Introduction:In papillary thyroid cancer (PTC), lymphatic metastases can be found in 50-60% of cases and distant metastases in 7-23%, lung and bone being the most frequent sites. In rare cases, it can metastasize to the pleura and manifest as a malignant pleural effusion (MPE) and portend a poor prognosis. The physical characteristics of MPEs do not allow distinguishing between PTC and other malignant causes. Cytology is important for differential diagnosis as well as immunohistochemistry and immunostaining, which is positive in differentiated thyroid cancer.

Objective: To report a case of the use of Tg washout (TW) in pleural effusion in metastatic PTC.

Clinical Case:60-year-old female who underwent total thyroidectomy plus lateral neck dissection for PTC, classic variant; ATA high risk, AJCC stage IV A; in treatment with levothyroxine sodium and calcium and calcitriol supplements for postsurgical hypoparathyroidism. Iodine 131 ablation was indicated according to the NCCN guidelines, but it was pending at the time of her hospitalization. She was admitted for dyspnea at rest and cough of 2 weeks of evolution, on physical examination with hypoventilation, dullness and decreased vocal vibrations in the left lung field. Chest X-ray: left pleural effusion of approximately 70%, for which an endopleural tube was placed. Chest CT scan revealed a left pleural effusion, with multiple nodules in the right lung and mediastinum. Blood test: TSH 4.19 mIU/ml, FT3 3.82 pg/ml, FT4 0.85 ng/dl, Tg 2245.7 ng/ml, TgAb 0 IU / ml. Hormonal replacement with levothyroxine sodium was adjusted to 175 mcg/ daily. Pleural fluid studies: TW> 24450 ng/ml, cytochemistry oriented exudate and cytological positive for malignant cells.

Discussion: TW in lymph nodes was proposed since the cytopathological diagnoses of these may be inadequate or inconclusive; even the ATA and AACE guidelines have recognized its importance. This offers the advantage that even if epithelial cells are not found in the aspirate, Tg may be elevated both inside and outside the lesion. A Tg value greater than 10 ng/ml has been taken as highly indicative of disease. Until now, the use of TW has only been described in lymph nodes. The cytochemical and cytological studies in our case were not definitive for PTC, we performed TW of the pleural fluid using the same cut-off point for the lymph node, with a positive result.

Conclusion:TW is a useful tool for the diagnosis of lymph node metastatic disease in PTC, which can also be applied to other tissues.