The thyroid tumors in these dogs demonstrate a markedly increased iodine uptake needed to support their increased thyroid hormone production. Despite the absence of a circulating thyrotoxicosis, differentiated thyroid carcinomas in the dog are usually capable of concentrating iodine. Planar thyroid scintigraphy of a dog with a differentiated thyroid carcinoma. This carcinoma involves both thyroid lobes as well as demonstrates extensive regional and diffuse pulmonary metastasis. The diagnosis of thyroid neoplasia in most cases is based on a cytologic or histopathologic evaluation of tissue samples collected from the mass.
Cytologic evaluation is usually capable of determining the source as thyroid tissue.
However cytologic evaluation is frequently limited in its ability to accurately differentiate a benign thyroid adenoma from a well differentiated thyroid carcinoma. In these cases, histopathologic evaluation of a tissue sample is necessary to confirm a malignancy. Excisional biopsy samples typically provide the best opportunity to evaluate for capsular or vascular invasion as important criteria of malignancy as well as important factors in treatment planning. In the small percentage of dogs who develop a thyrotoxicosis secondary to a differentiated thyroid neoplasia, diagnosis can be aided by measurement of circulating thyroid hormone levels. Technetium pertechnetate is concentrated in thyroid tissue by the same mechanism as iodine.