DEVELOPMENTAL ABNORMALITIES OF THE THYROID

DEVELOPMENTAL ABNORMALITIES OF THE THYROID - DEVELOPMENTAL...

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DEVELOPMENTAL ABNORMALITIES OF THE THYROID To understand the different thyroid anomalies, it is important to briefly review normal development of this gland. The thyroid is embryologically an offshoot of the primitive alimentary tract, from which it later becomes separated 67–70 (Figs. 16 and 17). During the third to fourth week in utero, a median anlage of epithelium arises from the pharyngeal floor in the region of the foramen cecum of the tongue (i.e., at the junction of the anterior two thirds and the posterior third of the tongue). The main body of the thyroid, referred to as the median lobe or median thyroid component, follows the descent of the heart and great vessels and moves caudally into the neck from this origin. It divides into an isthmus and two lobes, and by 7 weeks it forms a “shield” over the front of the trachea and thyroid cartilage. It is joined by a pair of lateral thyroid lobes originating from the fourth and fifth branchial pouches (Fig. 17). From these
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