Canine Hyperthyroidism

Summary

Hyperthyroidism is an endocrine or glandular disease caused by overproduction and/or over-release of hormones by the thyroid gland. The thyroid gland is a small, 2-lobed organ that sits in the middle of the neck, one lobe on each side of the trachea. Hormones released by the thyroid gland help regulate metabolism by affecting heat production and use of nutrients (i.e. protein, carbohydrates, and fat). Thyroxine, also called T4, is the main hormone produced by the thyroid gland.

Thyroid problems in dogs are common, but they are usually caused by hypothyroidism, or not enough production of T4. Hyperthyroidism in dogs is almost always caused by a tumor in the thyroid gland. Canine thyroid tumors are primarily cancerous (as opposed to benign or noncancerous). The other most common cause of hyperthyroidism is overdosing a hypothyroid dog on his thyroid medication.

Symptoms and Identification

Significant weight loss and constant hunger are the most common symptoms. Excessive drinking and urinating, throwing up, trouble swallowing, voice/bark change, aggressive behavior, hyperactivity, and a lump on the dog’s neck (where the thyroid gland is located) may also be noticed. An overactive thyroid can also affect the heart (e.g. altered rhythm, murmurs, fast rate) and circulatory system (e.g. high blood pressure/hypertension), so trouble breathing, coughing, and collapse are potential symptoms as well.

To diagnose hyperthyroidism, a physical examination, complete blood count (CBC), and chemistry panel will be performed to check all body system functions. Dehydration and liver changes are common and will be seen with these types of testing. Thyroid tests will be run to check for high levels of T4 in the blood. If these tests are normal, but your vet still suspects hyperthyroidism, other thyroid hormone tests may be performed such as a T3 suppression test. If a thyroid tumor is suspected, testing for metastasis (cancer spread) can include chest x-rays and/or abdominal ultrasound. Advanced imaging such as magnetic resonance imaging (MRI) or computed tomography (CT) scans may be performed to see how big the tumor is and if it is invading any important structure like the carotid artery.

Affected Breeds

In dogs, the Siberian Husky, Golden Retriever, and Beagle carry an increased risk of thyroid cancer compared to other dog breeds. Most dogs develop cancer-associated hyperthyroidism at an older age (>9 years old).

Treatment

Since most dogs with hyperthyroidism have a tumor, surgery to remove the tumor or one or both thyroid gland lobes is commonly performed. Surgery on the thyroid gland is a tricky procedure, so the veterinarian may refer the pet to a specialized surgeon. Some pets will need to be on medication for hypothyroidism after the procedure since thyroid hormones won’t be produced anymore. Very small glands called parathyroid glands, which control calcium levels in the blood, attach to the thyroid glands. Thyroid surgery may damage these glands or result in their removal (called hypoparathyroidism), so calcium medication may also be needed after surgery. Luckily, many dogs with thyroid cancer do pretty well with surgery if the cancer hasn’t spread to other areas. Average survival times after recovery from successful surgery is roughly 3 years.

Radioactive iodine therapy is performed in cats to cure hyperthyroidism, but it is not commonly performed in dogs. Medications and prescription diets to control hyperthyroidism are also a possibility for dogs, but again, they are not as commonly prescribed. If a dog has become hyperthyroid because of thyroid medication overdose, the veterinarian will likely want to change the prescription dose and monitor the pet closely over the next 4-6 months.

Veterinary Cost

Cost varies depending on treatment. Diagnosis cost ranges from $75-350 ($750-1,500 if CT or MRI is needed). Surgery ranges from $750 – $3,000. Radiation therapy is $2,500 – $5,000 for the procedure, which may not include additional testing, boarding, etc. Recheck T4 testing is usually around $50.

Prevention

Little can be done to prevent thyroid tumors in dogs. If hyperthyroidism was caused by a medication overdose, frequent rechecks and giving medication exactly as prescribed will help prevent additional overdoses.

References

Mooney CT: Hyperthyroidism. Textbook of Veterinary Internal Medicine, 7th ed. St. Louis, Saunders Elsevier 2010 pp. 1761-79.

Ward CRL, Morgan RV: Diseases of the thyroid. Handbook of Small Animal Practice, 5th ed. St. Louis, Saunders Elsevier 2008 pp. 459-463.

Bezzola P: Thyroid carcinoma and hyperthyroidism in a dog. Can Vet J. 2002 February;43(2):125-126.

Turrel JM, McEntee MC, Burke BP, et al: Sodium iodide I 131 treatment of dogs with nonresectable thyroid tumors: 39 cases (1990-2003). J Am Vet Med Assoc. 2006;229(4):542-548.

Worth AJ, Zuber RM, Hocking M. Radioiodide (131I) therapy for the treatment of canine thyroid carcinoma. Aust Vet J. 2005;83(4):208-214.

Adams WH, Walker MA, Daniel GB, et al: Treatment of differentiated thyroid carcinoma in 7 dogs utilizing 131-I. Vet Radiol Ultrasound. 1995;36(5):417-424.

Frederick AN, Pardo AD, Schmiedt CW, et al: Outcomes for dogs with functional thyroid tumors treated by surgical excision alone. J Am Vet Med Assoc. 2020 February;256(4):444-448.

Looney A, Wakshlag J: Dietary management of hyperthyroidism in a dog. J Am Anim Hosp Assoc. 2017 Mar/Apr;53(2):111-118.