We present the case of an adult female patient without any previous medical history and with a firm mass on neck examination besides normal thyroid function and laboratory tests, who was diagnosed with ectopic intrathyroidal thymic tissue, which has been rarely reported, in a few case reports before. The clinical and pathologic features presented by this case provide evidence for the management of similar cases with this rare diagnosis. A systematic search found four similar papers previously reporting ectopic thymus in thyroid in adult patients [2, 4, 6, 7]. It is interesting that all of the reported cases were consistent in the female sex of the patients.
Moon et al. reported a 31-year-old female patient with left ectopic intrathyroidal thymic tissue and a right papillary thyroid carcinoma (PTC) that first presented with enlargement of the right lobe of the thyroid with low serum thyrotropin and elevated free thyroxin in laboratory tests. Their investigation with US found a hypoechoic mass in the right lobe later, confirmed as PTC, but nothing in the left lobe. Further examination by computed tomography (CT) revealed a small hypoattenuating nodule in the left lobe in the precontrast phase and no enhancement in the postcontrast phase. The patient later underwent total thyroidectomy, and histological examination of the left lobe found mature adipose tissue, lymphoid tissue, and Hassall’s corpuscles, compatible with thymic tissue .
Kim et al. reported a 29-year-old woman with history of right thyroidectomy due to PTC 3 years before her referral and a right neck mass. The initial examination with US showed a well-defined hyperechoic nodule in the left lobe of the thyroid. The patient underwent total thyroidectomy with right neck dissection. On gross examination, a well-defined pale-yellow nodule of 0.9 cm was seen in the left thyroid lobe, which on microscopic examination, mature adipose tissue, ectopic thymic tissue with Hassall’s corpuscles, and intrathyroidal parathyroid tissue was found .
O’Connor et al. reported a 23-year-old female patient with Grave’s disease refractory to medical management and a history of clinical hyperthyroidism, exophthalmos, and asthma. Cervical US showed a hypervascular and enlarged thyroid, and the gland had diffuse homogeneous radioiodine uptake on nuclear imaging. The patient underwent total thyroidectomy and right inferior parathyroidectomy, which was enlarged during the surgical exploration. Surgical pathology and histopathology examination of the excised tissues showed diffuse hyperplasia of thyroid tissue with colloid involution consistent with the treated Grave’s condition and focal ectopic intrathyroidal thymic tissue and some chondroid metaplasia .
Velimezis et al. reported a 26-year-old woman with hyperthyroidism controlled with levothyroxine and a heterogeneous 1.15-cm nodule containing microcalcifications in the right thyroid lobe, suggesting the presence of malignancy on the cytology examination. The patient underwent total thyroidectomy, and postoperative histopathology examination found multifocal PTC associated with ectopic intrathyroidal thymic and parathyroid tissues in the right thyroid lobe. The patient had no complications in the 2-year follow-up period after the surgery .
Ectopic intrathyroidal thymic tissue has been studied in more detail in pediatric patients because it is more prevalent at lower ages. Ectopic thymic tissue results from deviation in the normal migration path of the thymus, which originates from the third pharyngeal pouches and moves medially and caudally to locate at the superior mediastinum by the eighth week of embryogenesis [2, 4, 5]. Being adjacent to the origin of inferior parathyroid glands and developing in a path through the thyroid, the presence of ectopic tissues of these three organs near each other is possible, as presented herein and the reviewed literature [1, 7]. A large-scale study of 375 cases of pediatric patients with ectopic intrathyroidal thymic tissue proposed that the typical findings of this condition on US are irregular, triangular, or polygonal hypoechoic or hyperechoic areas with punctate, granular, and linear echogenic foci inside the lesions . A systematic review on ectopic intrathyroidal thymic tissue in children proposed that the cytology appearance of this lesion is benign-appearing lymphocytes, often with minimal or no epithelial cell components . Also, this systematic review found the classic appearance of a hypoechoic mass with sharp margins and multiple focal internal echogenicities without shadows, just like normal thymic tissue in the US exploration of the lesion .
Critical differential diagnoses of an ectopic intrathyroidal mass full of lymphocytes are acute lymphoid leukemia, thymic carcinoma, carcinoma showing thymus-like differentiation (CASTLE), spindle cell tumors with thymus-like differentiation, and thymoma . Thymic tissue present within the thyroid gland can go through hyperplastic and metaplastic changes and result in benign and malignant lesions such as intrathyroidal epithelial thymoma (ITET) and CASTLE [2, 6]. Despite the rare prevalence of ectopic presence of thymic tissue in the thyroid gland, transformation to malignancies in this ectopic tissue should be kept in mind as series of patients with tumors such as ITET/CASTLE have been reported, and the appropriate management of the condition requires adequate awareness of it .
As this condition is rare, overall and exceptionally in adults, the management of the lesion is subject to great doubt and remains unknown. However, all the reviewed cases, similar to the current reported case, underwent total thyroidectomy with no need for further intervention except follow-up and close monitoring of surgical complications. A previously mentioned study proposed an algorithm for diagnosing and evaluating ectopic intrathyroidal thymic tissue through a comprehensive systematic review of available literature on pediatric patients . According to this algorithm, if the lesion has definitive US features of thymic tissue, no further evaluation or follow-up is needed. Otherwise, if the lesion is smaller than 10 mm, deferring surgery and monitoring with serial US is suggested. For lesions ≥ 10 mm, US-guided FNA is suggested. Next, running flow cytometry using CD4 and CD8 markers is suggested to differentiate the lesion as mature thymic tissue from T lymphoblastic leukemia and thymic carcinoma . However, one should bear in mind that this algorithm is proposed based on evidence from pediatric patients, and applying it to adults should be done with caution.