Thyroid Neoplasia is probably more common in dogs than in almost any other species. Thyroid tumors account for approximately 1 to 4 % of all canine neoplasms. Thyroid tumors account for 10% to 15% of canine tumors of the head and neck. While thyroid carcinomas have been reported in virtually every canine breed, Beagles, Boxers and Golden Retrievers are at an increased risk when compared to other breeds.

Etiology - More than 90% of all detected canine thyroid tumors are carcinomas. Approximately 85% of thyroid carcinomas in dogs are of a compact or follicular (differentiated) type. Approximately 15% or less are anaplastic (undifferentiated). Less than 5% of thyroid carcinomas in dogs originate from the parafollicular or C-cells. The C-cells of the thyroid produce calcitonin, a hormone important in calcium homeostasis. Tumors of the C-cells are called medullary thyroid carcinomas. Most dogs with thyroid tumors are euthyroid (have normal thyroid hormone levels) because thyroid tumors uncommonly produce excessive thyroid hormone levels. Only approximately 5% of dogs with thyroid carcinomas demonstrate a clinical thyrotoxicosis (elevated thyroid hormone levels). 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.
Clinical Signs - The common clinical signs demonstrated by dogs with thyroid carcinomas include a visible mass in the neck, coughing, dyspnea (difficulty breathing), dysphagia (difficulty swallowing), and dysphonia (change in bark). A smaller number of dogs with thyroid carcinomas demonstrate weight loss, vomiting/regurgitation, listless and anorexia (decreased appetite). The most common clinical signs demonstrated by dogs with thyrotoxicosis secondary to thyroid carcinoma are polyuria (increased urination) and polydipsia (increased water consumption).

Diagnosis - 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.
Planar thyroid scintigraphy (or thyroid scanning) is a well established diagnostic aid in the evaluation of dogs with thyroid carcinoma. Technetium pertechnetate is concentrated by thyroid tissue, salivary glands and gastric mucosa (stomach). Technetium pertechnetate is concentrated in thyroid tissue by the same mechanism as iodine. By evaluating the uptake of technetium pertechnetate on a thyroid scintigraphy it is possible to better define the nature and extent of thyroid neoplasia. Thyroid scintigraphy is an important step in the staging of the canine patient with differentiated thyroid neoplasia. Thyroid scintigraphy utilizing technetium pertechnetate can also be used as a predictor of radioiodine uptake..