Overview?
The
thyroid gland secretes hormones to regulate many metabolic
processes, including growth, development and energy
expenditure. Hyperthyroidism
(also known as thyrotoxicosis) is caused by the
overactivity of the thyroid gland, which produces more than
the normal level of thyroid hormones T4 & T3, which are
released into the bloodstream. Most often,
hyperthyroidism is the result of a benign tumour (called an
adenoma) involving one or both of the thyroid lobes. Feline
hyperthyroidism is the most common disease of the endocrine
system.
What
is the endocrine system?
The
endocrine system is composed of glands which secrete hormones
into the bloodstream. Hormones regulate many body
processes.
What
does the thyroid do?
Consisting
of two lobes (left & right), the thyroid is located in the
neck on either side of the windpipe. It produces two hormones
T3 triiodothyronine & T4 thyroxine. Both hormones help
control metabolism.
What
are the symptoms of Hyperthyroidism in cats?
Some symptoms of
hyperthyroidism may include;
-
Weight loss
despite an increased appetite
-
Increased
thirst & urination
-
Vomiting
-
Diarrhea
-
Behavioural
changes (nervous/jittery behaviour,
aggression,
overgrooming & or bald patches, hyperactive,
increased vocalisation)
-
Rapid
heartbeat
-
Poor coat
-
Weakness
(occasionally)
-
Decreased
appetite (occasionally)
-
Hypertension
(high blood pressure)
Please note;
your cat may not display all of the symptoms listed above.
Any of the above symptoms warrants a visit to the
veterinarian.
What
effects does hyperthyroidism have on the cat?
Increased
levels of the thyroid hormones cause an increased heart rate.
Cats may also have a heart murmur. It can lead to heart
failure & kidney damage.
High blood pressure, which can
lead to blindness. It can be fatal if left untreated.
Secondary
hypertrophic cardiomyopathy can occur as a result of
hyperthyroidism.
How is
Feline Hyperthyroidism diagnosed?
Your
veterinarian will observe your cat's clinical signs, palpitate
the thyroid gland. Other diseases such as renal failure & feline
diabetes have similar symptoms, so it is necessary to take
routine & specific blood tests to detect elevated levels
of the hormones T3 & T4 are performed. Some cats with
hyperthyroidism may show normal levels of these hormones in
their blood test. If this is the case then a T3 suppression
test may be performed. This involves taking a blood test to
check the levels of T3 & T4, 7 oral doses of the thyroid
hormone T3 & a blood test after the hormone was given. In
a normal cat, the level of T4 will drop, in a cat with
hyperthyroidism the T4 levels will stay the same or increase
slightly.
Thyroid
imaging are another method. A small dose of a radionuclide
which concentrates in the thyroid tissue.
Are
some cats prone to it?
No
specific breeds are prone to Feline Hyperthyroidism, however is most often seen in older (8 years plus) cats.
There has been
talk of a possible link between the consumption of canned food
& hyperthyroidism in cats, but this doesn't appear to be the
only cause.
What
is the treatment of Feline Hyperthyroidism?
There
are three options to treat hyperthyroidism in cats. Each
treatment has it's pros & cons. Treatment should be
depends on other medical conditions your cat may have (heart
disease, kidney failure), availability of a nuclear medicine
facility & cost.
Drugs
(Methimazole/Tapazole):
The use of medication doesn't cure hyperthyroidism, only
controls it. Administered daily for the rest of your cat's
life. There may be side effects from the use of drugs
including vomiting & lethargy. Short term this is the
cheapest option, but in the long run it can prove costly
having to pay for daily medication for your cat. Methimazole
can be given either in oral or transdermal (applied to the
skin inside the ear) form. Advantages of medication are
that treatment is reversible if side effects occur (see
precaution at the end of this article). Other side effects may
include; anorexia & vomiting.
In some cats,
hyperthyroidism & kidney failure run concurrently. The
hyperthyroidism can act to 'protect' the kidneys. This is
because hyperthyroidism
produces a hyperdynamic cardiac state (increased blood flow),
which increases glomerular blood flow (GBF) and glomerular
filtration rate (GFR), improving renal function. Once
treatment begins & the hyperthyroidism is brought under
control, kidney failure may become worse. If kidney failure is
suspected, your veterinarian will recommend using drugs to
control the hyperthyroidism & closely monitor kidney function.
That way, if the kidneys do begin to deteriorate, medication
can be re-evaluated or stopped immediately, opposed to surgery
or radioactive iodine treatment which is permanent.
Surgery:
Removal of the enlarged thyroid lobe(s), known as
thyroidectomy. Pros of this option are that it is a permanent
cure. Cons include increased anaesthetic & surgical risks
due to the effect the disease may have had on the cat's heart
& kidneys. If both glands are removed, there is increased
risk of accidentally removing the small parathyroid glands,
(which among other things regulates the calcium supply in the
body), which can cause hypocalcemia (low blood calcium). If both
lobes are removed you will have to give your a daily thyroid supplementation.
Radioactive
Iodine Treatment: This
involves a single injection of radioactive iodine (radioiodine
I-131). This concentrates in
& destroys the diseased thyroid tissue, while leaving the
normal thyroid tissue intact. Pros of this option are that the
cat doesn't require anaesthesia, there is no need to give your
cat medication for the rest of it's life and in the majority
of cases it is a permanent cure. In a few cases treatment will
need to be repeated. No damage is done to the parathyroid
glands. Cons are that it is expensive in the short term, and
can only be performed at a specialist veterinary centre. After
treatment your cat will be slightly radioactive & will
have to stay at your veterinarian's until the radioactive
levels drop.
