Thyroid Carcinoma

In dogs and cats, the thyroid glands are a paired set of structures positioned alongside the windpipe (trachea), roughly midway down the neck. Their role is to produce hormones essential for normal body function. Thyroid growths in dogs may be benign (adenomas) or malignant (carcinomas). Benign lesions commonly enlarge over time and may overproduce thyroid hormones; malignant lesions may also spread to other parts of the body. In cats, benign thyroid tumors are common, whereas in dogs most thyroid tumors are malignant. Thyroid tumors are most often seen in middle-aged to older large-breed dogs such as Boxers, Beagles, Golden Retrievers, and Siberian Huskies.

Figure 1. The dog’s neck is shaved before surgery; the head is to the left. Large thyroid masses (blue arrows) made the neck appear swollen.

Dogs with thyroid tumors may have no outward signs or may develop a noticeable lump in the neck region (Figure 1). If the mass presses on the trachea, breathing difficulty or coughing may occur. If it compresses the esophagus, the dog may gag or have trouble swallowing. Some dogs may show a change in bark, weight loss, or reduced appetite. Although most canine thyroid tumors are malignant, they rarely produce excessive thyroid hormone that would cause clinical hyperthyroidism signs such as:

• restlessness
• increased appetite
• weight loss
• coat/hair abnormalities
• increased drinking and urination

In some cases, thyroid tumors are located away from the normal thyroid position in the neck, or even under the tongue or within the chest.

Thyroid masses can sometimes cause visible neck swelling on radiographs (x-rays), but ultrasound or CT (computed tomography) is better for evaluating tumor size and invasiveness. A definitive diagnosis requires microscopic evaluation of a tissue sample. Because these tumors are highly vascular, coagulation status should be checked with blood clotting tests before biopsy or surgery.

Before surgery, additional testing is commonly done for staging. Chest radiographs or CT, abdominal ultrasound, and blood tests are used to look for evidence of thyroid hormone production or metastasis (cancer spread) and to assess overall organ function.

Figure 2. This thyroid tumor extended into the jugular vein, a major vessel draining blood from the head. Tumor cells entering this vessel can spread to the lungs; however, this 10-year-old dog lived 2 years after surgery without clinical illness.

Surgical removal can be challenging because thyroid tumors may invade nearby blood vessels or surrounding tissues (Figure 2). Large or invasive tumors can be difficult to excise, so referral to an ACVS board-certified veterinary surgeon is recommended for any large or fixed masses. Radiation therapy or chemotherapy is often advised when tumors cannot be completely removed or are too large for surgical resection.

Radioactive iodine (I-131) has been shown to be an effective adjunct therapy for thyroid tumors. I-131 may be used in patients who are poor surgical candidates or when vascular invasion is identified despite surgical removal.

Review of the excised tumor by a pathologist is important to determine whether additional treatment (chemotherapy, radiation, or I-131) is likely to help. Pathology assessment may include special staining (immunohistochemistry) that supports tailoring follow-up management.

After surgery, a soft bandage may be placed around the dog’s neck. Collars and neck leashes should be avoided until healing is complete—typically 10–14 days—and a body harness should be used instead. Activity should be restricted during this recovery period.

Owners should follow the surgeon’s instructions regarding postoperative medications. If both thyroid glands are removed, calcium levels may need repeated monitoring during recovery because some parathyroid tissue is removed with the thyroid glands (the parathyroid glands regulate calcium).

Surgical outcomes are best when the mass is freely movable, smaller than 4 cm, nonmetastatic, and can be completely excised. Depending on pathology findings and early diagnosis before local invasion or metastasis, dogs may achieve long-term survival of 1–3 years. Dogs treated with surgery plus follow-up I-131 therapy have an average survival of 34 months.

General anesthesia always carries risk. Complications specifically associated with thyroid tumor removal in dogs include bleeding and injury to the recurrent laryngeal nerve, which controls laryngeal (upper airway cartilage) movement during breathing and swallowing. Dogs that have both thyroid glands removed may develop low calcium (hypocalcemia) or low thyroid hormone (hypothyroidism), which require medication. Swelling at the neck incision can also occur after surgery.

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