The thyroid gland appears as an epithelial proliferation in the floor of the pharynx between the tuberculum impar and the copula at a point later indicated by the foramen cecum (Figs. 15.17 and 15.18A). Subsequently the thyroid descends in front of the pharyngeal gut as a bilobed diverticulum (Fig. 15.18). During this migration, the thyroid remains connected to the tongue by a narrow canal, the thyroglossal duct. This duct later disappears.
With further development, the thyroid gland descends in front of the hyoid bone and the laryngeal cartilages. It reaches its final position in front of the trachea in the seventh week (Fig. 15.18B). By then it has acquired a small median isthmus and two lateral lobes. The thyroid begins to function at approximately the end of the third month, at which time the first follicles containing colloid become visible. Follicular cells produce the colloid that serves as a source of thyroxine and triiodothyronine. Parafollicular, or C, cells derived from the ultimobranchial body (Fig. 15.10) serve as a source of calcitonin.
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