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Background: Dermatomyositis (DM) and polymyositis (PM) are two rare autoimmune disorders occasionally described with dysthyroidism; however, no solid evidence still proves such an association. Aim: To evaluate the prevalence of dysthyroidism among DM/PM patients. Design and setting: A nation-wide case-control study was conducted. Methods: From the Clalit Health Services health records database, we extracted 2085 (DM = 1475 (70.7%), PM = 610 (29.3%)) PM/DM cases and 10 193 sex-age matched controls in the period 2000-2018. Both univariate and multivariate analyses were performed to evaluate the link dysthyroidism and PM/DM. Survival analysis was also performed. Results: The rate of hyperthyroidism was significantly (P = .0097) higher in cases (n = 40, 1.9%) with respect to controls (n = 123, 1.2%). Similarly, the rate of hypothyroidism was significantly (P < .0001) associated with cases (n = 234, 11.2%) when compared to controls (n = 853, 8.4%). At the multivariate logistic regression analysis, both DM (OR 1.31 [95%CI 1.07-1.60], P = .0087) and PM (OR 1.54 [95%CI 1.21-1.95], P = .004) were significantly associated with hypothyroidism, whereas DM (OR 1.70 [95%CI 1.10-2.61], P = .0165) but not PM (OR 1.45 [0.83- 2.55], P = .1947) was found to be associated with hyperthyroidism. Subjects with PM and positive for anti-Sjögren's syndrome-related antigen A (SSA) auto-antibody displayed a significant risk of developing hyperthyroidism (OR 5.85 [95%CI 1.02- 33.74], P = .0480), whereas individuals with DM and positive for antinuclear antibody (ANA) had a higher risk of developing hyperthyroidism (OR 2.65 [95%CI 1.00-7.03], P = .0498). Conclusions: Physicians treating PM/DM patients should consider screening for thyroid dysfunction on a regular basis. [ABSTRACT FROM AUTHOR]