- Embryologically, the primordium of thyroid gland appears as a diverticulum arising from the endoderm at dorsum of the tongue (foramen cecum), between the first and second pharyngeal pouches.
- The downward migration of the thyroid diverticulum forms the thyroglossal duct along the midline anteriorly to the hyoid bone and thyroid cartilage.
- Thyroglossal duct cysts appear due to failure of involution of a portion of the thyroglossal duct.
- Remnant of thyroid tissue (due to sequestration of thyroid tissue along the thyroglossal duct) and ectopic thyroid gland (due to incomplete descent of the thyroid into the lower neck) may be found anywhere this path of descent.
Fig. 1 - Illustration depicting the path of the thyroid descent. Thyroglossal cysts occurs along the migratory pathway of the thyroid gland and are always close to midline.
Before we talk about anatomy, to properly identify and scan the thyroid gland, some technical conditions must be fulfilled (Table 1).
Table 1. Thyroid ultrasound technique
NORMAL THYROID: ANATOMY AND SONOGRAPHY FEATURES
The thyroid is an “H” shaped gland located in the visceral space of the infrahyoid neck, about 1-3 cm above the breastbone. It is divided into two lobes connected by a narrow isthmus that crosses anteriorly the trachea. The anatomical relation between the thyroid and the neighborhood are the following:
- Anteriorly, the thyroid is covered by the thin strap muscles (sternohyoid, sternothyroid and omohyoid).
- Anterolaterally, we have the more bulky sternocleidomastoid muscles.
- Posteriorly to each thyroid lobe we have the longus colli muscles.
- Laterally to each lobe we have the common carotid arteries and jugular veins.
- Medially we have the trachea.
Table 2. Thyroid anatomic relashionships
Fig. 2 - Diagram of a thyroid transverse section at the level of C6 depicting the normal thyroid anatomic relations. st, sternothyroid muscle; sh, sternohyoid muscle; scm, sternocelidomastoid muscle; e, esophagus; jv, jugular vein, cca, common carotid artery; lcm, longus colli muscle.
Fig. 3 - Tranverse B mode ultrasound of the normal thyroid gland. *, thyroid isthmus; R, right thyroid lobe; L, left thyroid lobe; Tr, trachea; st, sternothyroid muscle; sh, sternohyoid muscle; scm, sternocelidomastoid muscle; e, esophagus; jv, jugular vein, cca, common carotid artery; lcm, longus colli muscle.
Fig. 4 - The esophagus is usually seen neighbooring the posterior and medial margin o left thyroid lobe. Note the "gut signature" also observed in the esophagus as different wall layers with alternating hypoechoic and hyperechoic appearance (arrows on zoomed upper right image). Note the echogenic foci (*) within the esophagus lumen representing air. Swallowing can help differentiating it from neck lesions.
ECHOGENICITY AND ECHOTEXTURE
The normal thyroid parenchyma is hyperechoic (bright) compared with the prethyroid muscles (strap muscles), with fine and homogeneous echoetexture and also a smooth echogenic capsule.
Fig. 5 - Upper image: Longitudinal gray scale ultrasound depicting the normal thyroid echogenicity and echotexture. Note the hyperechogenicity (brightness) of the normal thyroid (th) compared with the prethyroid muscles (*). Bottom image: Longitudinal gray scale ultrasound depicting a heterogeneous decreased parenchymal echogenicity, with a thyroid gland (th) echogenicity similar to the prethyroid muscles (*).
Fig. 6 - In addition to comparing the thyroid echogenicity with the strap muscles, we should compare its echogenicity with the submandibular gland. Both glands have similar echogenicity. Note the capsule depicted as a thin echogenic structure anteriorly lining the thyroid (arrowhead).
Measurement of thyroid lobes involves three measurements: length(L), depth(D) and width(W). The thyroid volume is calculated using the formula for a prolatte elipse: p/6(LxDxW). Normal ultrasound values for thyroid dimensios are 13-18 mm for width, 16-18 mm for depth and 45-60 mm for length with normal isthmus about 2-6 mm deep. The volume of a normal thyroid in adults is still source of debate, however, we can consider a normal volume of 7.7-19.1 cm3 in men and 4.8-15.1 cm3 in women.
Fig. 7 - Both the width and depth are measured in a transverse view of thyroid lobe (left image). The width (W) is measured from a horizontal imaginary line drawn along the lateral edge of the trachea to the lateral thyroid border. Depth (D) is measured as the maximum antero-posterior diameter at the middle third of thyroid lobe. Length (L) is measured in a sagittal view of the lobe (right image) as the maximum longitudinal distance from upper to lower thyroid poles. Bottom image shows the measurement of isthmus (i) anteroposterior diameter.
THYROID ARTERIAL SUPPLY
The blood supply of the thyroid gland is from the paired superior and inferior thyroid arteries (Fig. 8). The superior thyroid arteries usually are the first branches of external carotid artery. It is divided into an anterior branch, which runs toward the isthmus to anastomose with the contralateral artery and a posterior branch, descending through the back of the lobe to anastomose with an ascending branch of the inferior thyroid artery.The inferior thyroid arteries are the largest branches of the thyrocervical trunk, which arises from the subclavian artery. In about 10%, a thyroid ima artery arises from the brachiocephalic trunk, the arch of aorta or from the right common carotid. This small artery ascends anteriorly to the trachea and enters tthe inferior border of the isthmus (Fig. 8).
Fig. 8 - Arterial supply of thyroid gland detailed.
DOPPLER OF THYROID ARTERIES
Main thyroid vessels are better depicted at the thyroid poles (Fig. 9). The superior thyroid artery (STA) runs superficially to the anterior border of thyroid gland sending a branch deep to its upper pole (Fig. 9). The inferior thyroid artery (ITA) enters the tracheoesophageal groove posterior to carotid space and runs behind the thyroid gland penetrating its lower pole in its posterior surface (Fig. 9).
The reference values for doppler ultrasound parameters of thyroid arteries in healthy individuals have been described (Table 3). Usually the average systolic peak velocity (SPV) in STA is greater than in ITA, probably because of their afferent vessels (external carotid artery supplying the STA and thyrocervical trunk supplying the ITA). Another important observation is about the impedance indices, which are higher in STA. Perhaps these may be explained by the fact that the efferents vessels from STA supply not only thyroid parenchyma, but also cervical structures with high impedance indices, such as muscles and the larynx.
Fig. 9 - Upper image: Longitudinal B mode shows the superior thyroid artery (white arrow) penetrating the thyroid upper pole. Bottom image: Longitudinal B mode shows the inferior thyroid artery (blue arrow) penetrating the thyroid lower pole.
Fig. 10 - Duplex-colour Doppler of inferior thyroid artery (upper image) and superior thyroid artery (lower image).
Table 3. Reference values for Doppler ultrasound parameters of the thyroid arteries
1. Sadler, T W (Thomas W); Langman, Jan. Medical embryology. 12th ed. / T.W. Sadler. Philadelphia : Wolters Kluwer Health/Lippincott Williams & Wilkins, c2012.
2. Macedo, T. A. A., et al. "Reference values for Doppler ultrasound parameters of the thyroid in a healthy iodine-non-deficient population." The British journal of radiology 80.956 (2007): 625-630.
3. Robert A. Sofferman , Robert A. Sofferman, Anil T. Ahuja. Ultrasound of the thyroid and parathyroid glands. Springer-Verlag New York,2012.