Hyperthyroidism in Dogs

What Is Hyperthyroidism in Dogs?

Hyperthyroidism is a rare condition caused by the overproduction of hormones made by the thyroid gland found in the neck region. This condition is most often caused by a cancerous tumor within the thyroid, but a benign thyroid mass, diet, and certain supplements can also be contributing factors.

What Does the Thyroid Do?

The thyroid is a butterfly-shaped gland that is on both sides of a dog’s windpipe (trachea). This gland makes thyroid hormones that aid in metabolism and regulation of calcium. When a dog has hyperthyroidism, the thyroid gland is overstimulated causing large amounts of thyroid hormones to be produced and released into the body.  These excess thyroid hormones lead to an increase in a dog’s metabolism and higher calcium levels.

Symptoms of Hyperthyroidism in Dogs

Hyperthyroid dogs have an increased metabolism, which leads to hyperactivity and weight loss despite a ravenous (large) appetite. They also tend to have increased calcium, which can cause the following symptoms:

• Increased thirst

• Increased urination

• Decreased energy level

• Vomiting

• Difficulty defecating (constipation)

If the cause of the hyperthyroidism is a cancerous mass in the thyroid gland, then this tumor can often be palpated (touched) within the neck region and cause coughing, difficulty breathing, difficulty eating, and alter the tone of a dog’s bark.

Causes of Hyperthyroidism in Dogs

The most common cause of hyperthyroidism in dogs is a cancerous thyroid mass that can be either a thyroid carcinoma or thyroid adenocarcinoma that overproduces thyroid hormones. Thyroid masses usually exist within the thyroid gland. However, there can be thyroid tissue in abnormal places. This is referred to as an ectopic thyroid tissue, and can be present under a dog’s tongue, or at the base of the heart. As a result, thyroid tumors can occasionally be found in those areas and lead to hyperthyroidism. Rarely, a benign mass called a thyroid adenoma within the thyroid gland can also cause hyperthyroidism.

Dogs that consume commercial raw meat diets contaminated with thyroid tissue can develop hyperthyroidism. Supplements that contain kelp or seaweed can trigger hyperthyroidism if ingested. This condition can also occur because of overmedication with thyroid supplements when a dog has hypothyroidism (underactive thyroid).

Any breed of dog is susceptible to getting hyperthyroidism. However, the dog breeds that have a higher incidence of developing a thyroid tumor are Beagles, Boxers, Golden Retrievers, and Siberian Huskies. Hyperthyroidism occurs most often in older dogs.

How Veterinarians Diagnose Hyperthyroidism in Dogs

The following diagnostics are needed to diagnose a dog with hyperthyroidism:

• Routine physical exam: A veterinarian may palpate a mass in the neck region or find a mass under the tongue. An elevated heart rate, elevated temperature, weak pulses, or rapid weight loss may also be noted. Enlarged lymph nodes may or may not be found during the exam.

• Comprehensive bloodwork: This consists of a blood cell count (CBC), chemistry panel that includes calcium level, and a thyroid panel. The bloodwork results usually include abnormal thyroid levels and an elevated calcium level.

• Urinalysis: Dogs with hyperthyroidism often drink more water and urinate more frequently.  A urinalysis test is needed to rule out the most common causes of increased thirst and urination such as a urinary tract infection, kidney disease, and diabetes. The urinalysis results for a dog with hyperthyroidism are normal. 

• Ultrasound of the neck: If a mass is palpable in the neck region, ultrasound is used to determine if the mass is within the thyroid gland and the size of the tumor.  

• Ultrasound of the heart: Also called an Echo, this may be needed to check for an ectopic thyroid mass that can be found at the base of the heart.

• CT scan or MRI: This advanced imaging may be needed to measure the mass and diagnose if the tumor has adhered to surrounding tissue. This will help figure out if the mass can be surgically removed.

• Thyroid scintigraphy: This diagnostic test, also known as a thyroid scan, may be offered to visualize the thyroid gland to determine if a mass is present.

