OBJECTIVE: There are few large cohort studies available on the association of systemic and thyroid autoimmune diseases. In this study, we wished to determine the association of Hashimoto's thyroiditis (HT) and Graves' disease (GD) with systemic autoimmune diseases. METHODS: One thousand five hundred and seventeen patients with systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), systemic sclerosis (SSc), mixed connective tissue disease (MCTD), Sjogren's syndrome (SS) and polymyositis/dermatomyositis (PM/DM) were included in the study. The HT and GD were diagnosed based on thorough clinical evaluation, imaging and fine-needle aspiration cytology (FNAC). The frequency of HT and GD in these diseases was assessed. In addition, 426 patients with HT or GD were assessed and the incidence of SLE, RA, SSc, MCTD, SS and PM/DM among these patients was determined. Prevalence ratios indicating the prevalences of GD or HT among our autoimmune patients in comparison to prevalences of GD or HT in the general population were calculated. RESULTS: Altogether 8.2% of systemic autoimmune patients had either HT or GD. MCTD and SS most frequently overlapped with autoimmune thyroid diseases (24 and 10%, respectively). HT was more common among MCTD, SS and RA patients (21, 7 and 6%, respectively) than GD (2.5, 3 and 1.6%, respectively). The prevalences of HT in SLE, RA, SSc, MCTD, SS and PM/DM were 90-, 160-, 220-, 556-, 176- and 69-fold higher than in the general population, respectively. The prevalences of GD in the same systemic diseases were 68-, 50-, 102-, 76-, 74- and 37-fold higher than in the general population, respectively. Among all thyroid patients, 30% had associated systemic disease. In particular, 51% of HT and only 16% of GD subjects had any of the systemic disorders. MCTD, SS, SLE, RA, SSc and PM/DM were all more common among HT patients (20, 17, 7, 4, 2 and 2%, respectively) than in GD individuals (2, 5, 5, 1, 2 and 1%, respectively). CONCLUSION: Systemic and thyroid autoimmune diseases often overlap with each other. HT and GD may be most common among MCTD, SSc and SS patients. On the other hand, these systemic diseases are often present in HT subjects. Therefore it is clinically important to screen patients with systemic autoimmune diseases for the co-existence of thyroid disorders.
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