Hyperthyroidism is the most common
endocrine disorder in cats, and is one of the most common diseases of older cats.
Most cats diagnosed with hyperthyroidism are over 10 years old. It is not unusual to
have both a diabetic and hyperthyroid cat. Hyperthyroidism is rare in dogs. The
information presented here is for cats.
Hyperthyroidism occurs when the thyroid gland, a small double-lobed gland in the neck,
produces an excess amount of thyroid hormones. The thyroid gland produces two
hormones, thyroxine (T4) and triiodithyronine (T3). In most cats, hyperthyroidism is
the result of too many active cells in the thyroid gland. Very rarely is it caused
by thyroid cancer (1-2%). Treatment of hyperthyroidism is usually very successful.
Typical signs of hyperthyroidism include
- weight loss
- increased appetite
- excessive drinking
- excessive urination
- the coat may look dirty and dull
- some cats have very rapid toenail growth or thick nails
- some cats display aggression
- a few cats (less than 10 percent) have decreased appetite,
depression, and weakness
In addition to the signs
listed above, hyperthyroidism causes
- increased blood pressure
- increased stress on the heart: rapid heartbeat and strong
heart contractions, or irregular heartbeat
Untreated hyperthyroidism can lead to
- congestive heart failure
- kidney damage
- diarrhea that is very difficult to treat
- detached retina (due to high blood pressure) which could
lead to sudden blindness
The levels of thyroid hormone in the body are measured by a blood test. Other blood tests
and methods of imaging the thyroid gland may be performed.
There are three treatment options available, and
the method used will will depend on the pet.
- Medication - to prevent the
thyroid gland from producing excess thyroid hormones. Methimazole (Tapazole) is a
commonly used antithyroid drug. Normal thyroid hormone levels are usually achieved
in 1-3 weeks. Side effects include anorexia and vomiting, but these often lessen
with time. It may be possible to decrease the dose to lessen the side effects while
still maintaining proper thyroid hormone levels. Additionally, some cats may become
hyperthyroid again, even while taking medication. Hospitalization is usually not
- Surgery - partial or total
removal of the thyroid gland. Normal thyroid hormone levels are usually achieved in
1-2 days. Side effects include surgical risks, and hypothyroidism (low thyroid
hormone). The parathyroid gland (a different gland next to the thyroid gland) may
also be removed or damaged, resulting in hypoparathyroidism. Because hyperthyroid
cats may have increased stress on their heart, they are usually started on antithyroid
medication prior to surgery so the thyroid hormone levels are normal and the effects on
the heart are minimized. The cat is hospitalized for 1-3 days.
- Radioactive iodine treatment -
to kill the overactive thyroid cells. Normal thyroid hormone levels are usually
achieved in 1-12 weeks. This is considered the best treatment if a facility is
available. It is very low risk, but not all cats are candidates, particularly cats
with other health problems that require them to be handled frequently. After
radioactive iodine treatment, the cats can't be handled right away because they must
eliminate the radioactive iodine first. This makes it difficult for a diabetic cat,
who requires insulin injections, to be a candidate for radioactive iodine treatment.
Also, few facilities are available for this treatment, and the cat must be hospitalized
for 1-4 weeks.
Since both hyperthyroidism and diabetes are common in older cats it is often necessary to
deal with both conditions. Usually, the cat has one disease, which is being treated,
then the second disease begins and must be dealt with.
If your cat is hyperthyroid first, you will probably have started therapy and gotten the
thyroid hormone levels near normal. Then if the cat then becomes diabetic, the
hyperthyroidism (which is controlled) probably won't play a major role in the diabetes
The situation is a bit more complicated if the cat is diabetic, then becomes hyperthyroid
too. Since hyperthyroidism has similar signs as diabetes (increased appetite,
drinking, urination, and weight loss), a cat that is diabetic who continues to show these
signs may be thought of as just needing additional diabetes management care. But the
possibility of hyperthyroidism should not be overlooked. Since hyperthyroidism
causes a general increase in the body's metabolic rate, it will have an effect on the
diabetes and diabetes management. Often, a cat that is hyperthyroid requires higher
doses of insulin in order to maintain the blood glucose levels near normal. As the
hyperthyroidism is brought under control, the metabolism slows to a more normal rate, and
the insulin dose needs to be adjusted. Pre-existing diabetes will probably have an
effect on which hyperthyroid treatment option is used.
Although not all of these cats are diabetic, the experiences with hyperthyroidism may be
valuable for you and show some of the options that are available.
- Barney - diabetic, hyperthyroidism treated
Barney was diagnosed with hyperthyroidism 6 months after being diagnosed diabetic.
The hyperthyroidism was detected during a routine pre-anesthesia blood test (he needed his
teeth cleaned). His thyroid hormone levels were moderately elevated, and his
heartbeat was faster than normal. The teeth cleaning was postponed
hyperthyroidism was placing a strain on his heart and that meant he could not safely
undergo anesthesia. Barney was started on Tapazole, 5 mg twice a day. Shortly
after starting Tapazole therapy, he started vomiting several times a day. Since he
is also diabetic, it is difficult to know if the vomiting was caused by the Tapezole, or
if it was due to his diabetes not being well controlled, or due to some unknown reason.
But vomiting is not something you want with a diabetic because you need to
keep their food intake as consistent as possible. Barney's thyroid hormone levels
were not extremely high, so the vet thought it would be ok to decrease his Tapezol dose to
1/2 a tablet twice a day. He showed an immediate improvement and the vomiting
stopped. A follow-up blood test was done, and his thyroid hormone levels were in the
low-normal range. Barney has continued on this low dose of Tapazole. Since the
hyperthyroidism increased metabolism and had an effect on the diabetes, during the time
when his thyroid hormones were returning to normal, we monitored his blood glucose levels
to be sure he was not getting too much insulin.
- Midnight - FIV, IBD, hyperthyroidism treated
Our cat who is on Tapazole had a very difficult time
tolerating the drug. Because she is somewhat fragile and is FIV+ and has IBD, she wasn't a
good candidate for the iodine treatment. We were finally able to reduce her Tapazole
dosage down to 1/2 tablet every other day and still stay within the normal thyroid ranges.
For owners whose cats can't handle daily Tapazole and can't have the iodine treatment or
the surgery (which I hear is very tricky), giving the minimum dosage possible to stay
within the acceptable range should be pursued.
- Desi - hyperthyroidism treated with
The radioiodine treatment is unbelievable! The worst part was having our cat stay at the
facility for a week with no contact. She was very thin when she was released (she's a
timid kitty, so I don't think she ate much) but regained her weight quickly after we got
her home. The vet radiologist who did the treatment called us every night to give us
updates. The first night he told us they were going to completely check Desi's heart,
because he said that by the time many hyperthyroid cats are diagnosed, they have already
sustained some heart damage, the worst of which is cardiomyopathy. Luckily, Desi was
fine. The other bad part was that we had to keep her pee and poop for 2 weeks before
disposing of it. I think this might be a California law, but it wouldn't surprise me if
other states also had this requirement.
- Pocket Companion to the Fourth Edition of Textbook of
Veterinary Internal Medicine Stephen J. Ettinger.
- The Cornell Book of Cats : A Comprehensive and Authoritative
Medical Reference for Every Cat and Kitten. Mordecai Siegal (Editor), et al.1997.
- The 5 Minute Veterinary Consult: Canine and Feline. Larry
Tilly and Francis W.K. Smith, Jr. 1997.
- Veterinary Drug Handbook. Second Edition. Donald
C. Plumb. 1995.
- The Well Cat Book Terri McGinnis, D.V.M. 1993.
Updated June 2002
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