Hyperthyroidism and hypothyroidism may upset function of circulation system. The aim of study was to determine the connection with congestive heart failure suffering from mitral endocardiosis and subclinical hypothyroidism. The research material included 27 dogs, patients of the Clinic of Horses, Dogs and Cats Diseases at the Faculty of Veterinary Medicine of the Agriculthural University of Wrocław. The control group included 13 healthy dogs, the clinical group included 14 dogs suffering from mitral endocardiosis and heart failury. The dogs was examined: interview, clinic examination, cardiological examination (echocardiographic and electrocardiographic examination,), blood examination (blood cell count, urea, creatinine, protein, Ca²⁺, Na⁺, K⁺, Cl^-, triglycerides, cholesterol, T₃, T₄, FT₄, TSH). The activity T₃ was statistically significant lowest, concentration of cholesterol was statistically significant lowest and higher R amplitude in klinical dogs group. Conclusion: in dogs suffering from mitral endocardiosis and heart failury did not diagnose subclinical hypothyroidism.

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