Excess Thyroid Hormone in Dogs

Hyperthyroidism is a rare condition in which excessive amounts of thyroid hormones are produced, leading to a high metabolic rate. Dogs with hyperthyroidism are usually hungry or thirsty regardless of how much food or water they have consumed. Because canine hyperthyroidism is often caused by thyroid carcinoma, early diagnosis and treatment is important for a good prognosis.

Dogs with hyperthyroidism are often asymptomatic, but potential clinical signs include:

• Excessive urination
• Increased water consumption
• Weight loss
• Increased appetite
• Increased heart rate
• Elevated heart rate
• Vomiting
• Diarrhea
• Difficulty and rapid breathing - especially during exercise
• Enlarged thyroid gland - the simplest way to identify is to feel around the neck for a small lump
• Lethargy
• Rapid heart rate ("gallop rhythm")

These symptoms reflect the increased metabolic rate that comes with overproduction of thyroid hormones.

Thyroid cancer is much rarer than hypothyroidism and is typically caused by thyroid carcinoma, or cancer. Older dogs are more at risk of hyperthyroidism, and Golden Retrievers, Boxers, and Beagles are especially predisposed to thyroid cancer. The condition can also arise as a response to medication for hypothyroidism or from consuming meat contaminated with thyroid tissue, which may occur in dogs that are fed a raw diet.

As with most hospital visits, the veterinarian will conduct a thorough physical examination and take a history, which will provide a clearer picture of your dog’s condition and overall health. With dogs whose hyperthyroidism is a result of thyroid carcinoma, the tumor can typically be felt as a firm mass in the neck, which may be movable or fixed depending on the extent of the cancer. During the examination, the veterinarian may run a chemical blood profile, urinalysis, and a complete blood count.

Hyperthyroidism is most commonly diagnosed when a high concentration of thyroid hormone is detected in the blood. This can be measured through a general blood test and by assessing the total T3 and T4 levels. If a tumor is present, further diagnostic tests will be necessary to discover how far the cancer has spread. Imaging and thyroid biopsy may be recommended in these cases.

Treatment for hyperthyroidism varies according to the underlying cause of the condition. For dogs that present with clinical signs as the result of a raw fed diet, a change in their meals may resolve the symptoms. Provide your dog with a variety of cuts to avoid feeding too much meat that may contain thyroid tissue.

Outpatient management is the usual outcome of a veterinarian visit resulting in hyperthyroidism. The veterinarian may prescribe anti-thyroid medication that is aimed at preventing an excessive production of hormones. With these drugs, however, you will need to be careful not to administer too much, as otherwise your dog may begin exhibiting signs of hypothyroidism once the symptoms of hyperthyroidism have abated. If the overproduction of hormones persists, part or all of the thyroid may be surgically removed as necessary.

If your dog is diagnosed with hyperthyroidism due to thyroid carcinoma, the cancer will need to be treated. Radioactive iodine, chemotherapy, and local radiotherapy may be helpful in combating localized tumors, although surgery may be required to remove the thyroid tumor. The radioiodine treatment will be completed in the hospital, usually in a special environment, as the treatment is radioactive and dangerous. If your dog receives cancer treatment, he/she may need to be hospitalized based on the city and state guidelines. Radioactive medicine needs time to clear the body as it can pose a danger to other pets, family members, and outsiders.

In cases where the hyperthyroidism arose because of carcinoma, the hyperthyroidism should correct itself once the cancer has been addressed.

As you adjust your dog’s diet or administer anti-thyroid medication, you will notice that your dog is gradually consuming less food and water. The clinical signs will continue to abate as the thyroid hormone production levels return to normal. Monitor your dog daily for additional symptoms, and if your dog is taking anti-thyroid medication, watch for signs of hypothyroidism – that is, the condition in which not enough thyroid hormones are produced.

If your dog underwent surgery or chemotherapy in response to thyroid carcinoma, make sure that he or she has a comfortable place to rest while recovering. The veterinarian will prefer to reexamine your dog upwards of every two weeks during the initial three to four months of treatment. The veterinarian will continue to run tests, including a complete blood count, to measure the concentration of T4.