by Amy Shojai
If a therapeutic diet is necessarily, make the switch
gradual y. Mix with the “regular” diet for the first several
days to slowly introduce the change, and then increase
the percentage of therapeutic diet day by day.
HYPERTHYROIDISM
Hyperthyroidism, though unrecognized in cats until 1979,
has since become the most commonly diagnosed disease
of the endocrine (hormonal) system in cats. “We see it in
middle-aged to older cats,” says Rhonda L. Schulman, DVM,
an internist at University of Il inois. “It’s definitely one of the
most common problems in older cats.”
The cause remains unknown, but Dr. Duncan Ferguson, a
veterinary scientist at the University of Georgia Col ege of
Veterinary Medicine, says the disease affects one in 300
cats older than seven. Risk factors for the disease include
being fed a predominantly canned food diet, especial y
those containing giblets (two- three-fold risk) and exposure
to cat litter (three-fold risk). Researchers don’t yet know what
this means—if eating the canned food leads to the disease
or the dry food helps protect from developing it.
Hyperthyroidism is the overactivity of the thyroid gland,
which is a double-lobed gland located in the base of the
cat’s neck. The gland manufactures and secretes the
hormones thyroxine and triiodothyronine, which help regulate
the body’s metabolism—the rate at which food and oxygen
are burned for energy. For unknown reasons, one or both
lobes of the thyroids of many elderly cats become enlarged
and create a toxic nodular goiter that causes an
overproduction of hormones. Basical y, hyperthyroidism
speeds up the metabolism, resulting in a variety of behavior
and physical changes.
“Many hyperthyroid cats also have high blood pressure,
and they may also have kidney insufficiency,” says Susan
Little, DVM, a feline specialist in Ottawa, Canada. A study
indicated that 87 percent of hyperthyroid cats had elevated
blood pressure. “We think of those diseases as a group, a
triple play. So if a patient has one, we look for the others.”
Diagnosis and treatment of feline hyperthyroidism
has made great strides over the years. “It used to be these
kitties would come in as a rack of bones,” says Me, DVM, a
radiologist in Carlsbad, California. Today, veterinarians tend
to catch the disease much earlier. “The earlier you catch a
disease process, the more likely we are of having a good
outcome.”
Dr. Hager considers feline hyperthyroidism highly
treatable. “It’s one of my favorite parts of the practice
because first of al , I’m a cat person. Second, there are few
things where the kitty comes in with something that’s
relatively bad, and we send them home with a real y high
probability of curing them. We’re about to treat a 22-year-old
kitty. That’s very satisfying.”
Senior Symptoms
Behavior changes are the most common symptoms of
hyperthyroidism. Cats with concurrent diseases such as
heart or kidney problems may have overlapping signs:
Increased appetite
Weight loss
Increased drinking and urination
Hyperactivity, pacing, short temper
Howling or yowling
Oily coat, hair loss, and/or rapid claw growth
Diarrhea or vomiting
Seeks cool place to rest
Diagnosis
Most commercial laboratories measure thyroid levels as a
routine part of blood test in cats, says Richard Nelson, DVM,
an internist at University of California-Davis. “That al ows you
to identify when those numbers start to go up.” However, the
thyroid level can be artificial y suppressed by other health
conditions, says Dr. Hager. “That’s where the scintigraphy
confirms and defines the problem.”
Also cal ed a thyroid scan, scintigraphy employs a
radioactive particle that seeks out and attaches to thyroid
tissue, which is then revealed on a gamma camera. “It tel s
you if al the thyroid tissue is only in the neck right at the
thyroid site, or other places,” says Dr. McCul ough.
Scintigraphy defines the extent of the disease, and helps the
veterinarian choose the best treatment options. For instance,
if the scan shows the thyroid tissue is limited to the glands in
the neck, surgery to remove affected tissue may be a good
option.
Dr. Hager says scintigraphy is currently the best method to
confirm a diagnosis of hyperthyroid disease, figure out how
much of the thyroid gland is involved, where the thyroid is
located, and also whether the enlargement is due to a
cancer or a benign adenoma (tumor). A tiny dose of a mildly
radioactive but inert metal cal ed technesium is injected into
the cat’s vein, travels throughout the body, and within about
20 minutes it is preferential y taken up by thyroid tissue,
salivary tissue, and the gastric mucosa.
“Technesium is the most commonly used imaging agent
for nuclear medicine,” says Dr. Hager. “It has a very short
half-life so it’s very safe, and it has a nice imageable photon
that emits so we can pick up on the gamma camera.” After
20 minutes, the cat is gently placed on the flat-faced camera,
and the inch-thick crystal inside the camera picks up the X-
rays emitted from the cat’s body and glows in response.
