cat is showing weight loss and poor coat - typical signs
Hyperthyroidism (overactive thyroid glands) is a very common disorder of older
cats. It is caused by an increase in production of thyroid
hormones from the thyroid glands, which are situated in the
neck. Clinical signs associated with hyperthyroidism can be
quite dramatic and cats can become seriously ill with this
condition. However, in most cases hyperthyroidism is treatable
and most cats will make a complete recovery.
hormones have an important role in controlling the body's
metabolic rate and thus the general activity level, so cats
with hyperthyroidism tend to burn up energy too rapidly and
typically suffer weight loss despite having an increased appetite
and increased food intake.
the vast majority of cases the increased thyroid hormone production
is due to a benign (non-cancerous) change. Both of the thyroid
glands are involved, although one gland may be more severely
affected than the other. The abnormal thyroid tissue becomes
enlarged, but the underlying cause of this change is currently
unknown. Cats usually respond extremely well to treatment,
and if the condition is recognised early and treated appropriately,
then the outlook for the affected cat is generally very good.
malignant (cancerous) tumour known as a thyroid adenocarcinoma
can also be an underlying cause of some cases of hyperthyroidism.
Fortunately this is rare, and is only the cause in around
one to two per cent of all hyperthyroid cats. When a thyroid
adenocarcinoma is present treatment is much more difficult.
is almost exclusively seen in middle- to old-aged cats, and
is rarely seen in cats less than seven years of age. Male
and female cats are affected with an equal frequency and although
no studies have shown any breeds that are particularly prone
to developing hyperthyroidism, there is some evidence that
it is less common in Siamese cats.
affected with hyperthyroidism usually develop a variety of
clinical signs, which are usually quite subtle at first, but
then become more severe as the disease progresses. Also, as
this disease occurs mostly in older cats, some affected cats
will have other diseases that can complicate and even mask
some of the clinical signs.
'classic' signs of hyperthyroidism are weight loss, usually
despite an increased appetite (polyphagia), increased thirst
(polydipsia), increased irritability, and restlessness or
even hyperactivity. Many affected cats have a rapid heart
rate (tachycardia) and develop an unkempt coat. Mild to moderate
diarrhoea and/or vomiting is also quite common. Some affected
cats will be noticeably intolerant of heat and seek out cooler
places to sit, and some (especially advanced cases) may pant
when they are stressed. Most hyperthyroid cats will show some
degree of polyphagia (excessive appetite) and restlessness,
but in some advanced cases there will be generalised weakness,
lethargy and loss of appetite and the signs will be less characteristic.
hormones have effects on virtually all the organs in the body,
and therefore it is not surprising that this disease can sometimes
cause secondary problems that may lead to the necessity for
additional investigations and treatment.
effect of thyroid hormones on the heart is to stimulate a
faster heart rate (more rapid beating of the heart) and a
stronger contraction of the heart muscle. Over time, with
hyperthyroidism the muscle of the largest chamber in the heart
(the left ventricle) enlarges and thickens – so called ‘left
ventricular hypertrophy'. If left untreated and unmanaged,
these changes will eventually compromise the normal function
of the heart and can even result in heart failure. This means
that in some cats with hyperthyroidism, additional treatment
may be required to control secondary heart disease. However,
once the underlying hyperthyroidism has been controlled, the
cardiac changes will often improve, or may even resolve completely.
(high blood pressure) is another potential complication of
hyperthyroidism and can cause additional damage to several
organs including the eyes, kidneys, heart and brain. If hypertension
is diagnosed along with hyperthyroidism, drugs will be needed
to control the blood pressure to reduce the risk of damaging
other organs. As with heart disease, following successful
treatment of the hyperthyroidism, the high blood pressure
will sometimes resolve and permanent therapy may not, therefore,
disease (chronic renal failure) does not occur as a direct
effect of hyperthyroidism, but the two diseases often occur
together because they are both common in older cats. Care
is needed where both these conditions are present, as the
hyperthyroidism tends to increase the blood supply to the
kidneys, which may improve their function. Thus blood tests
taken to assess kidney function in a hyperthyroid cat may
show normal or only mild changes, but potentially more severe
renal failure may be masked by the presence of the hyperthyroidism.
