Reviewed by Dr. Danielle Morosco, DVM
The thyroid gland is one of those things you don’t think about until there’s a problem. As a pet owner, you’re not likely to notice it unless it becomes inflamed or develops a lump.
The gland itself is made up of two halves (lobes) that sit on either side of your dog’s trachea (windpipe), like the wings of a butterfly.
Its job is to regulate metabolism, and it does this by releasing a thyroid hormone known as thyroxine (T4) into the bloodstream. If it’s not releasing enough hormones, the body gets tired and sluggish. If it’s releasing too many hormones, the bodily systems can become overactive.
Does Your Dog Have a Lump?
If you’ve noticed an unusual lump in your dog’s neck, make an appointment with the veterinarian ASAP. Thyroid cancer is relatively rare in dogs, but it’s always a good idea to have new lumps and bumps checked out.
The goal of this post is to help you gain a better understanding of the thyroid gland and its importance.
We’ll talk about the signs of thyroid cancer, how a diagnosis is made, the stages of cancer, treatment options, and the ultimate prognosis.
The Dog’s Thyroid Gland – What It Does
The hormones released by the thyroid play a significant role in your dog’s metabolism. In fact, without these hormones, your dog’s heart, liver, kidneys, muscles, and nervous system wouldn’t be able to function properly.
The thyroid functions as the body’s thermostat. The gland is vital in regulating the body’s metabolism, growth, and development. In fact, the hormones produced affect everything from breathing to heart rate.
The thyroid gland is one of the major glands of the endocrine system. Collectively, these glands include the:
- pituitary gland
- parathyroid glands
- islet cells of the pancreas
- adrenal glands
- Parathyroid Glands (located behind the thyroid gland where they produce the parathyroid hormone responsible for regulating calcium levels in the blood)
Thyroid Tissue Cells
Thyroid tissue consists of two types of microscopic cells known as follicular cells and parafollicular cells. The majority are follicular cells that store and secrete iodine-containing hormones. The parafollicular cells are a smaller subset. They secrete the hormone calcitonin which helps keep blood calcium low.
Two Important Hormones Released by the Thyroid Gland
There are two vital hormones released by the thyroid gland, each of which play a vital role in regulating the body’s metabolism.
These hormones, known as thyroxine (T4) and triiodothyronine (T3) work together and affect every cell and all the organs in the body.
Types of Thyroid Disease in Dogs
Disorders of the thyroid gland are complicated. If your dog has a thyroid disease, it doesn’t mean he/she will develop thyroid cancer.
However, some thyroid tumors will trigger thyroid disease.
Some examples of thyroid disease include:
Hypothyroidism in Dogs (Common)
Hypothyroidism (also known as Hashimoto’s thyroiditis or lymphocytic thyroiditis) refers to an underactive thyroid gland. It’s an autoimmune disorder that causes chronic inflammation of the thyroid gland.
Pets or people with hypothyroidism have thyroid glands that don’t secrete enough hormones. As a result, the metabolism slows.
Common signs of hypothyroidism include:
- Exercise intolerance
- Weight gain but no change in appetite
- Cold intolerance
- Changes to the dog’s coat and skin (increased shedding, hair thinning, and fur loss)
- Thickening of the skin
- Reproductive problems in intact dogs
Medium to large-sized breeds are more likely to develop hypothyroidism than toy breeds. The breeds that appear more predisposed to hypothyroidism include:
- Cocker spaniel
- Miniature schnauzer
- Doberman pinscher
- Golden retriever
- Airedale terrier
- Irish setter
Thankfully, hypothyroidism isn’t life threatening. It’s easy to diagnose with a blood test and is relatively inexpensive and easy to treat with a synthetic hormone.
Hyperthyroidism in Dogs (Rare)
This endocrine disorder causes the body to produce too much of the thyroid hormone. The excess hormones leave the body “amped up”. Signs include weight loss, anxiety, and diarrhea.
Hyperthyroidism in dogs is usually caused by a thyroid tumor.
This type of thyroid disease is an autoimmune disorder where the immune system attacks the thyroid gland. It’s the most common cause of primary hypothyroidism in dogs.
Clinical signs can happen at any point in a dog’s life but are more likely to occur between 2 and 5 years of age.
How Thyroid Tumors Develop in Dogs
Thyroid tumors develop due to abnormal growth of the cells that make up glandular tissue.
Glandular tissue (also known as glandular epithelium) is a complex system involved in the production and release of secretions like sweat, saliva, digestive enzymes, hormones, etc.
If a dog develops thyroid cancer, the growing tumor blocks the normal function of the thyroid gland. This makes it very difficult for the dog to keep up with daily activities.
