SUN-420 - Mimicking Malignancy: A Case of Perithyroidal Thymoma

Background:
The prevalence of thyroid nodules is 25% in the US population. Parsing the etiology of these, most often benign, growths is a core role of endocrinologists. Thyroid neoplasms are usually derived from thyrocytes, parafollicular cells, stromal, or lymphatic tissue in that order. Embryologically, the thyroid and thymus originate from neighboring pharyngeal tissues, similarly migrating inferiorly to their final anatomic locations at the base of neck and mediastinum, respectively. Thus, thymic remnants may appear as solid nodules in the region of the thyroid on ultrasound. Rarely, thymomas arise which may closely mimic thyroid malignancy both clinically and radiographically, generating clinician and patient concern. This report serves to inform an expanded differential diagnosis for thyroid nodules.

Clinical

Case:
A 32-year-old woman was seen for a thyroid nodule newly detected on physical exam one month prior. She denied compressive symptoms, pain, prior thyroid disease, radiation exposure or family history of thyroid cancer. Labs showed TSH 1.86 µIU/mL (n = 0.4–4.5), and free T4 0.94 ng/dL (n = 0.8–1.8). She was clinically euthyroid with a palpable 3 cm solid, fixed, firm left thyroid nodule of smooth contour and a 0.5 cm smooth mobile lymph node at right neck zone Va.
Ultrasound on referral delineated a 2.8 × 4.2 cm solid left inferior thyroid nodule. Limited ultrasound at FNA biopsy performed on referral congruently noted a 4.0 cm left thyroid nodule. Cytology showed chronic lymphocytic thyroiditis, Bethesda II.
One month later, she developed right-sided neck fullness and discomfort on phonation. Repeat ultrasound showed an exophytic hypoechoic lesion from the left inferior thyroid lobe now 4.3 × 3.6 × 2.8 cm with extension into neck zone VI. Surgical evaluation was clinically suspicious for mediastinal extension of the symptomatic mass therefore left hemithyroidectomy with paratracheal exploration was performed four months after initial presentation.
Final pathology revealed a 5 x 3.5 x 2 cm encapsulated type AB thymoma confined to ectopic thymic tissue resected from the left paratracheal region of the superior mediastinum, pT1aN0. A separate 2.3 x 1 x 1cm mass resected from the left paratracheal area was found to be benign thymic tissue. The 3 x 2.5 x 1.2 cm left hemithyroidectomy specimen comprised unremarkable thyroid tissue. Postoperatively, she reported mild fatigue and constipation, without symptoms of myasthenia gravis. Labs showed subclinical hypothyroidism.

Conclusion:
This case describes a thymoma adjacent to the thyroid gland presenting as a thyroid nodule mimic, leading to diagnostic uncertainty. Although rare, thymic remnants located close to or even within the thyroid should be considered in the differential when imaging shows atypical anatomy, as definitive diagnosis may require surgical excision notwithstanding Bethesda II cytology.

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