For this reason, irrespective of what treatment is chosen
for long-term management of the hyperthyroidism (see below),
it is usually advisable to start on medical treatment (tablets)
initially and to monitor the response with repeat blood and
urine tests to look at thyroid function and kidney function.
Just occasionally, successful treatment of the hyperthyroidism
results in a dramatic decline in kidney function. If this
is detected it may be necessary to reduce the dose of therapy
so that the hyperthyroidism is not fully controlled but renal
function is not too severely compromised.
with large visible swelling in its neck
you or your veterinary surgeon suspects hyperthyroidism, a
thorough physical examination and some blood tests will be
required to be performed by your vet to confirm the diagnosis.
On examination, one or two enlarged thyroid glands can often
be palpated (felt) as a small, firm mass in the neck (these
are often about the size of a pea or a baked-bean in hyperthyroid
cats). However, in some cats there is no palpable thyroid
enlargement, and this can be because the overactive tissue
is present in an unusual (ectopic) site (often within the
diagnosis is confirmed by determination of thyroid hormones
in the blood. A blood test looking at thyroxine (T4) concentration
is usually all that is required for the diagnosis as this
is usually elevated in clinical cases. Other laboratory tests
may also be abnormal – for example liver enzymes are commonly
increased secondary to hyperthyroidism, and assessment of
routine blood and urine tests is usually advised to help rule
out any other concurrent disease (such as renal failure).
Where possible, blood pressure should also be checked with
hyperthyroid cats, and if secondary heart disease is suspected
then an electrocardiogram (ECG – electrical tracing of heart
activity), and a chest X-ray or ultrasound may be helpful.
occasional cases, hyperthyroidism may be strongly suspected
on the basis of the clinical signs, but blood testing may
reveal a normal thyroid hormone (T4) concentration. There
are a number of potential reasons for this and usually on
a repeat test it will be elevated. If not, additional tests
may need to be undertaken to confirm or rule out hyperthyroidism.
scan of a cat showing bilateral hyperthyroidism
scanning is a technique that is available at a few specialist
centres and can be useful in the investigation of some hyperthyroid
cats. This technique can be used both to diagnose hyperthyroidism
and also to locate exactly where the abnormal tissue is. This
can be particularly helpful if surgery is being considered
but there is not a clearly identifiable enlarged thyroid on
clinical examination. It is also important as some cats have 'ectopic' overactive thyroid tissue located in the chest, and in these cases therefore surgical removal of the thyroid tissue in the neck would not cure the hyperthyroidism. With this technique a very small dose
of a radioactive chemical (technetium) is injected into the
cat's vein. The technetium is selectively taken up by abnormal
thyroid tissue, and this can be detected using a special camera
(a ‘gamma camera'). This is a simple, safe and easy procedure
that may be recommended in some situations.
are three main options for the treatment of hyperthyroidism,
each with advantages and disadvantages:
management (drug therapy)
drugs are available in tablet form and these act by reducing
the production and release of thyroid hormone from the thyroid
gland. They do not provide a cure for the condition, but they
do allow either short-term or long-term control of hyperthyroidism. Two options are available:
- Methimazole (Felimazole; Dechra) a twice daily medication, although once stabilised some cats only require once daily.
- Carbimazole (Vidalta; Intervet Schering-Plough) a slow release formulation meaning that it only needs to be given once daily. The tablets must not be broken or crushed.
Thyroid hormone concentrations
usually fall to within the reference range within 3 weeks.
Treatment is then adjusted
according to response. To maintain control of hyperthyroidism,
treatment needs to be given daily for the rest of the cat's
most cats methimazole and carbimazole are safe and effective treatments for hyperthyroidism. Side effects
are uncommon and if they do occur they are usually mild and
reversible. Poor appetite, vomiting and lethargy are the most
likely side effects and often resolve after the first few
weeks of treatment and/or by temporarily reducing the dose
of treatment and administering the tablets with food. More
serious problems, including reduced white blood cell counts,
reduced platelet counts (which help the blood to clot), liver
disorders, or skin irritation are rare, but if they do occur
then an alternative treatment must be used.
