Page 13 - 2014 Le Mag 2nd Qtr
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Total T4 – measures bound ( > 99%) and unbound (< 1%) circulating T4. While T4 is the most
widely used initial screening test for thyroid disorders, relying on this test alone is a primary
reason for the misdiagnosis of canine and feline thyroid disorders. T4 concentration is affected
by non-thyroidal illness (NTI), and certain medications (e.g. phenobarbital, corticosteroids, and
sulfonamides), and it fails to detect autoimmune thyroiditis.

FreeT4 – serum freeT4 represents the tiny unbound fraction (< 0.1%) of biologically active
thyroxine. It is much less likely to be inluenced by NTI or drugs, and so is a more accurate test
of thyroid activity. While some prefer the equilibrium dialysis (ED) radioimmunoasay (RIA)
method for measuring freeT4, newer technologies (improved analog RIAs and non-RIA
methods) offer alternative, accurate methodology. These new assays are faster and less costly.

Total T3 – represents both the bound and unbound forms of circulating T3. Measuring serum T3
alone is not accurate for diagnosing thyroid disorders, as it is inluenced by NTI. But, It is still
useful as part of a thyroid panel. If total T3 levels are very high (or very low, if measured at
MSU) in a dog not taking thyroxine, the dog most likely has a circulating T3 autoantibody which
is spuriously affecting the results.

Free T3 – measures the tiny biologically active fraction of T3. Levels may be elevated slightly in
thyroid-normal (euthyroid) dogs or cats with increased tissue metabolic demands, and are
typically spuriously high in dogs with T3 autoantibodies. Both total T3 and freeT3 should be
normal in hypothyroidism, unless the dog has concurrent NTI.

Canine Thyroglobulin Autoantibodies (TgAA) – elevated thyroglobulin autoantibodies are
usually present in the serum of dogs with autoimmune thyroiditis, the heritable form of
hypothyroidism. TgAA is important in screening breeding stock, as dogs testing positive for
TgAA should not be bred. The commercial TgAA test can give false negative results if the dog
has received thyroid supplement within the previous 90 days, thereby allowing unscrupulous
people to test dogs while on treatment to assert their normalcy, or to obtain certiication with
health registries such as the OFA Thyroid Registry. False negative TgAA results can also occur
in ~ 8% of dogs veriied to have high T3 and/or T4 autoantibodies. Furthermore, false positive
TgAA results may be obtained if the dog has been vaccinated within the previous 30 - 45 days
for rabies, or occasionally in cases of NTI.

T3 Autoantibody (T3AA)/T4 Autoantibody (T4AA) – these autoantibodies affect the accurate
measurement of T4 and T3. Results may be spuriously high (or low, if measured at MSU) for T3
and freeT3 or T4 and freeT4. However, if the freeT4 is measured by the ED technique, the
T4AA will be removed by the dialysis step and not be detected. Fortunately, most circulating
antibodies are against T3 (~70%), some affect both T3 and T4 (~25%), and only a few affect T4
alone (~5%).

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