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OBJECTIVE: Type 1 diabetes mellitus is frequently associated with autoimmune thyroid disease (ATD). Genetic susceptibility to autoantibody formation in association with ATD and type 1 diabetes mellitus has been described with varying frequencies, but there is still debate about the situation in the Chinese population. We have, therefore, investigated the prevalence of anti-thyroid peroxidase (anti-TPO) in type 1 diabetic patients, and compared the effect of anti-glutamate decarboxylase (anti-GAD) on the thyroid autoimmunity in patients with type 1 diabetes mellitus in Taiwan. SUBJECTS AND METHODS: Two hundred and forty-three subjects with type 1 diabetes mellitus and seventy unrelated normal controls were recruited for the detection of anti-TPO. Two hundred and seventeen sera from two hundred and forty-three type 1 diabetic patients were tested for anti-GAD. RIA and immunoprecipitation were used for anti-TPO and anti-GAD detection respectively. RESULTS: The intra-assay and interassay coefficients of variation of anti-TPO detected by the RIA method ranged from 5.5% to 11.1%. Among 243 type 1 diabetic patients, 53 (21.8%) were positive for anti-TPO. Compared with those without thyroid autoimmunity, there was a female preponderance for the type 1 diabetic patients with thyroid autoimmunity (female:male, 99:91 vs 37:16 respectively). Among the type 1 diabetic patients with thyroid autoimmunity, anti-TPO tended to occur in those of older age or with long-standing disease. The frequency of anti-GAD was 45.6%, (99 of 217), without gender preponderance (males:females, 18.0% vs 27.61%). Compared with those with negative anti-GAD, no significant difference of anti-TPO positivity for the type 1 diabetic patients with positive anti-GAD was found. CONCLUSION: Our data indicated that the RIA method for anti-TPO detection is sensitive and precise for routine clinical use. The presence of anti-TPO in 21.8% of our type 1 diabetic patients confirmed the strong association of ATD and type 1 diabetes mellitus without ethnic differences. The absence of correlation between anti-TPO and anti-GAD in our type 1 diabetic patients suggested genetic heterogeneity in the role of autoimmunity of type 1 diabetes mellitus and ATD among races.