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 classified as functional (produce excess thyroid hormone) or non­functional (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.

Thyroid Tumors in Dogs

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 fine-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 define 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/polydipsia, polyphagia, weight loss, exercise intolerance). The increasing numbers of head and neck CTs being performed mean 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 fixed 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 identified, 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 significant 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 postoperative survival.

Postoperative care: Monitoring for hemorrhage at the surgical site for the first 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.

Prognosis: Guarded to poor depending on the size of the tumor at the time of diagnosis, extent of disease, and possibly histologic type, as dogs with medullary tumors may have a better prognosis. Long-term survival is possible as dogs with freely movable tumors without evidence of metastasis have a median survival time of 20.5 months with surgical excision alone. At diagnosis, tumour diameter, tumour volume, tumour fixation, ectopic location, follicular cell origin and Ki67 were positively associated with local invasiveness. Tumour diameter, tumour volume and bilateral location were positively associated with the presence of distant metastasis. Macroscopic and histologic vascular invasion were independent negative predictors for disease-free survival. Sublingual ectopic thyroid tumours appear to have a less aggressive biological behavior compared to ectopic sublingual tumours. Many of these dogs have long survival even without treatment.

Thyroid Tumors in Cats

Background: Thyroid neoplasia in cats is a much different disease than in dogs. These tumors are almost always benign adenomas/adenomatous hyperplasia and are functional. They produce excessive amounts of thyroxine, and affected cats show clinical signs of hyperthyroidism (tachycardia, PU/PD, weight loss, polyphagia). There is bilateral involvement approximately 80% of the time and 5% are found to be ectopic (thoracic inlet or cranial mediastinum). Malignant thyroid tumors are rare.

Diagnosis: Based on history, clinical signs, elevated serum thyroxine levels and palpation of a thyroid mass. In normal cats you cannot feel the thyroid glands. 85–90% of affected cats have palpable tumors. Many cats also have hypertrophic cardiomyopathy and will have a gallop rhythm and sometimes a heart murmur. This can be confirmed by ultrasound of the heart. Diagnosis can be aided by radionuclide scanning of the thyroid glands, and is confirmed by increased uptake and size of the affected lobes. Because the disease is usually seen in older cats and older cats have a higher incidence of chronic renal failure, renal function (BUN and creatinine levels and urine specific gravity) should be evaluated preoperatively. Following therapy and the return to a euthyroid state, patients with renal failure or insufficiency should be monitored for worsening of renal disease that may result from a lower cardiac output (and therefore glomerular filtration rate).

Surgery: Thyroidectomy is a practical and usually curative procedure for hyperthyroidism. Preoperatively each cat should be treated medically (Methimazole/Tapazole) to make it euthyroid, improving its candidacy for anesthesia and surgery. Patients with hypertrophic cardiomyopathy and tachycardia should be started on propranolol preoperatively to lower the heart rate and lessen the chance of cardiac arrhythmias during surgery. The ECG is monitored closely because premature ventricular contractions are common.

The thyroid tumor should be removed by a modified extracapsular technique. A nick incision is made with a #15 blade, or bipolar cautery in an avascular area of the capsule adjacent to the parathyroid gland, and iris scissors used to extend the cut in the capsule around the parathyroid gland, removing thyroid tissue and capsule as one. If both thyroid glands are involved, they are each removed; however, removal of both glands during the same procedure increases the likelihood of postoperative hypocalcemia from damaging the blood supply to the parathyroid glands. For this reason, some surgeons elect to stage removal of the second gland. The resected tissue is submitted for histopathology.

Postoperative care and complications: With bilateral thyroidectomy postoperative monitoring for hypocalcemia (panting, nervousness, facial rubbing, muscle spasms, anorexia, depression) is extremely important. Signs of hypocalcemia indicate that the calcium level is probably less than 7.5 mg/dl and should be initially treated with intravenous 10% calcium gluconate (0.5–1.5 ml/ kg slowly over 10–20 minutes). During the treatment heart rate should be monitored and the infusion stopped if bradycardia is induced. When postoperative hypocalcemia occurs, long-term (usually a few months) treatment is sometimes necessary until the remaining parathyroid tissue revascularizes. Oral calcium (Tums) is often used along with vitamin D (dihydrotachysterol) for as long as clinically warranted based on serum calcium levels.

Even after bilateral thyroidectomy, many cats do not require long-term exogenous thyroid supplementation; although thyroid levels should be monitored periodically and thyroid supplementation instituted if levels remain low. The cardiac changes associated with the cardiomyopathy usually resolve following thyroidectomy. The prognosis for these cats is generally good, though relapse may occur 1–2 years later due to regrowth of the adenoma due to hypertrophy of tissue not removed during thyroidectomy.

Radioactive iodine treatment (¹³¹I) is a viable (and often preferred over surgery) option for treating hyperthyroidism in cats. Especially cats that are a poor anesthetic risk or who have hyperfunctional ectopic thyroid tissue. This is successful in approximately 85% of cats.