BACKGROUND:A common problem encountered in veterinary medicine is the euthyroid sick syndrome(ESS(,which refers to a state, where nonthyroidal illness(such as heart diseases)causes suppression of serum concentrations of thyroid hormone without true pathology of the thyroid gland. Chronic valve disease (endocardiosis) is the most common cause of heart failure in dogs. About a third of small-breed dogs older than10years of age are affected by heart diseases especially endocardiosis. OBJECTIVES: The aims of this study were to determine the effects of some acquired heart diseases (endocardiosis , dilated cardiomyopathy and congestive heart failure)on thyroid hormones, some serum biochemical and hematological indices in small breed dogs. METHODS:A total of 50small breed dogs‌ were selected.25 dogs were healthy and 15,7 and 3dogs had endocardiosis, congestive heart failure and dilated cardiomyopathy respectively which were confirmed by historical, physical examination, radiographic, and echocardiographic findings. Thyroid hormones (T4, fT4andTSH) were determined by using commercial canine Elisa kits. Serum biochemical analysis was carried out to determine the levels of creatine phosphokinase, aspartate aminotransferase,troponin I, blood urea nitrogen, creatinine, total protein, albumin, globulin, triglycerides, and cholesterol. Also Hematological indices (RBC,WBC,PCV,Hb and differential leukocyte count)were measured. RESULTS: One way ANOVA test showed that there were no significant changes in the serum levels of T4and fT4 in patients versus control group (p>0.05) ,only TSH concentration in dilated cardiomyopathy dogs compared to other groups was significant (p<0.05).Observations showed the creatine phosphokinase activity was significantly elevated in endocardiosis and congestive heart failure dogs in comparison with dilated cardiomyopathy and control groups. The serum concentration of protein significantly was decreased in Congestive heart failure dogs compared with endocardiosis and control dogs. Also, in comparison with the control group, serum level of triglycerides was diminished in Congestive heart failure dogs and blood urea nitrogen concentration significantly was increased in endocardiosis dogs compared with control.There were‌ no significant differences in other biochemical and hematological parameters. CONCLUSIONS:In our study,euthyroid sick syndrome or hypothyroidism were not seen in small-breed dogs suffering from heart disease. However in dogs with endocardiosis, increasing of serum creatine phosphokinase activity, could be symbol of complexity of heart disease.