Hyperthyroidism in Cats
What is hyperthyroidism?
A cat has two thyroid glands which lie either side of the windpipe in the neck. These
can become overactive, producing too much thyroid hormone. Usually thyroids
become overactive due to non-cancerous changes. This often affects both thyroid
glands, although one may be more severely affected. Rarely, changes can be caused
by a cancerous growth. This can make treatment more complicated.
How is hyperthyroidism diagnosed?
A cat may be suspected of having hyperthyroidism based on the clinical signs it is
showing. An enlarged thyroid gland is often palpable in the neck region. A blood
sample is needed to make a definitive diagnosis. This checks levels of circulating
thyroid hormones.
Clinical Signs
Thyroid hormones have an important role in controlling the body's metabolic rate.
Usually affected cats are middle to old age. Cats with hyperthyroidism tend to burn up
energy too rapidly and typically suffer weight loss despite having an increased
appetite and increased food intake. Sometimes cats will show increased thirst,
increased irritability, and restlessness. Occasionally cats may vomit or have diarrhoea.
The increased levels of thyroid hormones cause the heart to beat faster. Over time the
muscle of the heart can thicken. If left untreated, these changes can lead to heart
failure. Once the underlying hyperthyroidism is controlled, the heart changes will
often improve or resolve. Occasionally additional treatment may be required to
control secondary heart disease.
High blood pressure is another potential consequence of hyperthyroidism. This can
lead to organ damage including retinal damage (which may lead to blindness), kidney
damage, and damage to the heart or to the brain. Drugs may be needed to control
blood pressure, but high blood pressure will often resolve with successful treatment of
hyperthyroidism.
Possible complications
As mainly older cats are affected, it is not uncommon for hyperthyroid cats to have
kidney disease as well. Hyperthyroidism tends to increase the blood supply to the
kidneys, which may improve their function. Treating a hyperthyroid cat may unmask
an underlying kidney problem. Because of this it is important that kidney function is
monitored at the same time as thyroid levels. Occasionally, hyperthyroid treatment
results in a decline in kidney function. If this is the case it may be necessary to reduce
the dose of therapy so that the hyperthyroidism is not fully controlled but kidney
function is not too severely compromised.
Treatment options
MedicalAnti-thyroid treatment can be given in tablet form. These control thyroid levels but
are not a cure. There are currently two of these drugs on the market:
Felimazole- recommended to be given twice daily
Vidalta- recommended to be given once daily
Both tablets must not be crushed. Tablets will normally bring thyroid levels down into
the normal range within about 3 weeks. Blood tests are needed to check that the
dosage is correct and treatment is lifelong.
Side effects are uncommon. Occasionally treatment causes poor appetite, vomiting
and lethargy. This generally resolves if the dosage of treatment is reduced or
withdrawn. Rarely more serious side effects can occur, including anaemia, decreased
white blood cell production, decreased platelet production, liver failure or skin
irritation. If these occur, treatment must be withdrawn immediately.
Blood tests are recommended to check thyroid levels, liver and kidney function and
haematology (red blood cell count, white blood cell count and platelet levels) at
regular intervals. These are quite frequent during the stabilisation period as this is
when most side effects will be seen. The current recommendations (as advised by the
drug manufacturers) are that cats on Felimazole have blood samples taken before
treatment starts then at 3 weeks, 6 weeks, 10 weeks, 20 weeks and thereafter every 3
months. For cats on Vidalta it is recommended that bloods are taken before treatment
starts then at 3 weeks, 5 weeks, 8 weeks and thereafter every 3 months.
SurgicalIt is possible to surgically remove the affected thyroid gland. This is normally done
after initial stabilisation on medical treatment for 3 or 4 weeks. Sometimes both
glands are affected. If this is the case one gland is normally removed first with the
second gland being removed 6-8 weeks later. Sometimes a cat can remain
hyperthyroid even when both glands have been removed. This is because occasionally
thyroid tissue can be found elsewhere in the body (usually inside the chest).
A possible complication of surgery is damage to the parathyroid glands, which sit in
the capsule surrounding the thyroid. These are important for regulating calcium levels
in the body. To minimise the risk of damaging the parathyroid glands, the surgeon
will try to preserve the thyroid capsule where the parathyroid gland sits. Occasionally
this can mean some thyroid tissue is left behind, which can eventually regrow (over a
couple of years). Damage to the parathyroid gland is less serious if only one gland is
operated on, as the gland on the other side can compensate. If both sides are operated
on and both parathyroid glands damaged, there is a risk of low blood calcium. This
can be life threatening and may require lifelong supplementation with calcium and
vitamin D.
Radioactive iodine therapy
Radioactive iodine is an effective cure for hyperthyroidism whatever the location of
the overactive thyroid tissue. A 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).
Radioactive iodine is administered as a single injection given under the skin. The
radiation destroys the affected abnormal thyroid tissue, but does not damage the
surrounding tissues or the parathyroid glands. The radiation carries no significant risk
for the patient, but 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 in isolation
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 following treatment.