P. 387

N. Bacon1
1Fitzpatrick Referrals Oncology and Soft Tissue, University of Surrey School of Veterinary Medicine, 70 Priestley Road, Surrey Research Park, Guildford, GU23 7HT, UK
Surgery of the thyroid gland in small animals is almost exclusively related to thyroid neoplasia or hyperplasia. For the purposes of this discussion, both will be referred to as thyroid tumors. Thyroid tumors are classi ed as functional (produce excess thyroid hormone) or non- functional (no detectable hormone production). Thyroid tumors in dogs are usually malignant and nonfunctional; while in cats they are usually benign and functional.
Surgical anatomy: The thyroid gland in dogs and cats is divided into two lobes that lie just caudal and lateral to the larynx, adjacent to the trachea. The principal blood supply to each lobe is the cranial thyroid artery which arises from the common carotid artery. The caudal thyroid artery in the dog branches off the brachiocephalic artery; the caudal thyroid artery is not present in most cats. The cranial and caudal thyroid veins provide venous drainage.
The thyroid is normally pale tan in color. It has a distinct capsule that can be bluntly dissected from the gland. Usually two parathyroid glands are associated with each thyroid lobe. The external parathyroid gland usually
lies in the fascia at the cranial pole of the thyroid while the internal parathyroid is usually embedded within the thyroid parenchyma in the caudal aspect of the gland. The parathyroids also are supplied by the cranial thyroid artery.
Background: The majority of canine thyroid tumors are malignant. Adenocarcinoma (of follicular cell origin) is the most commonly seen tissue type. Boxers, beagles, and golden retrievers are at greater risk of developing thyroid carcinoma. The most common clinical signs reported are a palpable neck mass and a history of coughing. Approximately 7.5% have sublingual ectopic thyroid tumours (these dogs are typically younger and less
likely to have metastatic disease). Thyroid carcinomas in dogs most frequently metastasize to the lungs. Studies show that by the time of diagnosis approximately 40% of thyroid carcinomas have metastasized. The larger the primary tumor the greater the chance of metastasis, as tumors greater than 100 cubic cm are almost always associated with pulmonary metastasis. The cervical lymph nodes are the second most common site of metastasis and invasion into the jugular vein is common.
Diagnosis: Thyroid neoplasia is diagnosed by histopathologic examination. Biopsy may be performed by  ne needle aspiration, Tru-cut needle, or excision
but carries risks of hemorrhage into fascial planes of the neck. Ultrasound of the cervical area may be helpful to de ne the extent of the mass. Thoracic radiographs are essential to rule out radiographic evidence of pulmonary metastasis prior to attempting excision. Thyroid imaging (with the radionuclide technetium 99m) may reveal increased uptake with any thyroid tumor and when taken up intensely supports a functional tumor. A thyroid panel and thyroid stimulation test (TSH response) should be performed to evaluate thyroid function when indicated, as approximately 20% of dogs with thyroid carcinoma show signs of hyperthyroidism (polyuria/polydypsia, polyphagia, weight loss, exercise intolerance). The increasing numbers of head and neck CTs being performed means many thyroid incidentalomas are being diagnosed, the majority of which are thyroid carcinomas.
Surgery: Recommended only if the mass is relatively mobile, local lymph nodes are not involved, and the chest is free of metastasis. Although most  xed tumours are not amenable to curative-intent surgery, occasionally some can be resected. Small tumors may be completely removed by thyroidectomy. A ventral midline incision
is made from the caudal aspect of the larynx to just proximal to the manubrium. The paired sternohyoideus and sternothyroideus muscles are separated along the midline and retracted. The trachea is carefully retracted and each thyroid lobe is carefully examined. The parathyroid glands should be identi ed, though their visualization may be obscured by the tumor. The tumor is then carefully dissected from the surrounding carotid artery, jugular vein, vagosympathetic trunk, and recurrent laryngeal nerve. This process can be tedious and time consuming. These neoplasms are highly vascular, thus, strict hemostasis is important to prevent signi cant blood loss as well as impaired visualization of structures at risk. The resected tissues are submitted for histopathology. Bilateral thyroid carcinomas are well described in dogs and in these cases thyroidectomy combined with parathyroid sparing is appropriate and can result in long post-operative survival.
Postoperative care: Monitoring for hemorrhage at the surgical site for the  rst 24 hours is important. Serum calcium levels should be monitored daily for 2-4 days if a bilateral tumor was removed. Hypocalcemia may occur due to injury to the parathyroids and must be treated appropriately if seen. The patient should be re-evaluated in 2 weeks, 3 months, 6 months and 1 year and radiographs of the thorax should be obtained to monitor for metastasis.
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