Thyroid Tumors in Dogs

While many dogs develop lumps and bumps as they age and are often times harmless lumps of fat, there are some cases where the lump is actually a tumor.  If you notice a mass anywhere on your dog, especially one that is getting bigger, you need to have your veterinarian examine and evaluate it.  Dogs can develop cancer just like people can; it will affect both yours and his daily life.  If you notice a mass around your dog’s throat, neck, or mouth he may actually have what is known as a thyroid tumor.  

This type of tumor can affect any dog at any time of his life.  It may not bother your dog or it may be so large that if affects his ability to breathe normally.  Your veterinarian will perform a variety of diagnostic tests to check if it is malignant or benign.  Either way, prognosis of recovery is guarded due to the location of the mass.

If you notice any type of mass on your dog, especially one that has been increasing in size, you should have him evaluated by his veterinarian as soon as possible.

Symptoms of thyroid tumor may include:

• Firm palpable swelling 
• Respiratory distress
• Hyperthyroidism signs which are weight loss, polyphagia, restlessness, panting and weakness 
• Mitral insufficiency 
• Affected thyroid lobe distorted shape 

Types

Thyroid adenomas are typically tan or white in color, small and solid nodules that are demarcated from the thyroid parenchyma.  The thyroid lobe that is affected is only moderately distorted and enlarged.  If it is an adenoma, it is typically singular when present in the thyroid lobe. There is a distinct white fibrous connective capsule that separates the adenoma from the parenchyma.  Other adenomas are typically a thin-walled cysts filled with a red to yellow liquid.  The external surface is covered by blood vessels but is smooth.  The affected lobes parenchyma may be entirely destroyed. 

If the tumor is a carcinoma, it is typically larger than an adenoma.  Formation is typically multi-nodular with large areas of hemorrhage and necrosis near the center.  Unilateral involvement is more frequent than bilateral involvement of both thyroid lobes.  Unlike adenomas, carcinomas are very poorly encapsulated.  They tend to invade the wall of the trachea, esophagus, larynx, nerves, vessels, and cervical muscles.

Most adenomas affecting the thyroid lobes are small and noninvasive making them referred to as ‘clinically silent’.  This means you may not even know it is there.  However, if the tumor is clinically detectable, it is almost always malignant.  Thyroid tumors are diagnosed more frequently in older dogs.  Gender does not seem to play a role, but the more frequently seen affected dog breeds include Beagles, Boxers, and Golden Retrievers.  Both the right and left lobes are equally affected and some even have bilateral involvement.  The exact cause is unknown other than an abnormality within the endocrine system and immune system.

Malignant thyroid tumors can be found anywhere in the body.  They can develop at any location between under the tongue to the base of the heart.  However, the most common location to find a thyroid carcinoma in a dog is the cervical region.  

To begin her diagnostic process, your veterinarian will begin by collecting a verbal history from you.  She will want to know when you first noticed your dog’s lump, if it has changed in size or texture, if other pets in the household are experiencing similar symptoms, and any other details that may help with her diagnosis.  She will continue by performing a physical exam on your dog.  While his issue may be associated with a specific lump on his body, she will want to check him over entirely in order to evaluate him for other simultaneous symptoms he may be experiencing that seem unrelated to the untrained professional.  She will note all of his symptoms as it will assist her with her diagnostic process.

Laboratory diagnostic testing will consist mainly of blood work and endocrine testing.  A complete blood count (CBC) and chemistry panel are typically the first blood work performed when doing diagnostic testing.  It gives general information on your dog’s internal organ function and blood levels.  If your veterinarian suspects thyroid related issues or adrenal issues, she may then perform those blood tests which are more specific and therefore, a completely different test.  Your veterinarian may also want to collect a urine sample from your dog to perform a urinalysis.  This will provide more information on kidney and bladder function.

Depending on the results of her examination and blood work results, she may want to proceed with imaging or cytology of the mass.  In regards to imaging, your veterinarian may want to use an ultrasound to see exactly which tissues are being affected.  The trained professional will be able to tell if it is affecting the thyroid lobe itself, or if it is just very near it.  

As for the cytology, your veterinarian may attempt to collect a cell sample of the mass.  A fine needle aspirate (FNA) involves sticking a needle into the mass to collect a sample of cells from the area.  The cells are then examined under a microscope to check for cancerous cells.  However, this diagnostic test has been shown to give unreliable results when taken from the thyroid area.  The sample size is so small, there is no guarantee you will collect a cancerous cell even though the mass is indeed malignant.  The best way to diagnose what the mass is involves surgical removal of the mass with a biopsy.  This is the only way to get 100% definitive diagnosis.

If the tumor is freely moveable, you can opt to have it surgically removed.  The survival rate after this procedure is about 3 years.  If the tumor is more invasive, survival after thyroidectomy is only about 6 to 12 months.  The vascularity of the tumor, the depth of invasion into the adjacent blood vessels, and the local coagulopathies will all play a role in the surgical removal process and prognosis of recovery.  Radiation therapy is also a viable option for some dogs.  This can be in conjunction with surgical removal or just radiation treatment alone.  

If the mass is not malignant, you and your veterinarian will need to weigh the risks versus benefits of treatment options for your dog.  If it is not pressing on his trachea, not causing respiratory issues or eating difficulties, or anything that affects his daily life, leaving the mass alone is an option.  If it is very slow growing or does not grow at all, this may be your dog’s best treatment option.  You will just need to monitor it closely to ensure it does not begin to harm your dog’s quality of life.  

If the thyroid tumor is causing secondary issues such as thyroid disease, your dog will need to be treated for it simultaneously.  This typically involves administering an oral medication once or twice daily.  His thyroid levels will need to be evaluated every few months to see how he is responding to the therapy and if his levels are changing.

If your dog is experiencing any other symptoms or secondary conditions from the thyroid tumor, your veterinarian will administer medications and therapies in response to your dog’s need.

Prognosis of recovery from a thyroid tumor is guarded.  The malignancy, severity of vascularity involved, and the exact location of the mass will all play a role in your dog’s prognosis.  If the mass is benign, does not grow, and does not affect your dog’s quality of life, his prognosis of a normal life is decent.  However, the faster growing the mass, the worse your dog’s prognosis becomes as it typically indicates the tumor is malignant.  Offering your dog the best quality of life possible is sometimes the best you can do for him if he is diagnosed with a thyroid tumor.