Thyroid Tumors and Hyperthyroidism in Dogs

Large neck mass in a dog with
thyroid cancer (carcinoma)

In dogs, thyroid tumors are common, representing approximately 1 to 4% of all canine neoplasms. Most thyroid tumors in the dog are non-functional and do not lead to hyperthyroidism — completely different from the situation in cats, where most cats with thyroid tumors are hyperthyroid (for more information, see links to my related posts, below). 

As opposed to the relatively small, non-invasive and benign thyroid tumors associated with hyperthyroidism in cats, most clinically detected thyroid tumors in dogs are large, invasive tumors associated with thyroid cancer (carcinomas).

Nature of Thyroid Cancer in Dogs
In dogs with thyroid carcinoma, both local invasion of tumor into adjacent structures (e.g., esophagus, trachea, cervical musculature, nerves, or thyroid vessels) and distant metastasis (e.g., lung, liver, or regional lymph nodes) are common.

Approximately 50% of dogs with thyroid carcinoma have documented metastasis at time of diagnosis. During the natural course of disease, 65% to 90% of dogs with untreated disease will develop metastasis to a wide variety of tissue.

Thyroid Tumors and Hyper- and Hypothyroidism
In most dogs with thyroid tumors, their thyroid function remains normal. Up to a third of dogs with thyroid cancer will develop hypothyroidism from destruction of normal thyroid tissue.

Only 10% of thyroid carcinomas in dogs are overactive, producing clinical signs of hyperthyroidism. Although thyroid adenomas producing hyperthyroidism have been described, the incidence of benign thyroid tumors causing hyperthyroidism in dogs is very low. In dogs, a thyroid tumor causing hyperthyroidism should always be presumed to be a carcinoma until proven otherwise. Again, this is in stark contrast to the situation in hyperthyroid cats, in which thyroid carcinoma is rare (less than 5% of in cats).

Clinical Features 
Most dogs with thyroid tumors are of middle- to old-age (greater than 5 years; average age, 10 years). There is no sex predilection. Breeds reported to be at increased risk of developing thyroid tumors include boxers, beagles, Golden Retrievers, and Labrador Retrievers.

Many dogs with thyroid tumors are presented because the owner has noticed an enlargement of the neck. In more than 75% of dogs diagnosed in one survey, either the cervical swelling was the only reason for seeking veterinary care or the thyroid mass was detected by the veterinarian during an examination for another problem.

Because of the large tumor volume and high incidence of both local invasion and distant metastasis, clinical signs such as trouble breathing (dyspnea), cough, hoarseness, alternation in bark, vomiting, poor appetite, and weight loss may be reported, especially in dogs with nonfunctional thyroid tumors. A surprising number of owners, however, fail to report any clinical signs in their dogs.

Of all thyroid tumors in dogs, approximately 10% will become hyperthyroid, and in these dogs, the hyperthyroid state is usually the major reason for examination. Increased thirst and urination are usually the earliest and most predominant signs associated with hyperthyroidism in dogs. Weight loss, despite an increase in appetite, is also common.

Diagnosis
Thyroid imaging or scanning (thyroid scintigraphy) is useful in the evaluation of dogs with thyroid tumors because the procedure can help locate where the thyroid tissue is located (both normal tissue, as well as benign and malignant tumors).  Thyroid imaging can also be performed to help determine the extent of thyroid invasion or metastasis.  

Definitive diagnosis of thyroid carcinoma is best made by either incisional biopsy or surgical excision of the thyroid mass, followed by histological examination of the thyroid tissue. Despite the fact that most thyroid tumors detected in clinical practice are easily palpable, they are not simple to biopsy. Because of the vascular and invasive nature of canine thyroid carcinomas, hemorrhage is a common complication.

Treatment
Treatment of thyroid neoplasia in dogs is dictated by the size of the primary tumor, extent of local tissue invasion, presence of detectable metastasis, presence of hyper- or hypothyroidism, and available treatment options.

Because most clinically detected thyroid tumors in the dog are malignant, treatment is rarely curative. Nevertheless, one should generally advise some form of treatment because palliative relief and increased lifespan can usually be achieved in most dogs with thyroid carcinoma.

Surgical removal of a small thyroid tumor in a dog

Surgery, chemotherapy, cobalt irradiation, and use of radioactive iodine therapy, alone or in combination, may be indicated depending on the individual dog. Medical control of the hyperthyroid state can be achieved by the daily administration of an antithyroid drug such as methimazole or carbimazole (5 to 15 mg/dog, twice, daily), but such medical treatment will not do anything to prevent tumor growth or metastasis.

Suggested Reading:

1. Panciera DL, Peterson ME, Birchard, SJ: Diseases of the thyroid gland. In: Birchard SJ, Sherding RG (eds): Manual of Small Animal Practice (Third Edition), Philadelphia, Saunders Elsevier, 2006;327-342.
2. Peterson ME: Hyperthyroidism and thyroid tumor in dogs. In: Melian C, Perez Alenza MD, Peterson ME, Diaz M, Kooistra H (eds): Manual de Endocrinología en Pequeños Animales (Manual of Small Animal Endocrinology). Multimedica, Barcelona, Spain, 2008;113-125.
3. Mooney CT. Canine hyperthyroidism In: Mooney CT,Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association, 2012;86-91.

My Related Posts on Thyroid Disease in Dogs and Cats:

• Hypothyroidism: One of the Top 10 Diagnosed Diseases of Dogs 
• How is Canine Hypothyroidism Diagnosed? 
• How Do We Confirm a Diagnosis of Hypothyroidism in Dogs? 
• How Do We Treat Dogs With Hypothyroidism? 
• Hyperthyroid in Cats: Table of Contents 
• Treatment Options and Considerations for Hyperthyroid Cats 
• More Hyperthyroid Posts, etc