It’s difficult to pinpoint thyroid tumors to any one cause, although some breeds seem to be genetically predisposed.
Thyroid tumors are relatively rare in dogs.
According to the Animal Cancer & Imaging Centre in Canton, Mississippi, only 1.2% to 3.8% of dog tumors are related to thyroid cancer.
Benign Adenomas (non-cancerous tumors) in Dogs
Benign thyroid growths tend to get larger and may produce excess hormones. Fifty percent (50%) of dogs diagnosed with thyroid tumors end up being benign.
This type of tumor is hard to detect because there are no obvious signs. If there’s a mass big enough to be found, it’s usually malignant.
Malignant Tumors (cancerous and spreading) in Dogs
Malignant growths can grow larger in one area, but they can also spread to other parts of the body (metastasize).
Some dogs have thyroid tissue (known as ectopic thyroid tissue) in other areas of the body, like the base of the tongue. It’s rare, but this tissue does have the potential to develop into cancer.
Dog Breeds Predisposed to Thyroid Cancer
Golden retrievers are at increased risk of developing two types of canine thyroid cancer: anaplastic thyroid cancer and lymphocytic thyroid cancer. Anaplastic thyroid cancer is rare and very aggressive.
Thyroid cancer tends to affect medium to large breed dogs.
Although thyroid cancer can occur in any dog, the following breeds appear more susceptible:
- Bernese Mountain Dogs
- Golden Retrievers
- Standard Poodles
- Mixed-Breed Dogs
Medullary Thyroid Cancer in Dogs
Medullary thyroid carcinoma (MTC) is a rare type of cancer that forms in the thyroid gland. It starts in the parafollicular cells (C-cells) that release a hormone known as calcitonin. Calcitonin regulates calcium levels in the blood.
Too much calcium in the blood can cause a host of problems. It can:
- weaken bones
- create kidney stones
- cause problems with the heart and brain
Thankfully, medullary thyroid cancer is considered relatively rare in dogs and people.
Signs & Symptoms of Thyroid Cancer in Dogs
Unfortunately, dogs with thyroid tumors often have no signs of disease.
There could be a mass on the underside of the neck, but it may not be immediately noticeable to the pet parent. If the mass gets big enough to compress the windpipe (trachea) or esophagus, it can cause:
- difficulty breathing
- difficulty swallowing.
Clinical Signs of Thyroid Cancer in Dogs Include:
- a mass (fixed or moveable) in the neck
- change in the sound of their bark (hoarseness)
- weight loss
- increased thirst
- increased hunger
- muscle tremors
- frequent urination
Diagnosing Thyroid Cancer in Dogs
If you suspect your dog may have a tumor or an enlarged thyroid gland, do not hesitate to take him/her to the veterinarian.
The following are typical tests a veterinarian may suggest when seeking a diagnosis.
A thorough physical examination is performed to feel for lumps in the neck (and other parts of the body).
The veterinarian can detect signs of muscle wasting, pain, swollen regional lymph nodes, and a variety of other problems that may not be obvious to pet parents.
If cancer is suspected, chest x-rays can help determine whether it has metastasized (spread) to the lungs.
An ultrasound of the neck can be useful in determining whether the mass stems from the thyroid gland, whether it is encapsulated, or whether the surrounding lymph nodes appear enlarged.
A thyroid panel can determine whether T4 and T3 levels are elevated.
A chemistry panel can search for other changes that can indicate a thyroid problem
Tissue Sample/Tissue Biopsy
A definitive diagnosis of thyroid cancer is only made after analyzing a tissue sample obtained through a biopsy.
Thyroid masses are typically filled with blood vessels, and that makes it difficult to get a biopsy that isn’t diluted with too much blood. Some masses may invade the jugular vein or carotid artery.
Ultrasound guidance is used to perform fine-needle aspiration biopsy to avoid large blood vessels.
Treating Thyroid Cancer in Dogs
The type of treatment chosen will depend on the size of the tumor, whether it has spread to other parts of the body, how aggressive it is, and whether there are signs of thyrotoxicosis (when the gland produces excessive hormones).
Surgery to remove a dog’s cancerous thyroid gland is known as thyroidectomy. It can offer the best prognosis for dogs if the mass is small (less than 4 cm in size), is freely moveable, and hasn’t spread to other areas.
Surgical removal of a mass can be difficult if the mass has invaded nearby blood vessels or other tissues. Unfortunately, dogs with invasive tumors are poor surgical candidates.
If surgery is performed, but there’s a chance they couldn’t get all the cancer, radiation or chemotherapy may be advised.
