Hyperthyroidism in Dogs

Hyperthyroidism is a rare condition in dogs. It most commonly affects a small percentage of dogs with a particular type of thyroid tumor known as thyroid carcinoma. Affected dogs are typically seniors, ranging in age from 9 to 15 years. Occasionally dogs may also develop hyperthyroidism due to receiving too much thyroid medication or due to certain raw diets. 

What Is Hyperthyroidism in Dogs?

The thyroid gland is a butterfly-shaped gland located in your dog’s neck. It produces thyroid hormones, the most important of which is called thyroxine or T4. Thyroxine has many functions throughout your dog’s body, including regulating metabolism. Hyperthyroidism occurs when there is too much thyroxine in the body. This can occur due to an overproduction of thyroxine by the thyroid gland, or due to excessive supplementation of thyroxine from medications or diet.  

Hyperthyroidism vs. Hypothyroidism in Dogs

In dogs, hyperthyroidism—an excess of thyroid hormones—is uncommon. It is much more common for dogs to develop hypothyroidism, which is a deficiency of thyroid hormones. This occurs in dogs most commonly due to primary hypothyroidism, which develops due to atrophy of the thyroid gland or autoimmune destruction of the gland (thyroiditis). The resulting lack of thyroid hormone causes symptoms such as lethargy, weight gain, cold intolerance, and hair loss. This is in contrast to hyperthyroidism, which causes symptoms related to increased metabolism, such as weight loss despite an increased appetite, decreased muscle mass, increased thirst and urination, excitability, and increased heart rate.  

Causes of Hyperthyroidism in Dogs

In dogs, hyperthyroidism is almost always caused by an underlying thyroid tumor. Two types of thyroid tumors occur in dogs: thyroid adenoma and thyroid carcinoma. Thyroid carcinoma is the more common tumor type. A small percentage of thyroid carcinomas are functional tumors that produce excess thyroid hormone, leading to hyperthyroidism. 

Other causes of hyperthyroidism in dogs include iatrogenic hyperthyroidism and dietary hyperthyroidism. Iatrogenic hyperthyroidism occurs when a dog that is hypothyroid is placed on thyroid supplementation, but receives too much supplementation, thus becoming hyperthyroid. Dietary hyperthyroidism has been reported in dogs eating raw diets (1), likely due to thyroid tissue being incorporated in the food. Dogs receiving supplements that contain high levels of kelp or iodine may also be at risk for hyperthyroidism. 

Hyperthyroidism in Dogs Symptoms

Symptoms of hyperthyroidism in dogs may be difficult to spot and can often look similar to other conditions. Signs of hyperthyroidism in dogs can include:

• Weight loss
• Loss of muscle mass
• Increased thirst
• Increased urination
• Increased appetite
• Increased heart rate
• Agitation or excitability
• Mass in the neck (thyroid tumor)

Diagnosing Hyperthyroidism in Dogs

To diagnose hyperthyroidism in your dog, your veterinarian will first take a thorough history, including asking questions about your dog’s symptoms, your dog’s current diet, and any medications or supplements your dog may be taking. Then your vet will perform a full head-to-tail physical examination on your dog. 

Your vet will likely recommend some diagnostic tests such as blood work to evaluate your dog’s thyroid level. A thyroid panel may be performed, which often includes a T4 level, thyroid stimulating hormone (TSH) level, and possibly a T3 level as well. This allows your veterinarian to get a complete picture of your dog’s thyroid function.

If hyperthyroidism is suspected, imaging of your dog’s neck to look for a thyroid tumor will also be recommended. This is typically done by ultrasound with an experienced sonographer. Your dog’s thyroid gland will be evaluated for changes that could indicate the presence of a functional thyroid tumor, and the entire neck will be checked for any ectopic thyroid tissue—that is, thyroid tissue that is located in the wrong place, which may be producing excess thyroid hormone. Sometimes, ultrasonography is not enough to visualize a thyroid tumor or to fully evaluate how far it extends, so computed tomography (CT) or magnetic resonance imaging (MRI) may be recommended for further evaluation. 

Dog Hyperthyroidism Treatment

Hyperthyroidism in dogs is most often caused by a tumor on the thyroid gland, so the treatment of choice is surgical removal of the tumor. When possible, a thyroidectomy—complete removal of the thyroid gland—is performed. Following the procedure, the dog will be hypothyroid and will need thyroid hormone supplementation for life. This is given in the form of a pill taken twice a day. Chemotherapy may be used in addition to surgery for the treatment of systemic, microscopic disease.

If the tumor is too large to be removed surgically, radiation therapy may be used. Radiation therapy is also used for cases where the tumor has spread to other areas of the body, or in cases where the tumor is incompletely removed by surgery. 

