a lump in the throat: thyroid carcinomas in pets
Even though I’ve been out of veterinary school for (ahem) several years, there are a few patients from my clinical year that stand out, and whom I’ll never forget. One such patient was Clyde, a lovable brown mixed-breed dog, about the size and shape of a Beagle. Clyde came to the vet school for treatment because his regular veterinarian found a large lump in his neck, which ended up being a malignant tumor called a thyroid carcinoma.
We’ve talked about thyroid disease before, both in cats as hyperthyroidism, and in dogs as hypothyroidism. The thyroid glands are located in our pet’s neck, just below the larynx (or “voice box”). They produce the thyroid hormones that help regulate metabolism, which is why cats who are hyperthyroid are thin, and dogs with hypothyroidism tend to carry more than a few extra pounds.
In dogs, thyroid carcinomas usually don’t cause any change in the amount of thyroid hormone produced by the thyroid gland, so it’s unlikely that these tumors will show up on blood work. In most cases, either the owner or the veterinarian feels a lump in the dog’s throat during a routine exam.
The average age of a dog when diagnosed with a malignant thyroid tumor is about 9 years old, and Boxers, Beagles and Golden Retrievers are more likely to be affected than other breeds.
Once a lump is found, a fine needle aspirate may be performed to obtain a small sample of the lump for analysis. In about 50% of cases, this will lead to an appropriate diagnosis of a malignant thyroid tumor. A more reliable way to diagnose thyroid carcinoma is to surgically remove the mass and submit it for biopsy. In dogs, however, these lumps tend to be very deeply attached to the surrounding tissue. Furthermore, thyroid carcinomas are highly vascular, meaning that they have a lot of blood vessels running in and out of them, and for that reason they can bleed quite readily during surgery. When a highly vascular structure is deeply attached to surrounding tissues, life-threatening blood loss is a concern during removal.
Another way to diagnose malignant thyroid tumors is by using nuclear medicine. A radioactive substance is introduced intravenously and attaches itself to the thyroid tumor. A scanning process called scintigraphy picks up on the areas of radiation, showing where the cancer resides. This is helpful in determining if the tumor has spread to the lungs or local lymph nodes.
For those tumors that cannot be surgically removed, other treatment options exist. Radiation, like that used for other cancers, can be pursued, though thyroid carcinomas are known to be slow to respond. Radiation is a huge time commitment, as it should be performed several times a week to be effective. In one study, radiation had a two-year survival time, compared to the 20-month survival time associated with surgery.
Finally, radioiodine treatment, like that used to treat hyperthyroid cats, has been shown to be effective in treating malignant thyroid tumors and stopping their spread. This is a non-invasive treatment, though your dog will need to stay at the facility for several days after therapy until their radiation emission is at a safe level. The average survival time for pets who undergo radioiodine treatment is around 30 months. While these treatments can be expensive for pet parents on a family budget, having dog insurance from Petplan pet insurance can help reimburse you for the costs and allow you to focus on the treatments and not the vet bills.
In cats, thyroid tumors make the thyroid produce more thyroid hormone, mimicking hyperthyroidism. This can make malignant tumors more difficult to diagnose, because benign hyperthyroidism is relatively common in older cats. About 5% of hyperthyroid cats will have thyroid carcinoma.
In cats, thyroid carcinomas tend less likely to be deeply attached to surrounding tissues, so surgical removal is a more reliable treatment choice. Of course, radioiodine treatment can also be performed in our feline friends to control the spread of the tumor. In this case also, having cat insurance can open the financial doors for your pet to receive the best treatment possible.
I still think of Clyde from time to time, and though he’s no longer around, I’ll never forget the important lessons he taught me, both through his disease and his wonderful disposition.