CPD ACCREDITED ARTICLE
be first managed before proceeding with further
thyroid testing. Once concurrent disease is resolved, the “euthyroid sick effect is removed” and
most hyperthyroid cats will develop a clearly high
tT4, confirming the diagnosis. Conversely, in cats
without any overt underlying disease, simply repeating the serum tT4 concentration after 2 weeks
may be diagnostic if the tT4 is fluctuating in and
out of the reference interval. In some cats with preclinical disease, it may take a number of weeks or
even months for the serum tT4 concentrations to
increase into the range diagnostic for hyperthyroidism.
Test serum free T4
In cats with mild hyperthyroidism and tT4 values
within the upper third of the reference interval, serum free T4 (fT4) concentrations can also aid in diagnosis. Serum fT4 concentrations are more consistently elevated (less fluctuation) in hyperthyroid
cats than are tT4 concentrations.
Although fT4 is more sensitive than tT4 for diagnosing hyperthyroidism, the test specificity for fT4
is poor, with up to 20% of sick (and some clinically
normal) euthyroid cats having false-positive fT4 results. Caution is therefore advised in using serum
measurements of fT4 as the sole diagnostic test
for hyperthyroidism. As a thyroid function test, fT4
should always be interpreted with a corresponding tT4 measurement. A tT4 value within the upper
third of the reference range (30-50 nmol/L), combined with a high fT4 concentration, is consistent
with mild hyperthyroidism, whereas a low or low–
normal tT4 with a high fT4 is usually associated
with non-thyroidal illness.
Serum fT4 is currently measured by one of two
methods: Radio-immuno assay (RIA) using kits designed for use in humans and a modified equilibrium dialysis (MED) technique. The MED technique is
the most accurate method for determining serum
fT4 concentrations but unfortunately is not available in South Africa.
Test serum cTSH
The use of canine TSH (cTSH) assays in cats has
recently received attention in the diagnosis of feline hyperthyroidism. A reference range for cTSH of
0.03-0.15 ng/ml has been defined for older cats.
In cats with subclinical and occult hyperthyroidism, cTSH levels are low or undetectable. Therefore
if tT4 levels are in the upper half of the reference
range (>30 nmol/l) and cTSH levels are low or undetectable (< 0, 03 ng/ml), hyperthyroidism can be
diagnosed.
A recent study evaluating the usefulness of cTSH
as a diagnostic test for feline hyperthyroidism using
thyroid scintigraphy as the gold standard, conclud-
ed that measurement of serum cTSH concentration is a very sensitive, but nonspecific, diagnostic
test. Approximately 98% of hyperthyroid cats had
serum cTSH concentrations that are suppressed
below the limit of quantification (<0.03 ng/ml), but
approximately 30% of the older euthyroid cats in
that study also had undetectable serum cTSH concentrations.
The current commercial cTSH assay cannot accurately measure the very low concentrations needed to clearly distinguish between the low-normal
serum cTSH concentrations found in some euthyroid cats from the truly low or totally suppressed
concentrations found in most hyperthyroid cats. It
was shown that combining serum cTSH with tT4
or fT4 concentrations lowered the test sensitivity
of cTSH from 98 to 97%, but markedly increased
overall test specificity (from 69.9 to 98.8%).
The conclusion reached was that testing in parallel
by combining serum cTSH concentration with either tT4 or fT4 concentrations improved the ability
to correctly differentiate hyperthyroid cats with occult or mild disease from euthyroid cats suspected
of having thyroid disease, especially when serum
concentrations of tT4, T3, or fT4 were within the
upper limits of their reference interval or only marginally increased.
Dynamic thyriod tests
Other options for diagnosis of suspect cases include dynamic thyroid tests (T3 suppression, TSH/
TRH stimulations tests) and thyroid imaging (scintigraphy, ultrasound and CT). Dynamic thyroid tests
are not always straightforward to interpret and are
much less frequently performed since they require
multiple samples to be collected and may result in
side-effects.
Thyriod Imaging
Scintigraphy is performed to determine whether there
is increased activity in the thyroid glands relative to
the activity in the salivary glands 20-60 minutes after
intravenous or subcutaneous injection of pertechnetate. It is a very accurate and reproducible test used to
differentiate between cats with unilateral or bilateral
disease and to check for the presence of ectopic thyroid tissue not palpable in the neck. This is extremely
helpful when planning surgery. However as it requires
access to specialist facilities, it is not routinely available.
Ultrasonography has been used to document the
dimensions and volume of the thyroid glands in euthyroid and hyperthyroid cats and has been shown
to have 85.7% agreement with scintigraphy in defining normal and abnormal thyroid lobes. However, it
is technically demanding and very operator dependent. CT has been used to determine the dimensions
and volume of thyroid tissue in clinically normal cats;
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