What does the thyroid gland do normally?
The thyroid is a gland located in the neck on either side of the trachea. The hormones produced by this gland play an essential role in your dog’s normal metabolic regulation. These hormones are also important for maintaining normal cardiac and neurological function. Iodine uptake by the gland is needed for normal thyroid function.
The thyroid gland is controlled by a hormone (Thyroid Stimulating Hormone or TSH) secreted from the pituitary gland located at the base of the dog’s brain. This is an on/off button that signals to the thyroid when or when not to release its hormones.
So what happens when your dog has a thyroid tumor?
Thyroid tumors are relatively uncommon in the dog and represents about 2% of all canine tumors and 10-15% of tumors of the head and neck. Unfortunately, malignant thyroid carcinomas are more common then the benign form, representing up to 90% of palpable thyroid tumors. The abnormal cells of the carcinoma may not respond normally to the TSH from the pituitary gland.
The majority of dogs with thyroid carcinomas either have thyroid hormone levels within the normal range or do not produce enough, the remainder of affected dogs (about 10%) have functional thyroid tumors that secrete excess thyroid hormone. These tumors can frequently invade other local structures in the neck such as the jugular vein, the trachea or the oesophagus or even spread to other organs, such as the lungs or the liver. Dogs presenting with this tumor are usually older (about 7yrs ). The breeds that are most commonly affected are Beagles, Boxers and Golden Retrievers.
How do we know that your dog has a thyroid abnormality?
The first indications we get that your dog’s thyroid may be abnormal is from the history and symptoms that you tell us about. These include:
- Swelling/ neck mass
- Inability to breathe comfortably
- Inability to swallow normally
- Decreased appetite
- Weight loss
- Changes in the sound of the bark
- Neurological abnormalities
- Swelling/ neck mass
- Increased appetite
- Weight loss
- Increased/more frequent drinking
- Increased/ more frequent urination
- Change in behavior – over activity or aggression
- Change in the sound of the bark
The next step is to perform a blood test called a “T4”, where we measure the level of the thyroid hormones in the blood. This test is a good indicator of thyroid function but does not always tell us the type of pathology that is going on in the gland.
A biopsy of the thyroid is a definitive way of finding out if the thyroid is in fact cancerous and the extent of the malignancy.
An important tool for diagnosing an abnormal thyroid is a nuclear scintigraphy scan. This involves injecting the dog with a radioactive material, which is similar in structure to iodine (normally used by the thyroid), and then scanning the dog to see where the carcinoma is and where it may have spread.
A combination of these tests can give us a good idea of your dog’ prognosis and the best treatment choice.
What are the treatment options?
Surgical removal is the treatment of choice in cases where the tumor has not deeply invaded other local structures such as major blood vessels and nerves making the procedure too risky.
Thyroid carcinomas appear to be poorly to moderately sensitive to chemotherapy alone.
External beam radiation has been shown to have good results for locally invasive tumors; however this treatment is not readily available for veterinary use.
Thyroid ablation using radioactive iodine (131I) is an effective therapy in parts of the tumor capable of taking up iodine.
In order to ascertain whether this treatment will improve your dog’s prognosis, a nuclear scan is first done. The dog is injected with the radioactive material, which is similar in structure to the 131I, and then scanned to see if the carcinoma takes up an appropriate amount. If this is the case, then it is concluded that the 131I is likely to be effective in destroying the tumor, regardless of its position. A recent study at our hospital has shown that the average survival time with this treatment alone is about 30 months whereas the average survival time with no treatment at all is about 3 months.
It seems that the only real disadvantage of this treatment is that your dog may be slightly radioactive for a few days after treatment and so must be kept in strict isolation. After this period the dog can go home to its normal environment, but we advise that owners not have prolonged contact with the dog for another week or so and discourage the dog from sleeping on the bed or sitting on someone’s lap for a prolonged period of time.
Dogs often need to be on thyroid replacement hormone for the rest of their lives.