a larger study with an equal number of men and women.

The thyroid gland is the first endocrine gland seen in embryological development and takes its final shape and position at the end of the 7th week in front of the trachea.[18] Failure of the development of the entire gland, or part of the gland, results in agenesis or hemiagenesis, which may be unilateral or isthmic.[19] Many workers claim that the absence of isthmus is quite rare in humans.[9] Mikosch et al.[20] during 9 years studied 71,500 patients who underwent thyroid investigation. Sixteen of them had isthmus hemiagenesis. Melnick and Stemkowski[21] stated that a total of Ninety cases of partial absence of thyroid gland were noted in the available world literature. The absence of the left lobe was very common accounted for 80% of the cases, and the right lobe was absent in 20% of the cases. Won and Chung have reported that in their study population only 3% of the cases found with absence of isthmus.[22] Braun et al. reported that the isthmus was missing in four cases of the 58 cadavers they studied.[23] Ranade et al. reported the absence of isthmus in 35 of 105 cases (33%), of which 8 were female cadavers.[24] According to Grüber, the incidence is about 5%,[25] whereas according to Marshall, it is about 10%.[26] Based on the findings of our study, we report in this study that incidence accounted in total for about 20 (30.76%) [Figure 1] and [Figure 2]. The absence of an isthmus can be associated with other types of dysorganogenesis, such as the absence of a lobe or the presence of ectopic thyroid tissue.[27] Clinically, the diagnosis of absence of the isthmus can be done with scintigraphy, ultrasonography, computerized tomography, magnetic resonance imaging, or during a surgical procedure. The incidence of a pyramidal lobe was found to be 55% in the cadavers studied by Braun et al.[23] They reported that frequency is highly accounted in men than in women. In a study performed on Koreans, the frequency of the existence of the pyramidal lobe was 76.8%.[22] In our study, pyramidal lobe was found in 35 (53.8%) male cadavers [Figure 1] and [Figure 2] [Figure 3] and [Table 4]. Accessory thyroid gland at the carotid bifurcation may present as a carotid body tumor.[28] Ectopic thyroid tissue may undergo malignant changes.[18] An accessory thyroid gland was present in one specimen among 58 cadavers studied by Braun et al.[23] An ectopic tissue in the parotid gland has been reported by Mysorekar et al. who explained that the ectopic thyroid tissue in the parotid gland could be due to a common evolution of the thyroid and parotid glands, a heteroplasia or a metaplasia.[29] According to Standring, LGT is a fibrous or fibromuscular band that stretches from the pyramidal lobe or upper border of the isthmus of the thyroid gland, usually on the left side, to the body of the hyoid bone above.[30] Merkel thoughts that the levator glandulae was constant and glandular though usually surrounded by muscle fibers.[31] Huschke spoke of the structure only as glandular and mentioned nothing about the muscle.[32] Finally, Godart reported a case in which the structure was indeed muscular, on the basis of the nitric acid test for muscle.[33] According to Allan,[34] LGT is fibromusculoglandular in nature and innervated from branches of external laryngeal nerve. Microscopic examination revealed that LGT was mainly made up of glandular, muscular, and fibrous tissue. Soemmerring's muscle is the same as the hyo-thyro-glandulaire of Pointe, the LGT superficialis medius et longus of Krause,[35] and the musculus thyroideus of Merkel; its usual full name in the literature is LGT of Soemmerring. Saadeh et al. reported an unusual LGT on the left side of the neck in a female cadaver, originated from mastoid process, extended superficially to the superior belly of the omohyoid muscle, and inserted into the connective tissue of the left lobe of the thyroid gland.[36] In this study, the presence of LGT was found to be 25 (38.48%) [Figure 4] and [Figure 5]. Congenital anomalies of the thyroid gland can be explained developmentally. A study by Gangbo et al.[37] reported that a fetus of 24-week gestation was detected on echography to have congenital anomalies: intrauterine growth retardation, facial dysmorphism, and ventricular septal defect with aortic displacement. Posttermination of pregnancy autopsy showed additional internal organ anomalies such as thyroid isthmus agenesis and absent gallbladder. The authors stated that chromosome number 22 could play in development of thyroid lobar gland.{Figure 1}{Figure 2}{Table 4}{Figure 3}{Figure 4}{Figure 5}[INLINE:1]

 Conclusions



We conclude that an anomaly of the thyroid gland varies based on race, population, and gender. In our study, the incidence rate of presence of pyramidal lobe (53.84%), LGT (38.48%) and agenesis of isthmus (30.76%) respectively. 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 and should not be confused for other pathologies such as autonomous thyroid nodule and thyroiditis.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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