Background: Some of the thyroid disorders (TD) and Myasthenia gravis (MG) are autoimmune related disease. The purpose of the study to evaluate the relationship of MG with all morphological and functional thyroid disorders. Methods: We constructed a population-based cohort study during the period from January 2000-December 2002 by using reimbursement data from the Bureau National Health Insurance (NHI) system in Taiwan. Patients with TD and MG were identified by referring to the ICD-9-CM codes. (ICD-10-CM as reference) .The association of TD with MG occurred only in the same person within the study period. The Q value was used to measure the strength of disease-disease associations. Results: We obtained 520628 TD and 7965 MG records for analysis. Diffuse toxic goiter had highest association rate, followed by nontoxic nodular goiter, simple goiter, chronic lymphocytic thyroiditis, thyroid cancer, and toxic nodular goiter. Female and older patients had a higher rate than their male and younger counterparts, respectively. Functional abnormalities revealed higher incidence of thyrotoxicosis and hypothyroidism in both sexes. We also found the strongest association in men with chronic thyroiditis, diffuse toxic goiter, thyrotoxicosis, acquired hypothyroidism, thyroid cancer, and simple goiter. While an intermediate association was observed in female with diffuse toxic goiter, in a male with toxic and nontoxic nodular/multinodular goiters, in female with thyrotoxicosis, thyroid cancer and acquired hypothyroidism. Conclusion: This population based cohort study showed potential association of all types of TD with MG, and observed a higher association rate in female autoimmune TD whereas males showed a higher strength of association.
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