Hyperthyroidism (Thyroid Cancer) in Cats
Hyperthyroidism in cats is almost always associated with a cancer-a benign adenoma (more
common) or a malignant adenocarcinoma. These cancers tend to occur in older
cats and the major effect is the increased thyroid hormone production they
stimulate. Exposure to secondhand cigarette smoke may be a factor. Himalayans
and Siamese have a lower risk for this problem.
The first signs may be dramatic. The increase in thyroid hormones tends to
cause an increase in appetite-the finicky eater now wolfs everything down and
may even open cupboards looking for more-and an increase in activity. The
sedate older cat who used to spend his days soaking in the sun on his window
seat is now flying around the house like a wild kitten. Weight loss, vomiting, and panting may all be noted.
Careful palpation of the area under the chin when the head is lifted may reveal
a small lump. Thyroid tumors may appear only on one side, but they can be
Blood work is important to see if thyroid hormone levels are truly elevated.
It is important to check kidney function, because the increased metabolic
rate may hide kidney failure. It is also
important to thoroughly evaluate the heart, which can be damaged from the
increased metabolic rate. High blood pressure or
hypertension is often seen in hyperthyroid cats.
Treatment tends to start with an oral medication called methimazole, which
lowers hormone production. This must be given daily and manufacturers are
working on palatable and easy-to-administer forms for cats, including a
compounded version that is applied to the ear and absorbed through the skin.
Medical treatment enables your veterinarian to evaluate the cat’s kidney and
cardiac status prior to the more definitive treatment of surgery or radioactive
Surgery is another option, with the cancerous gland removed. Care must be
taken not to damage or remove the parathyroid glands that regulate calcium
metabolism. If both thyroid glands are removed, the cat will need supplemental
thyroid for the rest of his life.
A third and popular option is to use radioactive iodine to destroy the
cancerous tissue. Cats who undergo this therapy must stay at a treatment center
for 7 to 25 days while the radioactivity shed in the cat’s urine and feces
decreases to a safe level. These cats may need supplemental thyroid for the
rest of their lives.
Caught early, before any residual damage is done to the heart or the
kidneys, this can be a very treatable disease. If the heart and kidneys are
already damaged, they will still need treatment after the cancer is removed.
For the rare cats who get an adenocarcinoma, as opposed to simply an adenoma,
the cancer has often metastasized by the time it is diagnosed and the prognosis
is very poor.