INTRODUCTION
Hyperthyroidism is the clinical syndrome that exists when
there is overproduction of circulating thyroid hormone. Feline hyperthyroidism is the
single most common endocrine disorder of cats, which are the only non-human species in
which spontaneous thyrotoxicosis develops. It has been reported that up to 31% of all cats
have thyroid adenomas on post-mortem examination. Some veterinarians believe that the true
incidence of this disease has increased recently. Others believe that veterinarians
realize the prevalence of the disease and are thus diagnosing it more often. The cause of
hyperthyroidism in cats is unknown, and it is the subject of heated ongoing research.
Heredity, environmental concerns, and diet have all been suggested as possible etiologies,
however nothing has been proven.
The hypermetabolic
state of chronically increased levels of circulating thyroid
hormone results in deleterious effects on multiple organ
systems. Eighty-seven percent (87%) of hyperthyroid cats will
have some degree of hypertrophic cardiomyopathy, although it is
rare for a cat to presentwith overt heart failure.
Most cats will have increased heart rates (>220
bpm) and a significant proportion will
have a gallop rhythm due to turbulent blood flow
in the heart. Thromboemholic disease ("saddle
thrombi") is also associated with the cardiovascular
disease. The majority of cats will have abnormalities on routine
chemistry panels reflecting damage to the liver, kidneys and
muscle. Ninety-eight percent (98%) of all affected cats have benign
disease (flinctional thyroid adenomatous hyperplasia), and as a
result have a good to excellent prognosis. Of the cats with
benign disease, 2/3
of them will have bilateral disease (both thyroid glands
affected) and 1/3 will have only one hyperfunctional gland.
Benign hyperthyroidism has been reported in cats as young as 6
years and as old as 22 years (average age is 14 years). There is
no sex or breed predilection. Only a very small percentage
(<2%) of all hyperthyroid cats have a malignant tumor, with
mixed, compact and follicular adenocarcinomas being the most
common. The few cats that have a thyroid adenocarcinoma carry a
poorer long-term prognosis.
CLASSIC (AND NOT SO CLASSIC) CLINICAL SIGNS
OF BENIGN HYPERTHYROIDISM
The classic clinical signs of feline hyperthyroidism are a
result of the multisystemic effects of increased levels of
circulating thyroid hormone. The most commonly seen
manifestations of the disease are significant weight loss, loss
of the normal hair coat luster and patchy hair loss, normal to
increased appetite, irritability and/or restlessness. It is
important to remember that up to 30% of hyperthyroid cats will
actually be anorexic (the so called "apathetic"
hyperthyroid cat). Many cats will have palpably enlarged thyroid
glands, however many cats with very large glands will have
glands that cannot be easily palpated. This is caused by caudal
displacement of the glands due to the effects of gravity, as
they are commonly located in the thoracic inlet or thorax.
Although most cats exhibit many of the clinical signs, it is
rare for any individual cat to present with all of the clinical
signs. Many cats will also demonstrate intermittent
vomiting and diarrhea as well as increased water consumption and
urination.
DIAGNOSIS OF HYPERTHYROIDISM
While most cats with hyperthyroidism are diagnosed based on
clinical signs and increased T3 or T4 values, up to 42% of cats
that are proven to be hyperthyroid will have T3 and/or T4 values
that are in the normal range. It has been shown that circulating
levels of thyroid hormone can fluctuate up to 24% during any 24
hour period and up to 28% over a 2 week period. Thus if a cat is
mildly hyperthyroid, a single T4 assay may fall within the
normal range at any time. Also, the circulating thyroid levels
may be falsely decreased secondary to many other concurrent
disease processes (the "euthyroid sick syndrome").
Therefore, the diagnosis of feline hyperthyroidism
should not be excluded based on a single normal serum T3 or T4 value.
diagnosis of feline hyperthyroidism should not be
excluded based on a single normal serum T3 or
T4 value.
A diagnostic thyroid scan is the most sensitive and specific
method to diagnose feline hyperthyroidism. The scan is easily
performed, inexpensive and quick. To perform a scan, the cat is
injected with a very small amount of a radioactive solution
called technetium (99mTc). This radionuclide is an
iodine analog and localizes in glandular tissue (thyroid glands,
salivary glands, and glands that line the stomach mucosa), thus
allowing the thyroid glands to be imaged. In normal euthyroid
cats, the amount of uptake in the thyroid glands is equal to
that of the salivary glands. In hyperthyroid cats, there is much
more uptake in the thyroids when compared to that of the
salivary glands. When properly performed, there is little to no
chance of a misdiagnosis. Besides confirming the diagnosis of
hyperthyroidism, imaging of the thyroid glands allows us to
evaluate the size, location, shape, and pattern of uptake for
each thyroid gland, which is highly predictive of benign disease
versus that of a malignant thyroid tumor. This is important
because circulating thyroid hormone levels cannot differentiate
benign versus malignant disease and treatment options and
prognosis are drastically different for benign adenomas versus a
malignant adenocarcinoma.
