Objective To investigate the changes in serum level of myocardial enzymes in patients with primary hypothyroidism and to evaluate the relationship between the level of those enzymes and clinical or experimental parameters.Methods We measured several myocardial enzymes in serum of 108 patients with primary hypothyroidism including 31 cases of Hashimoto's thyroiditis,28 cases of atrophic thyroiditis,24 cases of Graves' hyperthyroidism treated by 131I,and 25 cases of Graves' hyperthyroidism treated by antithyroid drugs,as well as 50 normal controls.Results All the myocardial enzymes,including creatine kinase(CK) and its isoenzyme CK-MB,aspartate aminotransferase(AST),lactate dehydrogenase(LDH) and its isoenzyme HBDH,especially CK,were elevated in serum of patients with hypothyroidism.In addition,significant negative correlations were found between CK and FT3,CK and FT4,CK-MB and FT3,CK-MB and FT4.No difference in enzyme level was found among different hypothyroidism groups.Conclusion Hypothyroidism may cause elevation of myocardial enzymes in serum of the patients.The degree of CK and CK-MB increase was associated with the severity of hypothyroidism,and thus can be regarded as indexes to determine the severity of hypothyroidism.