CPD ACCREDITED ARTICLE
or three times daily. The dose should initially be low
(especially when renal insufficiency is suspected), and
the renal values should be monitored as the dose is
gradually increased. When cat and owner compliance
is good, the successful response rate is approximately
85%.
Difficulty in administration causes many treatment failures. In an attempt to make medicating simpler, many
clinicians are now using topical transdermal applications of methimazole, and initial studies seem to show
promise. Advantages of medical therapy are numerous, including easy implementation, no requirements
for special facilities, good availability and a reasonable
cost to the client. Almost all cats are potential candidates and very few contra-indications, other than
presence of thyroid carcinoma, exist.
The main disadvantages of medical therapy in this
species, is the high degree of owner and patient compliance required and the rapid recurrence of signs if
compliance is lost. In addition adverse and serious
side effects occur in about 8-15% of cats treated with
carbimazole or methimazole, which may require
complete drug withdrawal. The most common is
vomiting, anorexia and depression. Blood dyscrasias,
facial excoriation, hepatotoxicity, renal decompensation, coagulation abnormalities, generalised lymphadenopathy and even acquired myasthenia gravis occur
less commonly. These side effects normally occur
within the first 3 months of treatment.
Other medical treatments available include the use of
iopanoic acid and beta blockers. The results of studies
evaluating the efficacy of cholecystographic agents
such as iopanoic acid, in the treatment of feline hyperthyroidism have been disappointing and the drug
is not recommended for long term management of
the disease but may be suitable for short -term management prior to surgery or radioiodine therapy. Beta
blockers such as propanolol and atenolol as well as
potassium iodate have also been used successfully in
pre-surgical stabilisation but longer term studies are
required to evaluate their use for long-term medical
management.
Control of hyperthyroidism by percutaneous injection
of ethanol into solitary thyroid nodules as well as ultrasound-guided thyroid radio-heat ablation has been
described in a small number of cats, but these techniques have been associated with serious side-effects
and are unlikely to be a viable option for the majority
of hyperthyroid cats.
Surgical Thyroidectomy
Surgical thyroidectomy is highly effective, almost always curative and widely available for cats, but the
procedure can be associated with significant morbidity and mortality. Hyperthyroid cats also represent
an anaesthetic risk due to the systemic effects of the
disease. The most common serious post-surgical
complications in cats include hypocalcaemia, due
to hypoparathyroidism when the parathyroid glands
are inadvertently removed, laryngeal paralysis, voice
changes and Horner’s syndrome. Treatment failure
can occur due to inappropriate unilateral thyroidectomy where there is bilateral disease, incomplete
removal of tissue or less commonly the presence of
hyper-functional ectopic thyroid tissue. This can be
avoided by performing thyriod scintingraphy pre-operatively.
Radioactive Iodine Treatment
Radioactive iodine treatment, the therapy of choice
in human toxic nodular goiter, is also successfully
used in the cat. It is simple, safe and effective and is
possibly the best curative treatment for most hyperthyroid cats. Permanent hypothyroidism is rare as is
recurrence after successful treatment and other side
effects are minimal. The main disadvantages of such
treatment are that it is potentially extremely hazardous to personnel, requires prolonged hospitalisation,
is costly and is only available in a limited number of
centres.
Iodine Restricted Diet
A novel option for treatment of feline hyperthyroidism now exists in the form of a commercially available
iodine restricted diet (Hills Y/D®). An iodine restricted
food was developed based on the hypothesis that
feline hyperthyroidism can be managed nutritionally
by limiting the amount of dietary iodine available for
production of thyroid hormones. Feeding an iodine
restricted food has been shown in multiple feeding
trials over the past 10 years, in over 100 cats with naturally occurring disease, to decrease thyroid hormone
concentration and alleviate clinical signs of hyperthyroidism.
Three additional studies documenting the safety and
efficacy of iodine restricted food as the sole therapy
of hyperthyroid cats showed that a food with iodine
levels at or below 0.32 ppm per dry matter basis was
able to convert 90% of the hyperthyroid cats in the
study to a euthyroid state within 8-12 weeks. In all
these studies biochemical features of renal function
remained stable and no other biochemical abnormalities were observed. The only drawback in using an
iodine restricted diet is that feeding compliance has
to be 100% for the diet to have an effect. Any exposure to products or foods containing iodine, such as
compounding agents for medications, treats, supplements, cleaning agents, other pet food, prey food
or home prepared meals or treats and even in some
cases municipal water, should be discontinued.
The diet has been shown not to have any adverse
effects if fed to healthy cats and as such, the maintenance of normal thyroid concentrations and lack
of clinical signs of hypothyroidism in these cats indicates that adequate amounts of iodine is provided to
healthy animals fed the diet.
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