Introduction: Hashimoto's thyroiditis is a common autoimmune thyroid disease; however, pulmonary involvement is a rare manifestation. Lymphocytic interstitial pneumonitis is commonly associated with autoimmune diseases, but its association with Hashimoto's thyroiditis remains poorly understood. Case: A 63-year-old male with prior diagnosis of Hashimoto's thyroiditis presented with recurrent massive pleural effusion and progressive dyspnea. He also experienced fatigue, cold intolerance, and constipation. Physical examination revealed facial puffiness, diffuse goiter, and dry skin. Thyroid function tests performed 6 months before admission confirmed overt hypothyroidism (thyroidstimulating hormone [TSH] 80.30 µIU/mL, free thyroxine 4 [FT4] 0.46 ng/dL). One month prior to admission, laboratory findings showed subclinical hypothyroidism (TSH 9.09 µIU/mL, FT4 1.42 ng/dL). Despite levothyroxine treatment TSH levels remained elevated at 21.8 µIU/mL during admission. Anti-thyroid peroxidase antibodies were positive (13.95 IU/mL). Repeated thoracentesis revealed exudative pleural effusion with lymphocytic predominance and no evidence of malignancy in cytological examination. Infectious etiologies and malignancy were ruled out. Thyroid ultrasonography demonstrated diffuse hypoechoic enlargement of the thyroid gland. Chest radiography showed massive unilateral pleural effusion, while thoracic computed tomography revealed ground-glass opacities with a reticulogranular pattern and interlobular septal thickening, accompanied by multiple thin-walled cysts, highly suggestive of lymphocytic interstitial pneumonitis. Thyroid immunohistochemical analysis was performed (CD 20, CD 3, CD 10, CD 138, Ki 67, BCL 2, BCL 6, Thyroglobulin, MUM 1, Cyclin D1), consistent with lymphocytic thyroiditis. Significant clinical and laboratory improvement was achieved after combined treatment with levothyroxine and corticosteroid. Conclusion This case underscores the importance of considering autoimmune-related lymphocytic interstitial pneumonitis in patients with Hashimoto's thyroiditis who present with unexplained recurrent pleural effusion after exclusion of more common etiologies, such as infection and malignancy. Early recognition may lead to favorable improvement.
Lung Diseases, Interstitial ; Pleural Effusion ; Thyroiditis