Primary hypothyroidism due to thyroid carcinoma in a feline /Hipotireoidismo primario devido a carcinoma de tireoide em um felino

Hypothyroidism is rarely observed in cats. This article reports the case of a thirteen-year-old feline with a one-year history of bilateral cervical swelling that had majorly grown in size two weeks prior to the animal's admission. The cat presented with dysphonia, lethargy, weight gain, and constipation. On physical examination, hypothermia was observed, with a rectal temperature of 36.5 [degrees]C. Fine-needle aspiration cytology suggested thyroid neoplasia. The serum levels of total T4 and thyroid stimulating hormone (TSH), along with the exclusion of non-thyroid diseases, confirmed the diagnosis of hypothyroidism. This is the first report of feline hypothyroidism in Brazil secondary to thyroid neoplasia, classified as a follicular-compact carcinoma and confirmed by histopathology. Key words: feline, follicular-compact carcinoma, hypothyroidism. Hipotireoidismo e uma doenca rara na especie felina. Esse trabalho relata um felino de treze anos de idade com historico de inchaco cervical bilateral ha um ano, que havia crescido principalmente duas semanas antes da admissao do animal. O felino apresentava disfonia, letargia, ganho de peso e constipacao. No exame fisico, hipotermia foi observado com temperatura retal de 36,5 [degrees]C. Citologia aspirativa de agulha fina sugeriu neoplasia de tireoide. A dosagem serica de T4 total e TSH junto com a exclusao de doencas nao tireoidianas confirmaram o diagnostico de hipotireoidismo. Esse e o primeiro relato de hipotireoidismo felino no Brasil devido a uma neoplasia de tireoide, classificado como carcinoma folicular-compacto, confirmado na histopatologia. Palavras-chave: felino, carcinoma folicular-compacto, hipotireoidismo.