TREATMENT OF BENIGN FELINE HYPERTHYROIDISM
Once the diagnosis of benign feline
hyperthyroidism is made, there are essentially three treatment
options, each of which has advantages and disadvantages. The
three options are medical management using anti-thyroid drugs,
surgical removal of the thyroid gland(s) and radioactive iodine.
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The most common medication used to treat feline
hyperthyroidism is tapazole. The advantage to tapazole is the drug itself
is inexpensive. However) this can be misleading due to the number of
recheck appointments and serial thyroid hormone assays needed
to assure the patient is being dosed properly. Tapazole does have several
disadvantages, the most important of which is that it is not a cure for
the disease, but merely a treatment. Because of this, the drug must be
administered at least once a day (many
times more often) for the remainder of the
cat's life. The usual result is that the cat and owner become enemies,
with the cat running the opposite way when the owner comes near it.
Tapazole also has several undesirable side effects that, although not seen
in every case, are all too common. These include vomiting, diarrhea, blood
dyscrasias, and hepatotoxicity. As stated in the Physicians Desk Reference
(PDR) and the package insert, "tapazole is intended to be used to
ameliorate hyperthyroidism in preparation for a more definitive treatment"
and as such tapazole is not intended to be used as a long term treatment.
The second option for the treatment of hyperthyroidism
is the surgical removal of the thyroid gland(s). Sub-capsular
thyroidectomy has the advantage of being curative, with low rates of
recurrence, however there are several disadvantages of thyroidectomy. Once
the ventral neck incision is made, a decision must be made to take out
only one gland (usually the largest) or both glands. This decision is
complicated by the fact that many hyperfunctional thyroid glands are not
physically enlarged, thus many hyperfunctional, normally sized glands will
not be removed, and the cat will still be hyperthyroid, requiring a second
surgery. Also, if both glands are removed, there is increased risk of
accidentally removing the small para-thyroid glands (causing permanent
hypocalcemia), or in the least damaging their blood supply (causing
transient hypocalcemia). Also, if all thyroid tissue is removed, the owner
is back to giving at least one pill per day for the remainder of
the cat's life, this time thyroid supplementation. Another disadvantage to
surgery is that it is not uncommon for an animal to have functional
extrathyroidal tissue in abnormal locations (most commonly in the thorax).
Obviously, this tissue would be missed from a ventral incision in the
neck. Lastly, in the largest study done to date on the "Surgical Treatment
of Feline Hyperthyroidism" involving 85 cases, 9% of cats died in the
pen-operative period due to complications.
The third option for the treatment of hyperthyroidism is
the administration of radioactive iodine. The advantages of radioiodine
cure are many. The overall success rate is 96% following a single
injection of iodine- 131. Recurrence rate is extremely low. The return to
a euthyroid state is rapid, as circulating thyroid hormone levels drop
precipitously within 48 hours post-administration, and there are
essentially no side effects. Not only does the radioactive iodine only
localize in thyroid tissue, it only destroys hyperfunctional thyroid
tissue (wherever it is). Normal thyroid cells are suppressed through
negative feedback loops, and
as such do not concentrate any of the iodine. These spared normal thyroid
cells then can "turn back on,' and make normal amounts of thyroid hormone
following treatment. In this way, cats treated with radioactive iodine
typically do not need any medication and are euthyroid. Because the
killing effects (beta particles) that are released from the radioiodine
travel such a short distance, there is no effect on the adjacent para-thyroid
glands and post-therapy hypocalcemia is not seen. The only disadvantage to
the use of radioiodine is that the cat must be hospitalized in a special
facility for about 4-5 days, while the animal is excreting the
radioactivity.
The typical hyperthyroid cat treated with radioactive
iodine
will
gain significant weight during the short hospitalization period, have a
good appetite and
do well. Continued weight gain over the next
2-3 weeks is expected
and averages 3/4 pound (or about 15-20% of body weight).
TREATMENT OF MALIGNANT FELINE THYROID
CARCINOMAS
Malignant
thyroid adenocarcinomas are rarely diagnosed in cats, occurring in
less than 1 - 2% of all hyperthyroid cats. Cats with malignant
thyroid tumors will
have identical clinical signs as do cats with benign
hyperthyroidism. In addition, T3 and T4 levels do not correlate
with malignancy; and therefore cannot be used to differentiate
malignant versus benign tumors. Although excisional thyroidectomy
followed by histopathologic analysis is
the confirmatory method for malignancy;
imaging of the thyroid glands is often highly predictive of
malignancy. On a thyroid scan, malignant
adenocarcinomas will often
times be found in abnormal locations and have very abnormal shapes, sizes
and focal areas of increased or decreased uptake ("hot" or "cold" spots).
When
this is
seen, surgical removal of the questionable
thyroid tissue is performed, followed by histopathologic
confirmation. Once confirmed, "high dose" radioactive
iodine therapy is typically recommended for possible metastatic
disease. Long term prognosis in cats with malignant thyroid tumors
is dependent on the size, degree of invasiveness, and metastatic
potential of the primary tumor, but can range from weeks to over 3
years (average of about 1 year).