“That information goes to a computer that forms an image
that tel s us where most of the counts come from,” says Dr.
Hager. “It gives us the anatomic definition. With cancer the
radioactivity is scattered al up and down the neck, versus a
localized benign process that’s sharp and wel defined and
in one spot in the neck.”
Hyperthyroid disease in cats can be treated three ways.
“One is medical, two is to cut it out surgical y, and three is do
the iodine-131 [I-131 or radioactive] therapy,” says Dr.
McCul ough.
Surgery
When a benign adenoma is localized to the neck, surgery
can be a permanent fix. But because most cats develop
hyperthyroidism during their senior years, the risk of
anesthesia and other health considerations are a concern.
“There are two thyroid glands, so sometimes it’s a big
issue, do you take out just the abnormal one?” says Dr.
Schulman. Over time, she says the normal lobe that is left
behind in the surgery often also becomes abnormal, and
begins causing symptoms al over again. The surgery can
also be tricky. “The parathyroid gland sit right next to the
thyroid, so if you take out both thyroid glands you run the risk
of taking out al the parathyroid glands as wel . Then you
have a whole different problem!” The parathyroid glands
produce parathyroid hormone (PTH), which maintains a
normal blood calcium level in the body.
If both thyroid lobes are surgical y removed, Dr. Schulman
says most cats do very wel and bounce back pretty quickly.
&n
bsp; Interestingly, although a few need thyroid supplements, most
do not need further medication at al .
Medical Treatment
“The drug methimazole (Tapazole) is the most common
medical management for hyperthyroidism,” says Steven L.
Marks, BVSc, an internist and surgeon at North Carolina
State University. Depending on its severity and the size of
the cat, the dose ranges from about 2.5 to 7.5 mil igrams
twice a day. The drug only suppresses the thyroid hormone
production, so it must be given every day for the rest of the
cat’s life. However, many cats can be treated with 2.5–5 mg
once daily or every other day if side effects occur. Some
cats are given Tapazole on a temporary basis until other
treatment can be scheduled.
Dr. Hager warns that up to a fifth of the cats treated with
Tapazole wil have an adverse reaction. “They become
terribly il with problems with their liver. It can affect their
blood, cause itchy face. They can have lots of problems,” he
says. Also, because the drug is not a cure and little
adenomas continue to grow, he says the cat needs more
and more Tapazole as time goes one.
“Kitties have an aversion to being pil ed,” he cautions,
which can interfere with the bond you share with your pet.
Repeated unpleasant associations with medicating can
cause the cat to hide when he sees you. Another option is
transdermal application of the medicine. Tapazole is
compounded into a cream or gel that is able to carry the
medicine through the skin when smeared on the inside of the
ear.
Tapazol, a human drug, has been successful y used off-
label for hyperthyroid cats more than thirty years. In 2009,
The U.S. Food and Drug Administration announced approval
of Dechra Veterinary Product's Felimazole (methimazole)
specifical y for treatment of hyperthyroidism in cats with
coated tablets in 2.5 to 5 mg dosages. However, in a
Veterinary Information Network report, Margie Scherk, DVM,
DABVP (Feline) notes the new drug is identical to the
methimazole already in use. However, in Great Britain (and
some states in the US), veterinarians must use available
licensed drugs over generics, so this likely wil make a
difference for some cats and their owners.
Dr. Scherk and others would like the FDA to investigate
carbimazole that is converted to methimazole after
metabolism. This common oral treatment for feline
hyperthyroidism is sold as Vidalta in the UK by Intervet (part
of Schering Plough Animal Health).
Transdermal medication has been successful y used for
years in human medicine, and has been used for some time
in veterinary medicine especial y for pain control (fentanyl
patches) and in hyperthyroid cats. Research by Dr. Marks
and others looks at the benefits of transdermal application of
several products, including methimazole in cats. “Pets don’t
like to receive pil s if they don’t have to,” says Dr. Marks. The
struggle and stress of medicating cats can make them even
sicker, so ask your veterinarian about less stressful
medication options. More transdermal medications should
become available when results of these studies are
published.
When a hyperthyroid cat has other health concerns, those
may also require medical treatment. For instance, Dr. Little
says that hypertension is very common but highly
manageable with the proper medication. “The most
commonly used one is Norvasc (amlodipine). It’s a human
heart drug but it’s the best drug for hypertension in cats.” She
says control ing high blood pressure makes a clear
difference to their quality of life day to day.
Radioactive Iodine
The third option for feline hyperthyroidism is radioactive
iodine treatment, I-131. “That’s a one time only deal, but it
has limited availability, typical y in veterinary universities,”
says Dr. Schulman. A commercial enterprise, cal ed
Radiocat, has established a chain of clinics throughout the
United States that are mostly in large metropolitan areas.
