CPD ACCREDITED ARTICLE
however the value of such imaging in the diagnosis of
hyperthyroidism remains undocumented. Although
both CT and ultrasonography can provide information regarding thyroid volume and morphological
information, including invasiveness and can guide
sampling, they are unable to provide functional information and may not identify ectopic thyroid tissue or
metastatic disease.
When investigating a cat for possible hyperthyroidism,
it is also important to consider all possible differential diagnoses and to look for evidence of multiple
interacting diseases. This is because hyperthyroidism
is seen most commonly in older cats and this group
of patients is often affected by more than one disorder. Diabetes mellitus, renal disease, malassimilation
syndromes (including inflammatory bowel disease,
early intestinal lymphosarcoma, pancreatitis, and exocrine pancreatic insufficiency), acromegaly, and hyperadrenocorticism are perhaps the most important
differential diagnoses.
Treatment
Spontaneous remission of hyperthyroidism has not
been reported and prevention is not possible as the
aetiology still remains elusive. Because of the benign
nature of the thyroid lesions, if treated appropriately,
hyperthyroidism carries a favourable prognosis. Failure to institute therapy will result in insidious progression of clinical signs to emaciation, severe metabolic
and cardiac dysfunction and ultimately death.
Treatment options include medical management with
anti-thyroid drugs, surgical thyroidectomy, thyroid ablation using radioactive iodine (the gold standard in
human medicine) or most recently, feeding an iodinerestricted diet. Each treatment option has its own advantages and disadvantages. Selecting one therapy
over the other depends on the age of the cat, severity
of thyrotoxicosis, presence of concurrent illnesses,
facilities available, potential complications, costs and
the owner’s lifestyle and willingness to accept the
form of treatment advised. There are a few studies
directly comparing the outcome of each of the treatment methods available.
assess the cat’s renal function (check blood urea nitrogen [BUN], creatinine concentrations, serum SDMA
levels and urine specific gravity). This is because resolution of the hyperthyroid state often is associated
with an increase in BUN and creatinine concentrations
and a decrease in glomerular filtration rate (GFR) and
effective renal blood flow. Because of this, some cats
without prior evidence of renal insufficiency or with
only mild renal impairment develop signs of uraemia
after treatment for hyperthyroidism. To determine
what effect resolving the hyperthyroid state may have
on renal function, a short course of medical therapy
is recommended before considering radiotherapy
or surgery. Cats which show significant azotaemia
should then be maintained on medical therapy and
should not be considered suitable candidates for radiotherapy or surgery. Similarly, cats which develop
significant azotaemia after radiotherapy or surgery
should be given L-thyroxine to maintain a euthyroid or
mild hyperthyroid state.
Interestingly in a recent study that compared the survival of cats that developed mild azotaemia (> 177
nmol/L) with cats that did not develop azotaemia
following medical therapy, no difference was found
between the two groups. These results suggest that
the importance of the mild azotaemia that commonly develops after treatment for hyperthyroidism may
have been overemphasized previously and does not
change long-term outcomes in these cats. The same
study however did find that in cats that were inadvertently over-treated and made hypothyroid, the development of azotaemia was of significance as it negatively affected prognosis.
Medical Management
Medical management entails the use of anti-thyroid
drugs. It is used in cats for long term control and is
advised prior to surgery to decrease the metabolic and
cardiac complications associated with anaesthetising
hyperthyroid cats and to control clinical signs in cats
awaiting radioactive iodine therapy.
One study of 167 hyperthyroid cats suggested that the
median survival time (MST) in cats treated with radioactive iodine was significantly longer (4 years) compared with that of cats treated with anti-thyroid drug
therapy alone (2 years). Owner/cat compliance and
adverse drug reactions play a significant role in this
shorter survival time. Pre-existing renal failure also adversely affects survival time irrespective of treatment
type and results in a MST of about 6 months.
Drugs of the thiurylene class, mainly methimazole and
carbimazole, are most frequently used for both the
pre-operative and long term medical management of
hyperthyroidism because of their consistent and potent effect in lowering thyroid hormone concentrations. Methimazole and carbimazole are antithyroid
drugs that block T3 and T4 synthesis. They share the
same mechanism of action; carbimazole is almost entirely broken down to methimazole in vivo (carbimazole, 5 mg, is broken down to methimazole, 3 mg). It
usually takes 1 week of treatment to achieve a significant decrease in tT4 concentration.
Before deciding which treatment to use, the cat
should be assessed for concurrent disease, especially
renal disease, systemic hypertension, and heart disease, all of which occur commonly in association
with hyperthyroidism. It is particularly important to
The dose for both drugs is 2.5 to 5.0 mg per cat administered orally every 12 to 24 hours initially, increasing to every 8 to 12 hours as necessary, adjusted to
maintain a euthyroid state, and given for the rest of
the cat’s life. The drugs are most effective given twice
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