drug treatment has the advantage of being readily available
and economical, but it is not curative. Life long treatment,
usually involving twice daily oral dosage, will be required
and for some owners, and some cats, this may be difficult
to achieve. Routine blood tests should be checked periodically
during treatment to monitor the effectiveness of therapy,
to monitor kidney function (see above) and to look for potential
removal of the affected thyroid tissue (thyroidectomy) can
produce a permanent cure and is a common treatment for many
hyperthyroid cats. In general this is a very successful procedure
and is likely to produce a long-term cure or permanent cure
in most cats. However, surgery will not be successful if 'ectopic' thyroid tissue is present and even after successful surgery, occasionally
signs of hyperthyroidism develop again at a later time if
previously unaffected thyroid tissue becomes diseased.
reduce anaesthetic and surgical complications, where possible
it is always recommended that hyperthyroid patients are initially
stabilised with anti-thyroid drugs for three to four weeks
before surgery. Any associated heart disease must also be
treated where necessary. Good success and avoidance of postoperative
complications depend on good surgical skills, and experience
is necessary to achieve good results. The major risk associated
with surgery itself is inadvertent damage to the parathyroid
glands – these are small glands that lie close to, or within,
the thyroid glands themselves, and have a crucial role in
maintaining stable blood calcium levels. Damage to these glands
can result in a life-threatening fall in blood calcium concentrations
(hypocalcaemia). This is most likely to occur when both thyroid
glands are removed at the same time, since this can result
in damage to both parathyroid glands. To minimise the risk
of this complication in those cats that require removal of
both thyroid glands it may be appropriate to perform the procedure
in two stages, removing the most affected gland first and
allowing six to eight weeks for recovery of parathyroid hormone
production before removing the second thyroid gland.
is usually recommended that cats remain hospitalised for a
few days after surgery so that blood calcium concentrations
can be monitored and any treatment given, if required. Clinical
signs of low blood calcium include muscle twitches and weakness,
which can progress rapidly to convulsive seizures. Treatment
is with supplemental calcium by injection and then by mouth.
Additional treatment with vitamin D3 is also needed to allow
the calcium given by mouth to be used effectively. Once stable,
continued treatment can be given at home, but in most cats
the damage to the parathyroid glands is only temporary and
treatment may only be needed for a few days or weeks.
iodine (I 131) is a very safe and effective cure for hyperthyroidism wherever the location of the overactive thyroid tissue. It has the advantage of being curative in most
cases with no ongoing treatment required.
iodine is administered as a single injection given under the skin
– the iodine is then taken up by the active (abnormal) thyroid
tissue, but not by any other body tissues, resulting in a
selective local accumulation of radioactive material in the
abnormal tissues. The radiation destroys the affected abnormal
thyroid tissue, but does not damage the surrounding tissues
or the parathyroid glands.
advantages of radioactive iodine are that it is curative,
has no serious side-effects, does not require an anaesthetic
and is effective in treating all affected thyroid tissue at
one time, regardless of the location of the tissue. However,
it does involve the handling and injection of a radioactive
substance. This carries no significant risk for the patient,
but precautionary protective measures are required for people
who come into close contact with the cat. For this reason,
the treatment can only be carried out in certain specially
licensed facilities and a treated cat has to remain hospitalised
until the radiation level has fallen to within acceptable
limits. This usually means that the cat must be hospitalised
for between three and six weeks (depending on the facility)
following treatment. Most treated cats have normal thyroid
hormone concentrations restored within three weeks of the
treatment, although in some it can take longer.
single injection of radioactive iodine is curative in around
95 per cent of all hyperthyroid cases, and in the few cats
where hyperthyroidism persists the treatment can be repeated.
Occasionally a permanent reduction in thyroid hormone levels
(hypothyroidism) occurs following radioactive iodine treatment,
and if this is accompanied by clinical signs (lethargy, obesity,
poor haircoat) then thyroid hormone supplementation may be
required (in the form of tablets).
The licensed facilities currently available in the UK are at the Animal Health Trust near Newmarket, the University Veterinary Schools at Bristol, Edinburgh, London and Glasgow, the Barton Veterinary Hospital in Canterbury, and Bishopston Veterinary Group in Ripon, North Yorks.
of thyroid adenocarcinoma
rare cases of thyroid adenocarcinoma (malignant tumour) are
more difficult to treat but can be successfully treated using very high doses of
radioactive iodine. However, currently only certain centres such as the University of Bristol and the University of Glasgow are licensed to administer this high dose.