Thyroid aplasia presents with CH and an absence of any thyroidal tissue. The defect is usually sporadic, but there are some familial forms, and both autosomal dominant and autosomal recessive inheritance patterns have been reported. Neck ultrasound fails to reveal any thyroid tissue in the thyroid bed, and no uptake is seen with I-123 scintigraphy.

Ultrasound in children with CH may sometimes demonstrate a small but otherwise normal-appearing thyroid gland (Fig. 5.2). This is particularly common in those children who have CH and trisomy 21 (children with trisomy 21 are also more likely to have small benign thyroid cysts) (Fig. 5.3). There are published charts of normal thyroid volumes for different populations, and these should be used to evaluate the size of the thyroid in children with hypothyroidism, congenital, or otherwise.

Thyroid hemiagenesis is a rare congenital anomaly in which one lobe of the thyroid fails to form. It is the most innocuous form of thyroid dysgenesis and is much more common in girls. In hemiagenesis it is almost always the left lobe that is absent; the right lobe has a normal size and appearance, and, although blunted, the isthmus is usually present (Fig. 5.4). For unknown reasons, the isthmus will be absent in rare cases of right hemiagenesis (Fig. 5.5). Although hemiagenesis is considered an incidental finding, numerous studies have shown that these patients have a slightly increased rate of all forms of thyroid pathology, from hyperthyroidism to carcinoma. The diagnosis of thyroid hemiagenesis should be clearly communicated to the surgeon in patients who are to undergo any type of neck surgery.

Ectopic, or aberrant, thyroid tissue can be found anywhere along the normal path of thyroid descent but is most commonly found at the base of the tongue, in which case it may be referred to as a lingual thyroid (Fig. 5.6). Ultrasound of the thyroid bed may reveal a hypoplastic thyroid or no thyroid tissue at all. Further imaging workup varies according to local preferences and practice; although I-123 scintigraphy is very sensitive for the presence of ectopic thyroid tissue, it fails to localize exactly where along the path from the base of the tongue to the inferior neck any potential aberrant thyroid tissue exists. Contrast-enhanced CT is especially helpful for the evaluation of thyroid ectopia because of its excellent spatial resolution and ability to provide multiplanar images, which best identify smaller rests of ectopic thyroid tissue (Fig. 5.7), or any potential thyroglossal duct remnants.

The thyroglossal duct is a normally transient structure that involutes during fetal life; if it fails to do so, any remnants may slowly collect fluid and eventually manifest as a cystic neck mass. Ninety percent of thyroglossal duct cysts (TGCs) present before age 10, and the typical history is a young child with a painless and compressible midline or paramidline mass. There is often a history of waxing and waning size, which occurs when there is irritation from recurrent upper respiratory tract infection or minor trauma. Sometimes TGCs become secondarily infected and present as an acutely painful, inflamed mass. If a TGC has not been complicated by prior infection or bleeding, then it appears as an anechoic cystic mass with increased through transmission and sharp, well-defined margins. They are usually spherical but may have tubular configuration (Fig. 5.8). About half are found at the level of the hyoid bone, a quarter above it, and another quarter below it; they should never be found below the level of the thyroid. If complicated by prior hemorrhage or infection, TGCs may have more echogenic contents, small septations, and a thickened, irregular margin; they occasionally appear solid (although should still cause increased through transmission) (Fig. 5.9). Complete surgical resection is needed, or TGCs will recur.

Another embryologic anlage that may cause pediatric thyroid abnormalities is the branchial apparatus. Branchial apparatus anomalies (BAAs) include a spectrum of epithelial-lined remnants that only involve the left lobe of the thyroid. Third and fourth BAAs may become infected and cause suppurative thyroiditis, abscess, cystic mass, or sinus tract to the pyriform sinus; imaging depends on the specific patient’s complication. Suppurative thyroiditis—which can be caused by a BAA or occur primarily—begins as a focal area of abnormally decreased echogenicity within the thyroid (Fig. 5.10). With worsening infection and abscess formation, a complex focal fluid collection may develop, usually with heterogeneous but hypoechoic internal contents; if caused by a BAA, a sinus tract may extend up into the deep neck toward the pyriform sinus. Such complications are best visualized by contrast-enhanced CT imaging (Fig. 5.11).