Heart failure in a cat due to hypertrophic cardiomyopathy phenotype caused by chronic uncontrolled hyperthyroidism.

A 16-year-old castrated male Persian cat was presented with weight loss, anorexia and dyspnoea. Tachycardia and tachypnoea were observed upon presentation. The cat was previously diagnosed with hyperthyroidism and left ventricular hypertrophy and received methimazole, but was subsequently not followed up and treated appropriately. Thoracic radiography revealed mild pleural effusion, interstitial lung pattern, moderate cardiomegaly and moderate-to-severe dilation of the pulmonary artery and pulmonary vein. On echocardiography, the left ventricular hypertrophy, identified earlier, shoed partial regression. Therefore, the previous myocardial hypertrophy was diagnosed as a hypertrophic cardiomyopathy phenotype related to hyperthyroidism. ST-segment elevation was identified on electrocardiography, and the thyroid profile examination revealed increased total thyroxine and free thyroxine and decreased thyroid-stimulating hormone levels, suggesting myocardial injury and uncontrolled hyperthyroidism, respectively. In addition, normal N-terminal pro-B-type natriuretic peptide and high cardiac troponin I levels were found. Based on these findings, the observed congestive heart failure was considered as a sequel of myocardial injury caused by uncontrolled hyperthyroidism. Clinical signs resolved after intravenous administration of furosemide and butorphanol, oxygen supply and thoracocentesis. Furosemide and pimobendan were additionally administered, and the cat was discharged. This case demonstrates that myocardial damage due to chronic uncontrolled hyperthyroidism may cause heart failure in cats.