THYROID DEVELOPMENT AND DIAGNOSTIC IMAGING
Thyroid development begins during the third week of gestation as a midline focus of endodermal thickening on the floor of the primitive pharynx. This thyroid diverticulum "descends" as the head and neck of the embryo grow, and reaches its normal location in the inferior aspect of the neck by approximately 7 weeks. The thyroid diverticulum becomes solid and divides into right and left lobes, with a connecting isthmus. The developing thyroid gland temporarily connects via the thyroglossal duct to its embryonic site of origin at the base of the tongue, the foramen cecum. The thyroglossal duct degenerates, and the foramen cecum persists as a small blind pit.
The most common developmental abnormalities of the thyroid gland involve failure of appropriate embryonic thyroid descent. Thyroglossal duct cyst is a remnant of the thyroglossal duct. An ectopic thyroid gland is due to failure of embryonic dissent. Accessory thyroid tissue can occur at any site along the embryonic descent pathway from the base of the tongue to the thyroid gland isthmus. In approximately 40% of children, there is a persistent inferior segment of the thyroglossal duct. This pyramidal lobe is a midline superior extension of the isthmus that can attach to the hyoid bone by fibrous or muscular tissue.
The normal thyroid gland is in the infrahyoid compartment of the neck. The right and left lobes are located lateral to the trachea, and there is an anterior connecting isthmus. Structures adjacent peripheral to the thyroid gland include the esophagus, carotid arteries, jugular veins, strap muscles, and longus coli muscles. The normal newborn thyroid gland is 1.8 to 2.0 cm long and the anteroposterior diameter is 0.8 to 0.9 cm. At 1 year of age, the mean length is 2.5 cm and the diameter is 1.2 to 1.5 cm. The length in adults is 4 to 6 cm and the diameter is 1.3 to 1.8 cm.
The normal thyroid parenchyma is homogeneous on diagnostic imaging studies. The iodine content results in a slightly hyperattenuating character on unenhanced CT images. Thyroid tissue is slightly hyperechoic relative to neck muscles on sonography. The thin hyperechoic capsule is often visible on high-resolution images. Colloid follicles are occasionally visible in the normal thyroid gland as 1 or more small cystic areas, less than 3 mm in diameter. Inspissated colloid sometimes results in an echogenic focus within the cyst (Figure 32-1).
A longitudinal sonographic image shows multiple small hypoechoic cysts in an otherwise normal-appearing thyroid. The central echogenic foci represent inspissated colloid.
Of the available iodine isotopes, iodine-123 (123I) is optimal for standard functional and scintigraphic examination of thyroid. 123I has a short half-life and no β-emission; therefore, ...