A biopsy or fine needle aspirate and cytology of a thyroid mass are not recommended because the thyroid gland is very vascular (comprised of blood vessels), and aspirates or biopsies of the tumor are often inconclusive because of the amount of red blood cells in the sample. There is also a risk of severe hemorrhage if a needle or biopsy tool is inserted into a thyroid tumor.

Treatment of Hyperthyroidism in Dogs

• Surgery: If a thyroid tumor is found and is not attached to surrounding tissue, then surgical removal of the thyroid tumor, called a thyroidectomy, is recommended. Surgery can also be performed to remove a thyroid mass under the tongue or at the base of the heart. If surgery cannot be performed quickly, then an oral medication called methimazole may be prescribed to manage thyroid levels until surgery.

  • If a thyroid tumor is attached to surrounding tissue, then surgical removal of the entire tumor is not possible. A surgeon can surgically remove most of the tumor (debulk the tumor) and then schedule chemotherapy and/or radiation to treat the tumor cells that remain.

  • Aspiration pneumonia is a common complication after a thyroidectomy, so the patient pet needs to be watched closely for signs of respiratory distress.

• Radiation therapy: Radiation can be pursued if metastatic cancer is present, or if surgery is too dangerous to pursue based on the size of the mass or how invasive it is to surrounding tissues.

• High-dose radioactive iodine (I-131) therapy: This is an effective treatment option for cancerous thyroid masses that cannot be surgically removed or have metastasized. It is also used if some residual cancerous cells are left behind following a thyroidectomy.

• Chemotherapy: There are several chemotherapy drugs that have been used for treating hyperthyroidism caused by a cancerous thyroid mass. The drugs that have been found to be most effective are toceranib phosphate (Palladia), doxorubicin, and cisplatin.

• Diet change or discontinuing seaweed/kelp supplements: For dogs that have hyperthyroidism due to consuming commercial raw meat diets containing thyroid tissue—or supplements with seaweed or kelp— the best treatment involves stopping the diet or supplement.

• Adjusting thyroid supplement dose: Dogs receiving an overdose of thyroid supplementation for management of hypothyroidism need to have their dosage decreased to prevent symptoms of hyperthyroidism.

• Palliative care for cancerous thyroid tumors: If a pet parent does not wish to pursue surgery, chemotherapy, or radiation to treat their dog, Methimazole medication and Hill’s y/d diet can be used to manage hyperthyroidism by decreasing production of thyroid hormones. This treatment option provides palliative care and helps to reduce symptoms for a period. It does not address the thyroid tumor, so growth of the tumor continues, and it can still spread to other areas of the body.

Recovery and Management of Hyperthyroidism in Dogs

Prognosis depends on the cause of the hyperthyroidism. If there is a benign tumor, then surgery is curative. If diet or a supplement is the cause of the hyperthyroidism, discontinuing these will resolve the condition. Decreasing the dosage of the thyroid supplement for hypothyroid dogs that are overmedicated will resolve the hyperthyroid symptoms.

Treating a cancerous thyroid tumor can be challenging and will depend on a few factors. If the tumor is moveable and not adhered to surrounding tissue, then surgical removal can prolong the dog’s life by approximately three years. In contrast, if the tumor is large or adhered to surrounding tissue, then surgical removal is unlikely to remove the entire tumor, and further treatment is needed to manage the cancer. Radiation or radioactive iodine therapy for non-metastatic thyroid tumors can also yield a survival time of between one and three years.

Cancerous thyroid tumors have a high probability of spreading to other areas of the body, like the regional lymph nodes. This is called metastasis. If metastasis is present, then the prognosis is poor, and chemotherapy would be needed to try to treat the cancer in various locations of the body. Survival time is typically less than a year with treatment for metastasized thyroid tumors.

Dogs that have a thyroidectomy may become hypothyroid after surgery, which means that they have low thyroid hormones and need to take a long-term thyroid supplement to manage hypothyroidism. Also, some dogs will develop low calcium levels after surgery and need to be started on a calcium supplement.