Radioactive Iodine therapy (I-131 Therapy) is sometimes used for tumors that cannot be removed surgically or those that have spread to other areas of the body. Radioactive iodine destroys thyroid hormone-producing cells, making the dog hypothyroid. Thyroid hormone supplementation is necessary following treatment and is typically given in the form of a pill taken twice a day.

Some hyperthyroid dogs may be treated with a medication such as methimazole. Methimazole is an antithyroid thioamide that inhibits the formation of thyroid hormones. This medication is given in the form of a pill, typically dosed twice daily. This medication would be given for life, or until more definitive treatment such as thyroidectomy or radioactive iodine therapy is performed.

Dogs with iatrogenic hyperthyroidism due to over-supplementation may be treated with a dosage adjustment of their normal thyroid medication. Similarly, dogs with hyperthyroidism due to diet or supplements may be cured simply by changing the diet or discontinuing the supplement that caused the hyperthyroidism.

What to Feed a Dog with Hyperthyroidism

If your dog’s hyperthyroidism was caused by diet, changing your dog’s diet will be necessary to control the hyperthyroidism. Hyperthyroidism has been reported in dogs eating raw diets, likely due to thyroid tissue being incorporated in the food. In these cases, changing the diet caused the dogs’ thyroid levels to return to normal. If your dog’s hyperthyroidism is due to diet, your veterinarian will recommend a commercial cooked diet from a reputable company that is best for your dog’s health.

Cost to Treat Hyperthyroidism in Dogs

Treatment for hyperthyroidism in dogs often involves treating an underlying thyroid tumor. This may involve surgery, chemotherapy, radiation therapy, or radioactive iodine therapy. The cost to diagnose and treat a thyroid tumor can vary widely depending on the size of the tumor, its invasiveness, whether it has spread to other areas of the body, and the type of treatment used. Pet owners should expect to spend at least $2,000-$4,000 on thyroid tumor treatment, with costs increasing the larger and more invasive the tumor is.

Dogs with iatrogenic hyperthyroidism are often more easily treated with changes to their medication or supplement regimens and the cost to treat these cases is quite low. Similarly, dogs with dietary hyperthyroidism are typically treated with diet change and the cost to treat is minimal. 

Hyperthyroidism in Dogs Life Expectancy

For patients with thyroid tumors, prognosis varies depending on the invasiveness of the tumor and the type of treatment used. Patients with freely moveable tumors that are able to be surgically removed have a good prognosis, with a median survival time of three years (2). More invasive tumors have a reported survival time of 6-12 months with surgery alone (3).  

In one study of 25 dogs with localized, nonresectable tumors treated with radiation therapy, the progression-free survival rate was reported to be 80 percent at 1 year and 72 percent at 3 years (4). In a study of dogs receiving I-131 therapy, median survival times were comparable to those of patients receiving radiation therapy. Dogs with moderate disease and no metastasis had median survival times over two years. Unfortunately, those with metastatic disease survived less than 1 year (3).

Survival times vary for patients treated with chemotherapy depending on the type of chemotherapy treatment used. 

For patients with iatrogenic or dietary hyperthyroidism, prognosis is good to excellent, as these types of hyperthyroidism can often be reversed with changes in medication or diet regimen.

How to Prevent Hyperthyroidism in Dogs

Iatrogenic hyperthyroidism can be prevented by following your veterinarian’s dosing instructions carefully to ensure your dog receives the correct dose of thyroid medication. Regular follow-ups with your veterinarian will be necessary to ensure your dog’s thyroid level is within the normal range. If your dog is hypothyroid, he or she will need thyroid supplementation for life, so do not change or discontinue your dog’s medication without first consulting your veterinarian.

Dietary hyperthyroidism can be prevented by feeding your dog a high quality, cooked commercial diet. Your veterinarian can help you choose a diet that is appropriate for your dog’s age and body condition.

Unfortunately, because we don’t know exactly what causes thyroid tumors to develop, there is no way to prevent hyperthyroidism that occurs secondary to thyroid tumors.

Related Conditions

• Thyroid adenoma
• Thyroid carcinoma

References

1. Köhler, B et al. “Dietary hyperthyroidism in dogs.” The Journal of small animal practice vol. 53,3 (2012): 182-4. doi:10.1111/j.1748-5827.2011.01189.x
2. Nadeau ME, Kitchell BE: Evaluation of the use of chemotherapy and other prognostic variables for surgically excised canine thyroid carcinoma with and without metastasis. Can Vet J 2011 Vol 52 (9) pp. 994-98.
3. Lunn KF, Page RL: Tumors of the Endocrine System. Withrow & MacEwen’s Small Animal Clinical Oncology, 5th ed. St. Louis, Saunders Elsevier 201 pp. 513-515.
4. Theon AP, Marks SL, Feldman ES, et al: Prognostic factors and patterns of treatment failure in dogs with unresectable differentiated thyroid carcinomas treated with megavoltage irradiation. J Am Vet Med Assoc 200 Vol 216 (11) pp. 1775-79.