BACKGROUND: Concentrations of creatine kinase (CK) and lactate dehydrogenase (LD) in serum are increased in patients with hypothyroidism, but evidences for any changes in hyperthyroidism is controversial. The association between hypothyroidism and atherosclerotic cardiovascular disease (ASCVD) has been described for many years. In this study we compare CK, LD activities and its isoenzymes in patients with hypo- and hyperthyroidism. In addition we trying to find the association between ASCVD and thyroid diseases by measuring cardiac troponin I (cTnl). METHODS: Serum CK and LD activities, its isoenzymes and cTnI were assayed on 20 patients with hypothyroidism, and 55 patients with hyperthyroidism, and 24 individuals as a healthy control group during a period of 4 months in 1999. Medical records were analyzed retrospectively in order to find any evidence of cardiovascular diseases. RESULTS: Mean CK and LD activities are 206 U/L, 497 U/L in patient with hypothyroidism and 41 U/L, 335 U/L in patients with hyperthyroidism. cTnI was within the reference ranges for all patients, and they have no history of ASCVD. CONCLUSIONS: CK and LD activities are elevated in patients with hypothyroidism, and shows no statistical differences in patients with hyperthyroidism compared to normal individuals. When evaluating myocardial injury in a hypothyroid patient, cTnI is the superior marker compared to other muscle derived enzymes.