Issue Description Thyroid tumors include cystic structures
called goitres, multinodular overgrowth (hyperplasia), benign
(non-spreading) cancers (adenomas) and malignant (spreading) cancers
(carcinomas). Hyperplasias and adenomas grade into each other and most
produce excessive quantities of thyroid hormones. These induce complex
clinical syndromes. Malignant thyroid tumors rarely produce hormones
but they may spread both locally and to the lungs. Up to a third of
thyroid carcinomas in dogs may originate from specific cells in the
thyroid glands (C-cells) that act in combination with the hormones of
the parathyroid glands to regulate blood calcium Other Names Thyroid Cancer, Thyroid Neoplasm
Causes The reason why a particular pet may develop
this, or any cancer, is not straightforward. Cancer is often seemingly
the culmination of a series of circumstances that come together for
the unfortunate individual.
One non-cancerous tumor of the
thyroid called 'colloid goitre' is due to inactivity of the gland.
This is often caused by tumors in the pituitary gland preventing
production of the pituitary hormone that controls the thyroid. There
are numerous drugs and illnesses that can produce similar
non-cancerous goitre but such goitres are rarely large enough to see
Prolonged stimulation of the thyroid gland
often causes cancerous changes with a continuous spectrum from small
areas of overgrowth (hyperplasia) to benign thyroid adenoma and then
malignant cancer (thyroid adenocarcinoma). This multi-step process is
called tumor progression. Some cancers never progress past the first
stages so remain benign.
Only about 10% of canine thyroid tumors are functional, meaning that
they secrete excessive levels of thyroid hormone. This can result in
hyperthyroidism, a condition in which the body's metabolism is revved
up, resulting in symptoms such as rapid heart rate, elevated blood
pressure, hyperactivity, and weight loss despite a voracious appetite.
These thyroid tumors are usually located within the main thyroid
glands, which sit on either side of the trachea (windpipe), just below
the larynx (voice box). However, carcinoma can also arise anywhere
there are thyroid tissue remnants, extending from the base of the
tongue, down the neck, through the thoracic inlet (where the windpipe
and esophagus enter into the chest) and into the mediastinum (the area
within the chest cavity in front of the heart and between the lung
Symptoms Many dogs with thyroid cancer don't show many
symptoms. Some symptoms of thyroid cancer in dog may include urinating
more than usual, weight loss, increased appetite, and nervousness. The
thyroid gland, located at the base of the neck, may appear enlarged.
Some breeds of dogs are more susceptible to thyroid tumors than
others, including beagles, boxers, and golden retrievers.
Diagnosis About one third of thyroid cancers have already
metastasized (spread) to the draining lymph nodes and lungs at the
time of initial diagnosis. The best way to evaluate the location and
extent of functional thyroid tumors is by using a nuclear medicine
scan. This test uses a radioactive tracer, such as technetium or
iodine, which binds to any residual thyroid tissue. The areas of
radiation uptake are then measured with a gamma camera. If this test
is not available, an alternative is to perform a contrast CT
(computerized tomography) scan of the neck and chest, which uses
intravenous dye and digital X-ray pictures to outline any cancerous
tissues. If a CT scan is not available, radiographs (X-rays) of the
neck and chest combined with an ultrasound of the neck are a
Treatment There are many options for treating thyroid
cancer in dogs. The best option for your dog will depend on the size
of the tumor, how it is attached to the surrounding tissue, and
whether or not it has spread to other parts of the body.
it is possible to separate the tumor from other tissues without
damaging any major blood vessels or other vital parts, surgery is the
best treatment. This is the case whether or not the cancer has spread
to other parts of the body. If the cancer has spread to other parts of
the body, additional treatment will be needed, however.
the tumor involves the surrounding tissue and therefore cannot be
removed surgically, radiation therapy is often used. Radiation is
directed at the tumor, causing it to shrink. Sometimes after it
shrinks sufficiently, the remaining tumor can then be surgically
Finally, there is chemotherapy. Chemotherapy is
often prescribed along with surgical or radiation therapy. It is used
when cancer has spread to other organs of the body to slow its growth
and is also used to prevent the recurrence of cancer. There are a
number of drugs available for these purposes.
Prognosis Early diagnosis and treatment is key to
survival. There is a high rate of success in treatment that is begun
in the early stages of thyroid cancer in dogs.