Hyperthyroidism
Hyperthyroidism:
Hyperthyroidism
(also called thyrotoxicosis) is one of the most common diseases of the middle-aged
and older cat. It is a multi-system disorder caused by an increase in the amount
of thyroid hormones (T3 and T4) produced by an enlarged thyroid gland. It was
first documented in cats almost 30 years ago but the cause of the disease has
been elusive. Although the enlargement in the thyroid gland is caused by a tumour,
called an adenoma, it is non-cancerous. The
most common clinical signs of hyperthyroidism in cats include weight loss, increased
appetite (although some patients have decreased appetite), vomiting, increased
thirst and urination, hyperactivity, and diarrhea. The high levels of thyroid
hormones can cause the development of heart disease, and these patients may have
a heart murmur, difficulty breathing, high heart rate and arrhythmias. Once hyperthyroidism
has been confirmed, there are several treatment options. They include treatment
with radioactive iodine, surgical removal of the gland, and treatment with antithyroid
medications.
What
is hyperthyroidism?
Hyperthyroidism
is the most common hormonal disorder of cats. It’s rarely seen in cats under
eight years of age, and there is no sex or breed that is more commonly affected.
It’s due to an increase in production and secretion of thyroid hormone by
the thyroid glands in the neck as a result of the development of a small localised
tumour. This is invariably due to a "benign" tumour (i.e one which is
unlikely to spread elsewhere in the body) but rarely may be caused by a cancerous
growth. What
causes hyperthyroidism? We
don't yet understand why we see cats developing this disorder. We think it is
likely to be multifactorial and includes factors such as genetics, immune systems,
diet, environment and infectious agents. The Siamese and Himalayan breed have
a reduced risk of developing the disease.
What are the signs of hyperthyroidism? Cats
may present with a combination of the following signs which tend to develop gradually:
1. Weight loss.
2. Increased appetite.
3. Hyperactivity and restlessness.
4. Moderate elevation of body temperature.
5. Increased heart rate and
possible signs of heart muscle disease.
6. Increased frequency of bowel movements,
with abundant, bulky stools.
7. Increased thirst and urination.
8. Occasional
vomiting.
9. Panting.
10. Matted, greasy and unkempt coat.
How is hyperthyroidism
diagnosed?
In hyperthyroidism an enlarged thyroid gland may be felt either
side of the neck. As the enlarged gland may be freely movable and can slide along
and behind the windpipe, it may be difficult to detect and requires careful examination
by experienced hands. In the normal cat the thyroid glands are not usually detectable.
Once hyperthyroidism is suspected on the basis of any signs, the diagnosis is
confirmed by a blood test which detects elevated thyroid hormone levels. Other
laboratory tests may also be abnormal (e.g. liver enzymes, blood glucose). On
occasions more detailed thyroid tests may be required. How
can hyperthyroidism be treated? There
are three options for the treatment of hyperthyroidism. Which treatment option
is most suitable for your cat depends on a number of factors and your vet can
discuss this with you.
1. Anti-thyroid drug therapy
Anti-thyroid
tablets are readily available and economical. They don’t destroy the abnormal
thyroid gland, but act by interfering with the production and secretion of thyroid
hormone. Their use doesn’t cure, but rather controls the condition. These
drugs are given initially three times daily and may be reduced to twice daily
once the thyroid hormone levels have returned to normal. Mild and usually transient
side effects can be seen in cats on this medication (~15% of patients) and can
include inappetance, vomiting and lethargy. More serious side effects are very
uncommon and can include a fall in the number of white blood cells, clotting problems,
or a liver disorder. Regular check ups of patients on anti-thyroid tablets is
advisable to ensure they are progressing well.
2.Surgery
Tablets
have to be given for the rest of a cat’s life to control the excessive thyroid
hormone production. This may be an acceptable option if we are treating a very
elderly individual but if a cat is just into double figures we may have several
years of medication ahead. If this prospect is too daunting, we can consider surgical
removal of the gland. Anaesthesia can be a problem in hyperthyroid patients, both
as a direct result of the condition being treated and because elderly patients
may have other diseases e.g. long term kidney problems. To reduce the surgical
risks, patients should be treated with anti-thyroid drugs for 3 to 4 weeks prior
to surgery to reduce their thyroid hormone levels back to normal. Any associated
heart disease should be carefully controlled.
Surgery to the thyroid glands
is quite delicate and we have to be aware of the possibility of complications.
Recognised side effects of the surgical procedure may include nerve damage or
damage to the parathyroid glands whose hormones control the level of calcium in
the blood. The resultant low blood calcium level can cause muscle twitching, weakness
and seizures. Affected cats may need temporary or permanent treatment to correct
the calcium function.
If both thyroid glands have to be removed, the cat may
develop an underactive thyroid and will need lifelong tablets to correct this
problem. Strangely this is quite a rare event. If only one gland is obviously
affected and removed there is a chance that the other gland will become overactive
at some point in the future so further surgery may be required. 3.
Radioactive iodine therapy.
A carefully calibrated injection of radioactive
iodine is administered under the skin. The iodine selectively destroys the abnormal
thyroid tissue, including any areas of thyroid tissue which may be inaccessible
to surgery. The primary advantages of iodine treatment are that it is curative,
has no serious side-effects, doesn't require an anaesthetic or sedative, is associated
with a low recurrence of hyperthyroidism and the location of the tumour is unimportant.
The cost of treatment is comparable to surgical treatment.
The disadvantages
of iodine treatment include:
a)
POOR AVAILABILITY-due to safety regulations that cover the use of radioactive
products. Glasgow University Veterinary School is the only site in Scotland which
can offer this facility.
b) HOSPITALISATION- if an animal has been treated
with a radioactive substance it has to be caged in a specialised unit and all
urine and faeces collected. Human contact with the cat is minimised and staff
wear protective lead clothing. It's not possible to visit your cat while this
is carried out. The typical length of stay is around a month.
c) PATIENT SELECTION.
It's not an advisable treatment for patients who may have other geriatric problems
because excessive handling of the cat must be avoided.
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