Morphological and Histological Features of Human Fetal Thyroid Gland

Introduction: The thyroid gland regulates the basal metabolic rate, somatic growth, and psychic growth. Hence, the thyroid gland plays a major role for the normal growth of a fetus during the prenatal period. It is the first endocrine gland which starts to develop by 24 days after fertilization. The aim of our study is to study the morphological and histological features of the thyroid gland in various age group fetuses. Materials and Methods: 40 intact fetuses of different gestational ages ranging from 11 to 36 weeks were studied in the Department of Anatomy, Sri Muthukumaran Medical College & Research Institute, Chennai. The fetuses were preserved in 10% formalin solution. Midline dissection of the neck was done to expose the thyroid gland. The situation, shape, and measurements of the gland were noted. Then, sections of the gland were taken for histological study. These sections which were preserved in 10% formalin was processed and stained with hematoxylin and eosin. Results: It was observed that the thyroid gland was situated in its definitive location anterior to 1st-6th tracheal ring in all fetuses. The gland was horse-shoe shaped in all fetuses, except in one fetus (22 weeks), where isthmus was absent. The isthmus was related to 1-4th tracheal rings. The fetuses were categorized into three gestational age groups as: Group I: 10-17 weeks, Group II: 18-22 weeks, and Group III: 23-36 weeks. The mean and ranges of the length, breadth, and thickness of the right and left lateral lobes were measured in each group. The observation of the histological features revealed that development of thyroid gland has three stages as: Colloid formation stage, folliculogenesis stage, and mature follicular growth stage. These stages were correlated with gestational age. Conclusion: The present study helps us to understand and correlate the morphological and histological changes during the development of thyroid gland. Such knowledge helps the clinicians to understand the anatomical and histo-pathological changes in certain thyroid gland disorders.