MON-402 - From Lung Nodule to Thyroid Diagnosis: A Case of Incidental Metastatic Papillary Thyroid Carcinoma

Background:
Papillary thyroid carcinoma (PTC) is the most common thyroid malignancy and typically presents as a thyroid nodule. Distant metastases at initial presentation are uncommon, and incidental pulmonary metastases are particularly rare. Diagnosis becomes even more challenging in patients with concomitant granulomatous or vasculitic lung disease, in which inflammatory nodules may mimic malignant lesions. We present a case of incidentally discovered metastatic PTC in a patient with longstanding granulomatous lung disease.

Case Presentation:
A 56-year-old woman with eosinophilic granulomatosis with polyangiitis (eGPA) was referred to endocrinology after a right lower lobe lung lesion was incidentally identified as metastatic PTC on lung biopsy. Laboratory evaluation showed thyroglobulin 21.5 ng/mL, thyroglobulin antibody < 1.0 IU/mL, and TSH 1.11 uIU/mL. She had previously undergone resection of a left lower lobe granuloma more than 15 years earlier and had remained stable.
Surveillance imaging revealed a new right lung nodule; core biopsy showed necrotizing granulomatous inflammation without vasculitis or sarcoidosis. Due to lesion enlargement and chest discomfort, she underwent VATS wedge resection, which demonstrated necrotizing granuloma with a 0.3 cm focus of metastatic PTC. This finding prompted further work up with thyroid ultrasound, showing a 1.3 cm solid, hypoechoic, taller-than-wide left mid-lobe nodule (TI-RADS 5). Fine-needle aspiration was consistent with atypia of undetermined significance (Bethesda III) but ThyroSeq detected a TPM3/NTRK1 fusion (>95% probability of PTC). CT neck imaging showed a mildly enlarged left level II lymph node, though biopsy was negative. She underwent total thyroidectomy with resection of the left recurrent laryngeal nerve due to tumor involvement. Pathology confirmed classic PTC, 0.8 cm, confined to the thyroid parenchyma but present at the posterior margin, consistent with stage II disease (T1bN1M1). Postoperatively, she developed left vocal cord paralysis, which responded to Botox injections. She was started on levothyroxine with a target TSH < 0.1 uIU/mL. Postoperative thyroglobulin was 0.9 ng/mL with negative antibodies. She elected to proceed with adjuvant radioactive iodine therapy and remains under surveillance with serial thyroglobulin levels and imaging.

Discussion:
This case highlights the rare presentation of metastatic PTC incidentally identified in the lung in a patient with pre-existing granulomatous lung disease. It underscores the importance of careful evaluation of pulmonary lesions in patients with vasculitic conditions and demonstrates the utility of early recognition and importance of molecular testing (NTRK1 fusion) for risk stratification and surgical planning.

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