Thyroid "claw sign" a useful diagnostic marker in the outsized lesions of isthmus: A large colloid cyst

Abstract

Thyroid isthmus lesions are generally small sized and can be solid or cystic. Discerning isthmic origin of a large nodule, especially if purely cystic, can become a diagnostic challenge because of thin thyroid tissue in it. We report a case of a 68-year-old male patient who had 6 weeks history of non- inflammatory central neck swelling associated with recent dysphagia, for which he underwent ultrasound and computed tomography (CT) scan examinations. Colloid nodules usually do not require further attention. Despite being commonest and benign thyroid nodules, they may require treatment if causing pressure symptoms. Its imaging characteristics can be variable, but they usually exhibit comet tail artifacts on ultrasound. In equivocal cases, claw sign on CT scan is diagnostic to confirm the site. Radiologists have a principle role to rule out other differentials of cystic neck lesions by careful examination of imaging features. In our case, CT scan allowed to rule out primary differential of thyroglossal cyst and guided clinicians for specific management plan.

Keywords: Claw sign; Colloid cyst; Comet tail artifact; Computed tomography (CT); Isthmus.

Fig. 1

(a,b): Ultrasound neck showing a large cyst having homogenously low echogenic content with few tiny echogenic foci showing no comet tail artifact (arrows). Thyroid lobes (T) and airway (A).

Fig. 2

Doppler ultrasound showing avascular cyst.

Fig. 3

(a,b): Ultrasound showing internal content of the colloid cyst. Basal homogenous low echogenic and upper anechoic content(a,b). Arrow heads are showing curved upper margin of the basal content (a).

Fig. 4

(a-c): Longitudinal ultrasound images of the thyroid lobes. Both lobes showing heterogenous echotexture (a-c). Solitary solid nodule in the left lobe (b,c,C), showing vascularity on doppler ultrasound (c).

Fig. 5

(a-f): Contrast enhanced computed tomography (CECT) neck: Coronal images showing colloid cyst (asterisks) from anterior to posterior (a-c). Sagittal image (d) showing extent of the cyst from thyroid cartilage (arrow) to just above the clavicles (open arrows b,f). Axial image (e) showing relationship between cyst (asterisk) and thyroid gland (T), note the anterolateral displacement of strap muscles (arrowheads).

Fig. 6

Sagittal CECT neck showing CT attenuation taken at different points of the colloid cyst (varied between 27-51 Hounsfield Unit.

Fig. 7

(a-c): Axial CECT neck showing colloid cyst (asterisks) showing “claw sign” in right lobe (a, arrow). Same axial section but with increased contrast for clear demonstration of the “claw sign” in right lobe (b). “Claw sign” in left lobe (c, arrow). Thyroid lobes (T) and strap muscles (S).

Fig. 8

(a,b): 3D CT volume rendered images for skin, showing midline neck swelling (arrows).

Fig. 9

3D CT volume rendered images for air spaces showing no obvious compression effect by the colloid cyst on airway (multiple small arrows). The surface bulge of the neck swelling (long arrow). Air seen in the lung apices (asterisk).

Fig. 10

Simplified diagrammatic representation of proposed mechanism of formation of colloid nodule.

Fig. 11

(a,b): Pictorial representation of difference between intrinsic and extrinsic thyroid isthmic mass. (a) Pointed claw like margins of thyroid tissue which partially wraps around an intrinsic mass (arrows). (b) Rounding of the thyroid margins (arrowheads) which does not wrap around an extrinsic lesion.