“We have a private radio iodine clinic in our area that one of
the local veterinarians set up,” says Dr. Little. “They’re
increasingly common. The vet should know where the
nearest radioactive iodine clinic is.”
No anesthesia is involved. Treatment consists of a
subcutaneous (beneath the skin) injection of radioactive
iodine, usual y at the back of the neck. “You could inject it
anywhere, because it circulates everywhere in the body in
the bloodstream,” says Dr. Hager. As thyroid tissue absorbs
the radioactive iodine, it is subsequently destroyed.
This treatment is ideal especial y if the disease is caused
by a cancer and has spread to other places in the body. “The
prognosis is pretty good. The nice thing about radio-iodine
is that it wil kil just the tumor cel s and leave the healthy cel s,
even the cancer cel s that have moved elsewhere.” The
cancers are treated with a high dose of radio-iodine, and the
benign disease is given a lower dose.
“Our dose for the radio-iodine is between 2 and 6
mil icuries [measure of radioactivity],” says Dr. Hager. The
dose depends on the size of the cat, as wel as size of the
thyroid glands—smal er glands get a smal er dose. Dose is
also based on the cat’s thyroid hormone (T4) levels. “Also, if
kitties have kidney disease, then I’l give them a lower dose.
Kitties that are frail and have been hyperthyroid for a long
time, we give them a lower dose because they have less
reserves.” He says using scintigraphy has helped take away
much of the guesswork.
This treatment option has a 98 percent cure rate for cats.
The government regulates the use of radioactive iodine,
requires that a treated cat be quarantined for a period of one
to four weeks, and have his urine and feces monitored for
radioactivity before being released. Dr. Hager says these
restrictions are the biggest drawback to the treatment. “The
kitty has to stay with us for five days—that varies between
states.”
Quarantine can be hard on the owners and on the cats.
“Mom and dad are worried about having them gone, and
what their kitty wil do. Cats are creatures of habit. They love
their routine. And the minute you take them out of their
routine, then they become upset,” says Dr. Hager.
For that reason, the centers where he works include a
viewing room for people to come in and see the cat, and for
the cat to see them. Although about half the owners are
reluctant to visit, fearing they’l just upset the pet, Dr. Hager
says he’s not noticed any cat becoming real y upset over
that.
A smal percentage of treated cats become hypothyroid
afterwards—that is, require thyroid hormone supplements,
but that’s rare, says Dr. Hager. Because most treated cats
are older, he advises
they be monitored for kidney function.
“This is going to be the best thing for the overal health of
your kitty in the long run. People are extremely happy. The
kitties go home and they live a good quality of life.”
Bottom Line
Cost of the scintigraphy varies across the country.
“With us it’s between $200 to $250,” says Dr. Hager.
Radioactive iodine treatment also varies from place to
place. Dr. Hager estimates on average it costs
between $600 to $700 for the iodine treatment alone.
With the scintigraphy included, the treatment costs
$1,000 to $1,100.
Medication at first look wil seem much less expensive.
However, daily Tapazole for the lifetime of the cat also
requires the cost of regular blood tests to monitor the
thyroid levels and make sure the liver isn’t damaged.
Compounded medication wil be easier on the cat—
you’l avoid the trauma of twice-daily pil ing—but wil be
more expensive. Price varies from practice to practice.
Golden Moments: Blanche’s Thirst for
Life
Eight years ago, when she lived in Jersey City, New
Jersey, Karen Al ison adopted a pair of three-year-old cats
that needed a new home. One was a petite Siamese she
named Stel a. “The other was a very chubby Russian Blue
with incredible green eyes,” says Karen. She named the
unusual blue 14-pound cat Blanche. “Russian Blues are
usual y very svelte. But not Miss Blanche.”
Blanche quickly became accustomed to the house and
loved her life. She got along wel with Stel a, and remained
healthy and retained her portly figure and outgoing
personality. Blanche was a bit upset when another feline
interloper, Stanley, joined the family. Now at the mature age
of 12 years old, Blanche is best friends with Stanley.
Last year, Karen noticed that the blue cat was drinking
more water. Although there was no increase in urination,
Blanche had also lost weight. “That concerned me, and I
thought it must be time for the first geriatric blood screen
anyway,” says Karen. She took the cat to see Dr. Cindy
Bressler at Symphony Veterinary Center in the upper west
side of New York City, and had a blood test run. “The vet got
back to me and said yes, something’s wrong. She’s
hyperthyroid.
Karen was concerned, but immediately knew what
was involved. She’d learned about hyperthyroidism by