Thyroid carcinomas in dogs – a literature review and retrospective study on the outcome of treatment for dogs treated at the Swedish University Animal Hospital (UDS), Uppsala 2008-2018

Abstract: Thyroid carcinomas is not the most common canine neoplasm, but since it is more common than the benign thyroid adenoma it is still an important disease to recognize. Most commonly it affects older dogs, with a mean age of about 10 years. No predilection for gender is seen, but it seems to be more common in certain breeds like Golden retriever, Beagle and Boxer. The etiology of thyroid carcinomas is still unknown. There are several theories, but no proven facts. This makes it difficult to treat the condition medically, as we are not sure what to direct the treatment at. The most successful treatment seems to be surgery, surgery combined with chemotherapy or surgery combined with external radiation therapy. There are a few studies on the subject which do not all agree with each other. They do however all agree that treatment improves survival time. The most common clinical sign of thyroid carcinoma is a lump in the ventral throat region. Dyspnea, difficulties to swallow, cough or signs of hyper-/hypothyroidism are other reported symptoms. Small studies on thyroid carcinoma and the outcome of treatment have been published abroad, but none on the dogs treated at the Swedish University Animal Hospital in Uppsala (UDS). The aim of this study was therefore to increase the knowledge of thyroid carcinomas by writing a literature review, and also to investigate the prognosis of treatment by comparing different treatment methods with no treatment. In addition, the study also investigated methods of diagnosis and tried to identify factors which increase/decrease survival time. The foremost goal was to give veterinarian clinicians more information about the diagnosis, ways of treatment and the prognosis of thyroid carcinomas. The hypothesis was that treatment (surgical, medical and/or radiation) increased survival time in dogs diagnosed with thyroid carcinomas. In agreement with previous studies, both groups of treated dogs showed a longer survival time than the dogs not treated. However, this difference could not be shown to be statistically significant. As for the factors increasing/decreasing survival time, the only significant result was that neoplastic ectopic thyroid tissue decreased survival time. All other analyzed factors such as clinical attachment to underlying tissue, presence of metastasis, tumor volume and hyperthyroid VS euthyroid dog showed a logical trend, but had no statistically significant impact. Further research on bigger study populations need to be performed to decide if these factors affect the survival time. The survival analysis for grade of tumor did not show a logical result. FNA biopsy was proved to provide a conclusive diagnosis in significantly less cases than tissue biopsy.