What is/are the best tests to diagnose hypothyroidism in dogs?
We recommend measuring a total T4 by RIA as a screening procedure in any
dog suspected with the disease. Total T3 along with the T4 is optional,
it does not provide as much information as T4. However, measuring T3 does
help in identifying the relatively small percent of dogs with T3 autoantibodies.
If the total T4 value in the suspect dog is well into the normal range
(i.e., > 20 nmol/L) hypothyroidism is unlikely. If the T4 value is below
normal ( < 12 nmol/L) and the dog has strong clinical signs supportive
of hypothyroidism, is not sick from another obvious problem, and is not
receiving drugs such as glucocorticoids, anticonvulsants, or potentiated
sulfonamides (i.e., Tribrissen, trimethoprim-sulfadiazine or Primor, ormetoprim-sulfamethoxine)
which are known to suppress levels of T4, you can feel reasonably certain
that the dog is hypothyroid. If the total T4 is borderline (11-19 nmol/L)
or if the T4 is low and the dog is receiving one of these drugs or is sick
from another problem, we recommend a follow up test with higher discrimination
such as free T4 by dialysis or endogenous canine TSH measurement.
What is the value in measuring free T4? Free T4 is the portion
of total T4 not bound to carrier proteins. It represents only about 0.1%
of the total T4 (for example, a normal total T4 in a dog is 20 nmol/L,
while the normal free T4 is 20 pmol/L which is 1000x less than the total).
Free T4 is the fraction of hormone that is thought to be active in cells
and therefore measurement of free T4 provides one of the best methods to
evaluate thyroid activity at the cellular level. Methods to measure or
estimate free T4 vary considerably. A common technique offered by some
labs is the analogue method or free T4 by analogue RIA. This technique
is not reliable in dogs with euthyroid sick syndrome in particular, and
is of questionable value overall in the evaluation of thyroid function
in dogs. This method is inexpensive, however. A better technique uses equilibrium
dialysis (available through our lab) to measure free T4. This method is
more expensive and labor intensive, however, it has been shown to give
reliable estimates of true free T4 concentrations. Free T4 by dialysis
can be the initial test for the diagnosis of hypothyroidism or it can be
used in dogs who have been found to have borderline total T4 concentrations.
One precaution with free T4 analysis: chronic glucocorticoid therapy or
dogs with Cushing's disease may show low free T4 levels.
What about the TSH response test? TSH for the response test
is no longer commercially available and we doubt that it will come on the
market anytime soon. Therefore, this test is (unfortunately) not a diagnostic
option at the present time.
What is the value in measuring canine TSH concentrations?
A new assay has appeared designed to measure endogenous TSH concentrations
in dogs (we do not believe that it works in cats). TSH levels should be
a very sensitive indicator of thyroid function and would be expected to
increase above normal as the thyroid gland fails in primary hypothyroidism.
Since this form of hypothyroidism (primary) is by far the most common type
in the dog, measurement of TSH should provide one of the best tools for
the diagnosis. This assay is so new (released in late 1995) that there
are few published studies showing how sensitive and specific the test is.
We do know that TSH levels increase in dogs in response to thyroid removal
(as expected because T4 negative feedback is gone) and that TSH levels
increase in dogs given thyrotropin releasing hormone (TRH). However, we
do NOT know if either non-thyroidal illness or drugs such as glucocorticoids
or anticonvulsants affect TSH levels. Based on our experience thus far,
TSH seems best suited to use in combination with free T4 measurement. A
low free T4 together with a high TSH confirms the diagnosis of hypothyroidism.
What form of T4 should be used to treat canine hypothyroidism?
Any brand of synthetic l-thyroxine can be used. We recommend initial therapy
at 20 ug/kg BID, and then convert to 20 ug/kg SID after 2-3 months of therapy.
Some dogs, however, require life-long BID therapy with T4. It is best to
check post-pill levels of T4 (sample taken 4-12 hours post-pill) after
6 weeks of therapy and thereafter about every 6 months to ensure correct
dosing. Ideal T4 levels post-pill are between 30 and 60 nmol/L.
Does lipemia affect T4 or T3 measurement? It affects the
apparent T3 level (raises it). The more severe the lipemia, the higher
the apparent T3.
How do I test for hypothyroidism in a dog who is on T4 therapy?
You must stop the T4 therapy for several weeks to allow the thyroid axis
to recover (assuming the dog actually has a normal thyroid). We recommend
awaiting about 1 month, and then measure total T4 or better, free T4 by
dialysis.
The total T4 level in a dog I am evaluating came back borderline
low. What are my options?It is possible that this dog is hypothyroid.
Alternatively, there may be a non-thyroidal cause for the borderline low
T4 or this dog may simply have a normal resting T4 level which is on the
low end of normal. Options to consider: 1)wait and retest in a few months.
If the dog is a true hypothyroid, the T4 level will continue to drop and
clinical signs should become more profound. 2)To pursue the diagnosis,
measure free T4 by dialysis and/or endogenous TSH. Assaying both is best.
These tests are more sensitive and specific for hypothyroidism than a total
T4. They are also more expensive. Nevertheless, given that the diagnosis
of hypothyroidism means that the dog will be on T4 replacement therapy
for life, the extra testing to definitively rule in or out hypothyroidism
is a worthwhile investment in the long run. 3) Put the dog on a therapeutic
trial of T4 replacement. If this option is used, be sure to reevaluate
the dog in 10-12 weeks and measure a post-pill T4 level at that time. If
the T4 is within the normal post-pill range, the dog should show a complete
recovery after 10-12 weeks of T4. A partial response may mean that the
diagnosis was in error. Some advocate taking the dog off of T4 at this
time, to evaluate if all the clinical signs return (confirming the diagnosis!!).
Among potential problems with the therapeutic trial approach are: what
to do with partial responders, what if clinical signs were due to a non-thyroid
problem that happened to resolve during the trial therapy, and possible
'pharmacologic' effects of T4 such as stimulation of hair growth. It is
more difficult to evaluate thyroid function in a dog on T4 replacement
since T4 will suppress the pituitary-thyroid axis. In general, therapeutic
trials are not the best option and often lead to a delay in the diagnosis
of the real problem.
*(obtained from Auburn University)