RESUMO
BACKGROUND: Mixed connective tissue disease (MCTD) is a connective tissue disorder characterized by high titers of distinct antibodies: U1 ribonucleoprotein with clinical features seen in systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), dermatomyositis (DM), polymyositis, and scleroderma. The association of SLE and DM with various cancers of the thyroid has been reported in the literature. However, there have been no reports associating MCTD with thyroid cancer. CASE REPORT: We present a 58-year-old woman diagnosed with MCTD with co-morbid interstitial lung disease that has remained stable for 10 years, who developed papillary thyroid carcinoma (PTC) 10 years after initial diagnosis. CONCLUSIONS: We theorize that: 1) MCTD may have been a primary diagnosis complicated by PTC, or 2) MCTD may have been an initial presentation of paraneoplastic syndrome of silent PTC, because her symptoms of MCTD significantly improved after total thyroidectomy. To the best of our knowledge, this is the first case report to associate MCTD with PTC. It highlights the importance of maintaining a high index of suspicion for thyroid malignancy in MCTD patients.