An Uncommon Presentation of Thyrothymic Thyroid Rests Presenting as a Lateral Lymph Nodal Mass

INTRODUCTION

Thyrothymic thyroid rests (TTR) also called Reeves rests are embryological remnants of the thyroid gland extending along the thyrothymic ligament from the inferior thyroid pole into the mediastinum. Other embryological remnants include the pyramidal lobe and the tubercle of Zukerkandl. These remnants are important as total thyroidectomy entails the removal of the thyroid gland along with all its embryological remnants to prevent recurrences. A detailed anatomical knowledge regarding all these remnants is crucial for the completeness of surgery. We present one such rare anatomical variation of TTR presenting as a separate supraclavicular mass lateral to carotid sheath in a follicular variant of papillary thyroid carcinoma (FVPTC) involving the right lobe of the thyroid.

CASE DESCRIPTION

A 19-year-old female presented with a painless progressive right supraclavicular mass of 8 months duration. Neck examination revealed a 5 × 4 cm firm mass in the right supraclavicular area, which was mobile and lower border plunging beneath the right clavicle. The thyroid was nonpalpable. Contrast-enhanced computerized tomography of the neck revealed a well-defined 5 × 4.5 × 2.7 cm lesion causing medial displacement of the right common carotid artery and right internal jugular vein (IJV) and another similar lesion of size 1.6 × 1.9 cm superior to it (Fig. 1). A well-defined nodule measuring 1 × 1.2 × 1.7 cm was found in the inferior pole of the thyroid gland abutting and displacing the right lobe anteriorly. Guided fine needle aspiration cytology (FNAC) from the thyroid nodule revealed and adenomatoid nodule (Bethesda 2), and the supraclavicular mass showed thyroid follicular cells without any lymph node architecture. A diagnosis of thyroid neoplasm with metastases in the right lateral level 4 lymph nodes was made and planned for surgical intervention. Intraoperatively after lateralizing the right sternocleidomastoid muscle, a 6 × 5 cm supraclavicular mass was noted lateral to the carotid sheath extending beneath the clavicle resembling TTR (Fig. 2A), separate from surrounding structures (Fig. 2B) which was excised (Fig. 2C) and sent for frozen study. After mobilizing carotid vessels laterally, the thyroid gland was approached that grossly appeared normal except a 1 × 0.5 cm nodule in the right lobe, near the tubercle of Zukerkandl. Additionally, grade 2 and grade 3 TTRs were found extending from the lower pole of the right lobe (Fig. 2D). Total thyroidectomy with right modified radical neck dissection and central compartment lymph node dissection was done (Fig. 3). Frozen study of the right supraclavicular mass revealed normal thyroid tissue also with thymic tissue. Patient had uneventful hospital course in the postoperative period. Histopathology showed FVPTC from the right lobe of the thyroid with metastatic deposits in 1 central and 1 right lateral node from 5 and 17 nodes respectively. Histopathology of the 5 × 7 cm right supraclavicular mass showed benign thyroid tissue without any lymph node architecture (Fig. 4). Four weeks after surgery with thyroid stimulating hormone (TSH) stimulation 65 mci of I-131 was administered, which did not reveal any metastatic deposits. Subsequently patient was started on thyroxine and completed 3 months of follow-up. Her stimulated serum thyroglobulin and antithyroglobulin antibodies were 12.9 ng/mL and <0.9 IU/mL, respectively.

DISCUSSION

The thyroid gland originates from the endodermal cells of the primitive pharynx by fusion between medial thyroid anlage (from tuberculum impar) and lateral thyroid process (from the endoderm of the ventral portion of fourth pharyngeal pouch) arising near the base of the tongue and migrates caudally to reach its usual position in the neck by the seventh week of gestation. The thyroid gland has three embryological remnants: the pyramidal lobe, the tubercle of Zukerkandl, and TTRs. Rests within the thyrothymic area occur in about 30–50% of the cases, and they can be mistaken as lymph nodes or parathyroid glands. Usually these are located between lower pole of the thyroid and the arch of the aorta along the thyrothymic tract. There are 80% of the identified rests that are connected to the thyroid proper either by a pedicle or by a fibrovascular band while 20% are separate. We did report a patient who underwent total thyroidectomy for papillary thyroid carcinoma with a missed grade IV TTR in the anterosuperior mediastinum. This case was diagnosed by non-elevation of TSH in the postoperative period in spite of thyroxine withdrawal for 6 weeks.¹ Majority of the TTRs are small in size (<1 cm). Based on its relation to the inferior pole of the thyroid, Sackett et al. classified TTRs as grade IV in Table 1.²

Total thyroidectomy is the most common thyroid surgery performed and is defined as the removal of all visible thyroid tissue including its embryological remnants. So, proper knowledge regarding anatomy and possible embryological variations is required to ensure the completeness of surgery and to prevent a recurrence. Our experience with these embryological remnants in 1,118 consecutive total thyroidectomy patients revealed 20.57% had some form of TTR of which the majority were grade I (49.13%) and grade II (32.17%).³ Grade IV TTRs were comparatively less (2.6%) and retrosternal/intrathoracic goiter presentations were not uncommon (0.43%). Of those, 230 TTRs in our case series, there was no thyroid tissue that was found lateral to the carotid sheath and all were within the thyrothymic area medial to carotid sheath with few extending into the superior/anterior mediastinum. This case is unique with regard to its lateral position of the TTR to the carotid vessels and IJV. This TTR mimicked a lymph nodal mass with suspected metastatic deposit from the thyroid tumor. Clinically a differentiating feature of this TTR from lymph nodal metastases was its soft to firm consistency. So, we did consider a possibility of TTR in the preoperative evaluation because of its consistency and absence of lymph nodal elements in the preoperative FNAC. Snook et al. reported that 10 patients out of 3,044 total thyroidectomies performed for multinodular goiter, developed recurrence, and required reoperation despite previous “total” thyroidectomy.⁴ Of these, only one was a true local recurrence in the thyroid bed and all others were related to the embryologic remnants of the thyroid gland, four in the pyramidal tract, and five in the thyrothymic tract. In <5% of cases, they present as nodular enlargement and is attached to thyroid gland proper through a narrow pedicle of thyroid tissue or can be detached from gland. True thyrothymic rests that are not connected to the thyroid gland should be distinguished from detached nodules that occur because of intraoperative manipulation during mobilization of the inferior thyroid poles. Sackett et al. examined 180 sides of the thyroid gland in 100 consecutive patients in which 53 patients had thyrothymic rest of which 57% had bilateral rests, 30% only on the right, and 13% only on the left.² About 8% of identified rests were attached to the thyroid properly by a pedicle of thyroid tissue and 20% were separated. Most of the TTRs were subcentemetric (88%). Gurleyik et al. studied 222 sides of the thyroid.⁵ Grade IV TTR was identified and excised in 8 of 134 patients (6%) with mean size of 36.4 mm. We could not find any literature supporting the occurrence of TTR presenting as a neck mass lateral to the carotid sheath and this case might add to its armamentarium of the vagaries of TTR presentation.

ORCID

Sahithi Priya Boddukura https://orcid.org/0000-0002-3585-5143

Dhalapathy Sadacharan https://orcid.org/0000-0003-4744-6395

REFERENCES

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