“White knight,” “giraffe hide,” and “Swiss cheese” appearance

Abstract

Sir, Recently, four women, being treated with levothyroxine sodium tablet for primary autoimmune hypothyroidism, were referred to us for further evaluation of “suspicious” features, detected on thyroid ultrasonography (USG). The first woman had one small hyperechoic space-occupying lesion (SOL) [Figure 1a], while the second woman had a larger SOL with similar appearance [Figure 1b and c]. Multiple echogenic nodules were noticed in the third patient [Figure 1d and e], while the fourth patient demonstrated multiple hypoechoic areas scattered throughout the gland [Figure 1f and g]. Thyroid glands were enlarged in all of them with lobular surfaces, and USG-guided fine-needle aspiration cytology (FNAC) was suggestive of chronic lymphocytic thyroiditis (CLT).Figure 1: USG appearances of the patients. (a) (Patient 1): Sagittal view showing hypoechoic gland with a small “white knight” nodule (black arrow). Note the heterogeneous appearance of the gland and the linear hyperechoic lines (represents fibrous bands). (b) (Patient 2): Sagittal view showing large “white knight” nodule (white arrow). (c) (Patient 2): Transverse view showing the same “white knight” nodule (white arrow) in the left lobe. Note the isoechoic nodule in the right lobe (notched white arrow) in the background of hypoechoic gland. (d and e) (Patient 3): Transverse and sagittal views showing focal hypoechoic area (notched white arrow in d) and “giraffe hide” pattern (e). (f and g) (Patient 4): Sagittal view showing the “Swiss cheese” pattern. USG: UltrasonographyCLT, also termed as chronic Hashimoto’s thyroiditis, is the most common etiology of primary hypothyroidism worldwide. The pathological hallmark of CLT is lymphocytic (B-cells and cytotoxic T-cells) infiltration of the thyroid gland; the degree of glandular involvement depends on the stage of the disease. The thyroid gland in CLT typically is diffusely enlarged without any palpable nodule. However, heterogeneous and patchy involvement produces regional parenchymal distortion resulting in a lobular or nodular appearance leading to ultrasonographic evaluation of the thyroid gland. USG in such glands often detects one or more “pseudonodule(s).” Normal thyroid parenchyma appears homogeneously hyperechoic compared to the surrounding strap muscles of the neck. Areas of lymphocytic infiltration appear hypoechoic compared to normal thyroid parenchyma. The thyroid gland in CLT usually has heterogeneous echotexture on USG; regions of lymphocytic infiltration appear hypoechoic, unaffected islands of thyroid parenchyma demonstrate normal echogenicity, and early fibrosis appears as linear echogenic bands. However, the sonological appearance in CLT varies, and the characteristic pattern recognition obviates unnecessary FNAC. A solitary, well-circumscribed, homogenous, markedly hyperechoic nodule of varying size, without internal vascularity, cystic degeneration, or calcifications termed “white knight” nodule [Figure 1a-c], may occasionally be encountered in such patients. Present on a background of a hypoechoic gland, these nodules seem to be regenerative nodules of CLT. Despite its remarkable appearance, such a nodule is considered benign and does not merit further evaluation.[1,2] When multiple echogenic nodules (represents uninvolved areas with normal follicular structure) dispersed across the gland are separated from one another by bands of hypoechogenicity, the ultrasound appearance is called “giraffe hide” (1E), reminiscent of giraffe’s skin. This distinctive pattern is always benign and characteristic of CLT.[1,2] Although the reverse pattern, i.e., multiple hypoechoic areas (represents aggregates of lymphocytes) with ill-defined margins surrounded by echogenic areas of thyroid parenchyma or fibrosis, more closely resembles giraffe skin, which consists of dark areas surrounded by bright lines, the term “giraffe hide” pattern was originally described for the former one. The later pattern, however, is also a characteristic of CLT. More discrete aggregates of lymphocytes appear as discrete focal hypoechoic regions on the background of an echoic gland. Numerous such well-defined hypoechoic areas give the gland a “Swiss cheese” appearance [Figure 1f and g], similar to the holes in Swiss cheese. Once these small hypoechoic areas become almost confluent and there is marked fibrosis with very little parenchyma separating the hypoechoic tissue, the appearance is called “honeycomb” variant. The vascular pattern distinguishes these hypoechoic “pseudonodules” from thyroid nodules. These hypoechoic areas do not demonstrate vascular flow, whereas vascular flow is seen over the isoechoic normal thyroid parenchyma, especially along the areas of fibrosis. The decision for FNAC from a thyroid nodule depends on the size of the nodule and the American College of Radiology (ACR) Thyroid Imaging, Reporting and Data System (TI-RADS) staging.[3] However, the ACR TI-RADS classification does not consider these patterns. Recognition of such characteristic morphologic patterns substantially decreases the number of unnecessary invasive interventions. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.