Download Feline Hyperthyroidism - Donnington Grove Veterinary Group

24-­‐hour Emergency Service 01635 47170 Feline Hyperthyroidism
What is it?
Hyperthyroidism is over-production of thyroid hormone from the thyroid glands in the neck
caused by a tumour of one or both of the thyroid glands on either side of the neck. These
tumours are often quite small and nearly always benign (non-cancerous) but produce
thyroid hormone in an unregulated and excessive manner.
Thyroid hormone is a substance in the blood which is involved in the regulation of the body
metabolic rate. Higher levels lead to faster metabolism, a bit like pushing the accelerator in
a car makes the engine run faster.
What are the signs?
This acceleration in body metabolism results in a number of signs. Not all cats will display
all the signs:
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Increased appetite
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Increased drinking and urination
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Weight loss
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Increased heart rate
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Intermittent vomiting and/or diarrhoea
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Palpable thyroid tumour
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Rather busy, restless nature
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Increased vocalisation (meowing)
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Poor coat quality
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May seek cooler places to sleep rather than seeking warmth
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In advanced cases: lethargy, weakness and poor appetite.
What will occur if the condition is not treated?
If the condition is left untreated the cat will eventually become emaciated and weak from
weight loss. In addition, the heart is over-stimulated causing a very high heart rate and
thickening of the heart muscle, making it work less efficiently. These heart changes occur
fairly early in the disease but eventually become irreversible. Heart failure is the final
cause of death in many cases. The disease course is often long, taking months or even
more than a year to lead to death, but is quite debilitating once it is advanced.
Nearly all cases occur in cats over 10 years of age. Unfortunately many people attribute
the changes that occur with this disease to old age and do not seek veterinary advice until
the condition is quite advanced. With treatment these cats can have many years of happy,
healthy life ahead of them.
How is diagnosis made?
If the clinical signs are sufficiently typical and the tumour can be felt, the diagnosis may be
made with a good degree of certainty on examination alone. A definite diagnosis requires
a blood sample to measure the levels of thyroid hormone in the blood. Such a blood
sample is also useful to check of other old age related problems such as kidney failure,
liver failure or diabetes.
How is hyperthyroidism treated?
There are two main options.
Medical treatment
There are two main drugs available – felimazole which is a twice daily treatment, and
Vidalta which is a once daily treatment. These drugs inhibit the release of thyroid hormone
from the tumourous gland and can reduce the levels in the blood to normal. This can be
used as a long term treatment in many cases, but requires that the cat will take that tablets
readily and tolerates them well. Side effects most commonly include vomiting, poor
appetite and/or lethargy. Rare side effects include itchiness of the skin, anaemia, jaundice
and suppression of the immune system. Most of these side effects are reversible by
stopping giving the drug.
Unfortunately, medical treatment does not eliminate the tumour, so they will be on tablets
for life and the dose required can increase as time goes by.
Surgical removal of the affected thyroid gland(s)
This is the best solution to the condition if the cat is well enough to take the anaesthetic.
Initially the patient is treated medically for 3-4 weeks to reverse some of the effects of the
disease (especially those on the heart) and thereby reduce the risks of the anaesthetic.
Once stabilised, a general anaesthetic is given and the affected gland removed. Both
glands are checked at the time of surgery to determine if only one or both are affected.
In most cases the surgery is very successful and no further medication is required. The
main side effect that can be seen is a problem with blood calcium levels. This is due to
effects on the parathyroid glands. There are two of these small glands on each side of the
neck attached to the thyroid gland. One sits directly on the side of the thyroid gland and
usually has to be removed with the thyroid gland. The other sits at one end and the aim is
to preserve this, but sometimes the gland is sufficiently abnormal that it cannot be
identified.
Problems with calcium levels will usually occur in the first week after surgery. They
manifest as inappetance, muscle tremors, twitching and in extreme cases, seizures (fits).
Prompt treatment is required but usually only requires calcium supplementation for a time
as the remaining parathyroid glands are able increase in size to restore sufficient function.
Problems with calcium are rare following removal of only one thyroid gland, but are
increased if both are removed at the same time. Consequently, if both thyroid glands are
affected it may be recommended that the removal of the second gland be delayed by
approximately one month to allow the parathyroid glands to recover between procedures.
Some surgeons, however, may remove both glands at once if they feel confident about
their preservation of parathyroid glands.
Following removal a proportion of cats may suffer a recurrence of disease at a later date.
In most cases this is due to the disease process affecting the other gland after the removal
of one gland and a repeat surgery on the other side is usually effective. Rarely re-growth
of tumour from a few cells left behind can occur, or even a tumour in a portion of thyroid
tissue at an abnormal site.