Macroscopic Study of the Isthmus of the Thyroid Gland in Bangladeshi People : A Postmortem Study

Background: The position and size of isthmus of thyroid gland varies considerably in human with age, sex, physiologic state, race and geographical location and sometimes the isthmus may be absent. So this study was designed to find out the macroscopic differences in isthmus of thyroid gland of different age and sex groups in Bangladeshi people. Objective: To record the macroscopic characteristics of isthmus of thyroid gland with advancing age in both sexes with a view to help establishing normal standard of Bangladeshi people. Materials and Methods: This descriptive cross-sectional study was carried out on 54 autopsied human thyroid glands aged 5 to 65 years. Thyroid glands were collected from unclaimed dead bodies autopsied in the morgue of Sylhet M. A. G. Osmani Medical College, Sylhet. The collected specimens were divided into groups –– A (20 years and below), B (21 to 50 years) and C (50 years and above). All specimens were examined morphologically by careful gross dissection method. Results: The isthmus was absent in 5.56% cases. In most of the cases (35.29%) it was against the 1–4 tracheal rings. There was significant difference in length between Group A and Group C (p<0.05) and in breadth between Group A and Group C and between Group B and Group C (p<0.05). No significant difference was found in length, breadth and thickness of isthmus of the thyroid gland between males and females. Conclusion: The presence or absence, positional change and variation in gross dimension of isthmus of thyroid gland were evident in human. The macroscopic difference was found with increasing age but not with sex.

fibromuscular band known as the levator glandulae thyroideae which usually replaces the upper part of the pyramidal lobe. 9 However, morphological study of thyroid isthmus bears a great importance. We have only few studies on human organs in Bangladesh, especially in their anatomical variation in site, size and gross dimensions in comparison to those of the Western population.

Materials and Methods
This descriptive cross-sectional study was conducted This study was done on 54 postmortem human thyroid glands collected from individuals of both sexes (male 43 and female 11) aged from 5 to 65 years at the autopsy laboratory of Department of Forensic Medicine, SOMC. Age and sex of cadavers were collected from records in the register book. All the samples were collected from medicolegal cases excluding hanging, poisoning, any cutting or crushing injury to the thyroid gland and known case of thyroid disease. The collected samples were fixed in 10% f o rmal saline solution. Dissection was carried out to expose the thyroid glands. The thyroid gland was examined for the following variables -a) Presence or absence of isthmus, b) Posterior relation of isthmus, c) Length, breadth and thickness of isthmus. Among them isthmus was found in 51 cases. The samples having the isthmus (n=51) were divided into three age groups --Group A (20 years and below), Group B (21 to 50 years) and Group C (50 years and above). 10

Measurement of length, breadth and thickness of isthmus
The length of the isthmus was measured transversely at its superior limit, the middle part and the inferior limit with the help of a scale. The mean measurement was recorded for further use. The breadth of the isthmus was measured from superior to inferior border with a scale. Three measurements were taken at its right end, left end and middle part. The mean value was recorded as breadth of the isthmus. The thickness of the isthmus was measured anteroposteriorly at its two ends and middle part with a slide calipers. The mean value was recorded as thickness of the isthmus. 11 Statistical analyses were done by unpaired Student "t" test and one way ANOVA test using SPSS 12.0 version.

Results
In this study it was observed that out of 54 thyroid glands 3 (5.56%) had no isthmus. In 18 (35.29%) cases the isthmus was related to 1 st , 2 nd , 3 rd and 4 th tracheal rings posteriorly. Table I shows the posterior relation of isthmus of the thyroid gland. There is significant difference regarding posterior relation of isthmus in relation to tracheal rings. Table II shows the length, breadth and thickness of isthmus of the thyroid gland in different age groups of study population. In this study it was found that the average length of the isthmus of the thyroid gland was 1.25 ± 0.54, 1.41 ± 0.64 and 1.80 ± 0.64 cm in Groups A, B and C respectively. The mean length of isthmus of the thyroid gland was highest in Group C and lowest in Group A. There was significant difference between Group A vs Group C (p<0.05).
In this study the average breadth of the isthmus of the thyroid gland was 1.53 ± 0.60, 1.68 ± 0.52 and 2.02 ± 0.52 cm in Groups A, B and C respectively. There was significant difference between Group A vs Group C and B vs C (p<0.05).
In this study the average thickness of the isthmus of the thyroid gland was 0.56 ± 0.22, 0.50 ± 0.19 and 0.55 ± 0.13 cm in Groups A, B and C respectively. There was no significant difference among the groups. There was no significant difference between males and females regarding length, breadth and thickness of isthmus of the thyroid gland (Table III)

Discussion
In the present study, it was found that 5.56% thyroid gland had no isthmus. This observation can be compared with study done by Enayetullah 11 (Table IV).   In the present study the average thickness of the isthmus of the thyroid gland was 0.53 ± 0.19 cm. The mean thickness of isthmus of the thyroid gland in male was 0.51 ± 0.17 cm and in female was 0.57 ± 0.23 cm.
Hoyes & Karshaw 24 described the breadth of isthmus of thyroid gland is 0.2 to 0.6 cm which is similar to the findings in the present study. Similar results were also found by Enayetullah 11 and Begum 28 .
The nature of specimens studied and the areas where studies are carried out, effect of different goiter zones, age, sex and race of population studied can contribute to the anatomical variations of the thyroid gland found in different reports by various authors.
In this study, the presence or absence, positional change and variation in gross dimension of isthmus of thyroid gland were evident in humans. The macroscopic difference was found with increasing age but not with sex. This study will also help in minimizing complications of thyroid surgery and tracheostomy. We would like to recommend further studies with larger samples and high technical support.