Hyperthyroidism in Cats
The number of cases of hyperthyroidism
in cats has been steadily increasing over the years, it occurs
when there is an excess of thyroxine (T4) and tri-iodothyronine
(T3) produced by the thyroid gland. Hypothyroidism is when there
is a deficiency of these hormones. The thyroid gland controls
metabolism. This is a very common endocrine disorder in middle
age-older cats but is rarely seen in dogs. There seems to be no
sex disposition, however Siamese and Himalayan breeds appear less
likely to develop the disease.
Hyperthyroidism is most commonly
caused by small benign tumour in either one or both of the thyroid
glands, the changes are benign but the reason behind it is still
unknown. Rarely, a thyroid carcinoma may be the cause. If left
untreated, hyperthyroidism can result in changes in metabolic
rates and may cause cardiac abnormalities.
It is important to know that many
other diseases of older cats can mimic the symptoms of hyperthyroidism
e.g. Diabetes mellitus and IBD.
IODINE LEVELS: At
the North American Veterinary Conference 2005 Dr. P. Schenck showed
studies which suggested that cats fed on a canned food were more
at risk from hyperthyroidism, this may because of substances in
the lining of the can or because of iodine levels. Iodine deficiency
in cats can cause the thyroid glands to be heavier and larger,
cats can be fed on very high or very low iodine levels and can
adapt quite well. However canned foods have a much larger range
of iodine levels than dried foods and the theory is that variability
in the levels of iodine intake by constantly switching brands
of canned foods (including flavours in the same brand) can overwhelm
the system and induce hyperthyroidism.
Other goitrogens (agents that cause
thyroid enlargement) include: sorghum, Soya beans, ascorbic acid
and copper sulphate.
There are no detailed studies to
determine what level of iodine is consistent with normal thyroid
function over a long period of time.
CAT LITTER: According
to David Bruyette speaking at the WSAVA 2001, researchers have
found a marked increase in hyperthyroidism in cats which use litter
trays. Unfortunately, they cannot explain this as yet. More research
is needed to determine whether the type of litter is an important
• Polydipsia (Excessive drinking)
In 10% of hyperthyroid cats:
• Polyuria (Excessive urination) - depression
• Polyphagia (Excessive appetite) - muscle weakness
• Weight loss - anorexia
• Hyperactivity This is known as ‘apathetic’
• Voluminous fatty faeces
• Heat intolerance
• Skin lesions, dry, greasy, matted coat, alopecia
Heart abnormalities are often present
Hyperthyroidism often masks concurrent renal failure
Liver enzymes are often raised in hyperthyroid cats
Proteinuria is often seen in hyperthyroid cats (may indicate renal
There are three main ways of treating
(1) Antithyroid drugs.
This treatment relies on the owner giving daily oral medication.
Some side-effects may occur (e.g. vomiting) but these are generally
seen only in the first few weeks of treatment. Symptoms will reoccur
if the treatment is stopped for more than 24-72 hours as this
is not a cure, it just controls the disease.
This involves removal of the thyroid glands. Both glands should
be removed, even if one looks normal (otherwise symptoms seem
to reoccur 6-12 months later). This is a very effective procedure
and is quite simple to perform. However, the operation itself
may be risky because of the use of anaesthetic with cardiac, renal,
hepatic and gastrointestinal problems that may be associated with
the hyperthyroidism. This surgery is most successful after the
cat has been stabilised with medication for a few months.
(3) Radioactive Iodine:
Radioiodine destroys the overactive thyroid cells without damaging
the normal ones. This is a very effective treatment but it is
expensive and not always widely available as special materials,
facilities and licences are needed.
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John Burns Pet Health