Background
The diagnosis of hyperthyroidism can be confirmed in the majority of cats
based on the presence of appropriate clinical signs and an increased circulating
total T4 concentration. At Axiom, a total T4 value > 60nmol/L is considered
supportive of hyperthyroidism. However, the upper end of the reference range
in cats extends to 40nmol/L. Those cases with total T4 values between 40-60
nmol/L are therefore in the equivocal range. There are several possible
explanations for equivocal results. The most common explanations are either
1. very early hyperthyroidism
2. early hyperthyroidism with concurrent nonthyroidal illness
3. random fluctuation into the equivocal range in a euthyroid cat
Various diagnostic options are available in clarifying the thyroid status
of cats with equivocal results.
"Evaluation in Context"
As in dogs, total T4 concentrations generally decrease in cats with significant
nonthyroidal illness. Thus most cats with obvious or chronic clinical abnormalities
are expected to have subnormal or low-normal values. The presence of an
equivocal result in these cases should be considered as inappropriate and
increase the suspicion of early hyperthyroidism (with/without concurrent
nonthyroidal illness). Certainly an equivocal result in a cat that looks
clinically severely ill is highly suspicious of early hyperthyroidism with
concurrent illness.
Wait and Retest
In otherwise relatively healthy cats it is often appropriate to wait for
4-6 weeks and re-test total T4 concentrations. If euthyroid, it is unlikely
that a second sample will be within the equivocal range. However, if hyperthyroid
it is quite likely that the result will have increased into the diagnostic
range. This approach is only appropriate for relatively clinically healthy
cats.
Free T4 Measurement
Free T4 estimation is generally considered to be a slightly more diagnostically
"sensitive" test for hyperthyroidism. i.e. it is slightly more
likely to give a positive result in early cases than is total T4 measurement.
Free T4 measurement may therefore be a useful second line test in equivocal
cases. However, note that some cats with nonthyroidal illness will have
temporary increases in free T4 and so this is not routinely recommended
as the sole test in routine investigation of feline hyperthyroidism
T3 Suppression Test
Administration of exogenous T3 to a euthyroid cat causes an increased negative
feedback effect on TSH secretion from the pituitary gland and consequently
circulating total T4 concentrations decrease. However, cats with hyperthyroidism
secrete T4 independent of TSH control and so T3 administration has little
effect on total T4 values in these cats. This test can therefore be used
to evaluate "equivocal" hyperthyroidism.
Test Protocol: Collect blood for total T4 measurement, administer
0.02 mg/kg T3 (Tertroxin) orally, TID for 7 doses; collect blood
2-4 hours after the last dose of T3 for total T4 and total T3 measurement.
Total T4 values should decrease to < 20nmol/L with >50 % suppression
compared to the baseline value to indicate euthyroidism. Failure to suppress
is consistent with hyperthyroidism. The T3 measurement is performed solely
to confirm adequate owner compliance in medicating with T3. |