Thyroid Cancer in Dogs: Not always a bad disease!

Thyroid neoplasia is fairly common in dogs and is seen more frequently in boxers, golden retrievers, and beagles.(1) The tumors are usually malignant and unilateral however bilateral tumors are possible. Carcinoma is the most common tumor and they can metastasize to lungs and regional lymph nodes.(2) They can also be locally invasive and spread into adjacent vasculature such as the jugular vein. Because of the complex nature of the cervical anatomy and potential invasiveness of thyroid tumors, surgical removal by a board certified veterinary surgeon is recommended.

Clinical Signs

Dogs may be minimally symptomatic and present for a mass in the neck that is either discovered by the owner or found on routine physical examination.(Fig. 1) 

Fig. 1: A West Highland White Terrier with a thyroid  tumor visible in the neck. (arrows)

If early diagnosis is pursued, the tumor may be small, encapsulated, and freely movable in the neck. More advanced tumors can be very large, fixed in position, and encompassing the trachea and larynx.

In rare cases the thyroid tumor is functional and secreting an excessive amount of thyroxine causing hyperthyroidism. Signs of hyperthyroidism include weight loss, increased appetite, polyuria and polydipsia. Cardiac abnormalities such as cardiomyopathy can result from the excessive thyroid hormone secretion. Submit blood samples for thyroid hormone panel to confirm the diagnosis of a functional thyroid tumor.

Diagnosis and Staging

An accurate preoperative diagnosis is important since masses in the neck other than thyroid tumors, such as those involving the salivary glands or lymph nodes, can occur. Carefully palpate the neck mass and regional lymph nodes. Small, well-circumscribed and freely movable tumors are usually amenable to surgical resection.

Fine-needle aspirate or tissue biopsy of the thyroid tumor helps establish the type of tumor. However, recognize that bleeding associated with these procedures can obscure the surgical field and make the procedure more difficult. Ultrasound examination may be helpful to identify the involved thyroid lobes, evaluate for local tissue invasion, and guide fine needle aspiration. Obtain thoracic radiographs to rule out pulmonary metastasis or other cardiopulmonary disorders.

Surgical Procedure

The objectives of surgery are to: completely remove the thyroid mass, preserve at least one parathyroid gland, minimize blood loss, and avoid injury to the recurrent laryngeal nerves, trachea, and esophagus.

Anatomy

The thyroid gland in the dog is divided into 2 lobes that are located adjacent to the trachea and just distal to the larynx.(3) (Fig. 2) 

Fig. 2: Normal anatomy of the cervical region in the dog. Arrows delineate the thyroid lobes.
(Illustration by Mr. Tim Vogt, Medical Illustrator, College of Veterinary Medicine, The Ohio State University)

The normal gland has a pale brown color and is approximately 2 to 2 and 1/2 cm in length.  The principle blood supply to each lobe is the cranial and caudal thyroid arteries, branches of the common carotid artery.  Venous drainage of the thyroid is via the cranial and caudal thyroid veins.

The thyroid gland has a distinct capsule that can be bluntly separated from the parenchyma.  Small blood vessels may be located on the capsule surface and between the capsule and the parenchyma of the gland. Two parathyroid glands are usually associated with each thyroid lobe.  The external parathyroid gland usually lies in the fascia at the cranial pole of the thyroid lobe.  The internal parathyroid gland is usually embedded in the thyroid parenchyma and is variable in location.  The external parathyroid glands are much smaller than the thyroid lobe and can be distinguished from the thyroid tissue by their lighter color and spherical shape. However, the parathyroid gland can be confused with fat.  The blood supply to the parathyroid glands also arises from the cranial thyroid artery.

Overview of Surgical Technique

The dog is placed in dorsal recumbency and the ventral cervical area prepared for aseptic surgery. A ventral midline cervical surgical approach is made from the larynx to the manubrium. The paired sternohyoideus muscles are divided on the midline and retracted. (Fig. 3) A stomach tube or small endotracheal tube in the esophagus helps identify this structure.

Fig. 3: Operative photograph of a unilateral thyroid tumor (arrows)
in a dog with thyroid carcinoma.

Ligate or cauterize the extensive vascular network surrounding the thyroid and carefully dissect out the tumor. Begin dissection at the caudal aspect of the tumor and gradually work cranially. (Fig. 4) 

Fig. 4: Dissection of a thyroid carcinoma in a dog proceeding from
caudal to cranial (black arrow). Yellow arrows indicate the trachea.

Identify the recurrent laryngeal nerves adjacent to the trachea and avoid trauma to them. If possible, identify and preserve the parathyroid glands especially with bilateral thyroidectomy. With large malignant tumors, the parathyroid glands may be difficult to indentify.

Close the muscle routinely with simple continuous, absorbable suture; the subcutaneous tissue with simple continuous, absorbable suture; and the skin with simple interrupted, nonabsorbable suture or an intradermal pattern using absorbable suture.

Postoperative Care and Complications

Short-Term

Closely monitor for hemorrhage or seroma formation at the surgical site. Cold compresses on the incision for the first 24 hours are helpful to prevent swelling.Monitor serum calcium concentrations for at least 2–4 days postoperatively if a bilateral thyroidectomy was performed. Monitor the calcium concentrations longer if the levels are decreasing. Treat hypocalcemia with oral calcium and vitamin D. If bilateral thryoidectomy was performed, evaluate thyroid function and treat hypoparathyroidism if necessary.

Long-Term

Reevaluate the dog frequently (every 3 months for the first year) with physical examinations and thoracic radiographs to monitor for recurrence of the primary tumor and metastasis.
Consider postoperative chemotherapy or radiation therapy if the tumor was malignant and incompletely excised.

Prognosis

Key Point:  Surgical resection of encapsulated, movable thyroid tumor in dogs has been found to result in median postoperative survival of 20 months.

In the study cited above the dogs were treated with surgery alone and had no evidence of metastasis at the time of surgery.(4) As with most cancers, early diagnosis and treatment increases the chances of a good outcome.

References
  1. Birchard SJ, Roesel OF: Neoplasia of the thyroid gland in the dog: A retrospective study of 16 cases. JAAHA 17:369-372, 1981.
  2. Brodey TS, Kelly DF: Thyroid neoplasms in the dog. Cancer 22: 406-416, 1968.
  3. Evans HE, Christensen GC: Miller's Anatomy of the Dog, The Endocrine System, WB Saunders, Philadelphia, 1979, pp. 611-618.
  4. Klein MK1, Powers BE, Withrow SJ, Curtis CR, Straw RC, Ogilvie GK, Dickinson KL, Cooper MF, Baier M. Treatment of thyroid carcinoma in dogs by surgical resection alone: 20 cases (1981-1989).J Am Vet Med Assoc. 1995 Apr 1;206(7):1007-9.