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Abstract
The thyroid is the largest endocrine gland in the body; its weight is about 20-25 g. It is slightly heavier in females in whom it becomes enlarged during menstruation and pregnancy. The glan.d consists of 2 lobes that are connected across the midline by a narrow isthmus.
The gland starts as a central evagination of the pharyngeal endoderm between the first and second pharyngeal pouches. The origin of this can be seen as the foramen caecum of the tongue. The evagination migrates down to join structures called the ultimobranchial bodies which may be derived from the fourth pharyngeal pouch. Ectopic throyid tissue and cysts may be found any^where along the track of this thyroglossal duct. Invasion of that endodermal tissue by the surrounding vascular mesenchyme leads to its fragmentation into small clusters of cells. By the third month of the intrauterine life, colloid starts to accumulate in the center of each cluster leading to the formation of thyroid follicles.
The gland is ensheathed by the pretracheal layer of deep cervical fascia. The posterior surface of the isthmus is firmly adherent to the second, third and fourth tracheal rings with the pretracheal