Only 25% to 50% of dogs with thyroid carcinomas are candidates for surgical excision at the time of diagnosis.Mayer MN, MacDonald VS. External beam radiation therapy for thyroid cancer in the dog. Can Vet J. 2007 Jul;48(7):761-3. PMID: 17824167; PMCID: PMC1899859.
Surgery is often only considered if there is a good chance that all cancer cells can be removed.
Stereotactic radiosurgery (SRS) has been successfully used to treat cancer in people, including thyroid cancer. The technique is extremely accurate and allows for the removal of the tumor with little harm to the surrounding healthy cells.
This non-invasive therapy helps maintain your dog’s quality of life. Compared to conventional radiation therapy, SRS has fewer treatment sessions and fewer side effects.
External Beam Radiation Therapy
External beam radiation therapy is a treatment option that uses a linear accelerator or cobalt therapy machine to administer radiation.
This treatment is used if the tumor has spread to adjacent structures and can’t be completely removed through surgery.
Radiation therapy can also be used relieve the symptoms and suffering caused by cancer (palliation).
This treatment option involves injecting radioactive iodine into the body.
Radioactive iodine is used in people and dogs with thyroid cancer. It’s performed by placing a radioactive tag on iodine and inserted into the body. The iodine is slowly absorbed by the thyroid gland which then destroys cancerous tissue.
This option can be used in dogs where surgery is not a great option. It can also be used if it’s found that the cancer has spread. Unfortunately, this type of therapy requires high doses of radioactive iodine to work effectively.
Used alone, radioactive iodine may not be as effective. As a result, it can be combined with chemotherapy and traditional radiotherapy. Complications of radioactive iodine therapy are rare but may include:
- Inducement of hypothyroidism
- The treatment may not work as planned
- Kidney dysfunction or renal failure
Important Sidenote on Radioactive Iodine Therapy Recovery for Dogs
Your dog may need to be temporarily confined once treatment has been administered. This is because not all of the radioactive iodine is taken up by the body. Small amounts will be released through the urine, saliva, and feces.
According to Southeast Veterinary Oncology and Internal Medicine, nearly all radioactive iodine will leave the body during the first two days after the initial dose.
Follow-up Treatment Post Surgery
Assuming all goes well during the thyroidectomy, dogs are usually given oral antibiotics and pain medication after surgery. Once the thyroid gland is removed, a synthetic hormone replacement is prescribed.
Thyroxine (also known as L-Thyroxine, Levothyroxine, and Thyro-tabs) is given by mouth in the form of a tablet or capsule. In some cases, it is compounded into a liquid.
Complications and Risks of Surgery
Although surgery can offer the best outcome for your pet, it also comes with risks. General anesthesia always comes with its own set of risks, but there is also a risk of excessive bleeding and laryngeal paralysis with thyroidectomies.
Laryngeal Paralysis Risk
Laryngeal paralysis is a serious condition. Normally, the larynx (voice box) allows us to take a deep breath. It opens and closes in a way that allows us (and our pets) to eat, drink, and breath freely. If it becomes paralyzed, none of these things can happen.
It’s treatable, but usually requires more surgery.
Of course, there are many things to take into consideration before surgery is performed. These things include:
Your dog’s age
Older dogs tend to be at higher risk of complications from anesthesia because of underlying disease. Some complications commonly seen in older dogs include:
- low blood pressure (hypotension)
- low heart rate (bradycardia)
- low blood oxygen (hypoxemia)
According to the American Kennel Club Canine Health Foundation, “experts estimate that the risk of anesthetic death increases as much as seven times for dogs older than 12 years of age.”
The tumor sizes
Larger tumors or masses can be challenging, depending on their location. Thyroid gland tumors can have a lot of blood vessels and can also be attached to nearby tissue.
Life Expectancy of a Dog with Thyroid Cancer
The median survival time of a dog with thyroid cancer depends on the type and stage of cancer, but in general, dogs with thyroid tumors may live anywhere from six months to several years.
Sadly, dogs with malignant thyroid tumors have an average survival time of 3 months.
On the opposite end of the spectrum are dogs diagnosed with small tumors that haven’t spread. If treated successfully through surgical removal, these dogs can live 3 years or longer.
The prognosis is usually better for thyroid tumors that are small and not too invasive.
Surgery isn’t always an option if the tumor has invaded nearby tissues. If performed, it can be tricky due to the mass location.
Radiation, radioactive iodine, or chemotherapy may be required if surgery cannot be performed or if cancer cells were left behind after surgery.
Sadly, the prognosis for dogs with thyroid cancer that has spread to other parts of the body is poor.
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