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VETERINARY HANDBOOK
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Canine
Hypothyroidism |
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There are a great many factors affecting
thyroid hormone levels in dogs. It is essential that a thorough clinical
examination and history is carried out prior to any diagnostic testing.
The following factors are also very important to consider when carrying
out thyroid function tests. |
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Concurrent Therapy - Dogs must be off all therapy
for at least 4 weeks prior to testing, many drugs can significantly affect
tT4 and cTSH levels.
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Breed Specific Data - Sight hounds are known to have lower tT4 levels compared
to other breeds. It is important to establish a reference range guideline
for a particular breed if screening for hypothyroidism is to be done effectively.
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Sick Euthyroid Syndrome (SES) - This can cause confusion as dogs ill for
other reasons will usually have depressed tT4 levels. Care must be taken
to evaluate the dog for other illness prior to testing for thyroid function.
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Diagnosing
Canine Hypothyroidism - Flow Chart (PDF
file) |
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Total T4 (tT4) |
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tT4 may be used as a general screen for thyroid
function and is used to monitor treatment for animals on thyroid therapy,
samples should be taken 4 - 6 hours post pill. Levels of tT4 should be
above 30 nmol/L but within the reference range of 20 - 60 nmol/L. |
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Canine TSH (cTSH) |
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A commercial assay is now available for cTSH
and this has provided extra information to assist in the diagnosis of canine
hypothyroidism. The tT4 may be low due to non-thyroidal illness. A normal
cTSH (< 0.41) makes hypothyroidism unlikely.
cTSH may be measured at 30 minutes as part of a TRH stimulation test
and can be used to diagnose secondary hypothyroidism. Normal dogs should
increase cTSH by at least 0.4 ng/mL.
cTSH should always be tested with tT4/fT4/fT4ED or TRH stim test and
not on its own as results maybe misleading (see flow chart for intrepretation
of tT4 & cTSH). |
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Total T3 (tT3) |
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The analysis of tT3 is of little diagnostic
value in the diagnosis of hypothyroidism. |
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Free T4 (fT4) |
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The analysis of fT4 is not thought to be of any greater
diagnostic value than tT4, but it is thought it may have some advantages
in dogs with non-thyroidal illness. fT4 should be measured using equilibrium
dialysis as some analogue methods may underestimate fT4 levels. |
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Thyroid Profile 1 (TP1) |
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Canine TSH is measured in addition to Total
T4 and this increases the accuracy of the Total T4. This profile is a very
useful single sample screen. |
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Thyroid Profile 2 (TP2) |
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Canine TSH is measured in addition to a TRH
stimulation test. This gives a very complete picture of thyroid status
as a stimulation test is likely to be more diagnostic. |
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Thyroid Profile 3 (TP3) |
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Canine TSH and Free T4 ED are measured in
addition to a TRH stimulation test. A complete picture of thyroid status
and is the test of choice where possible. |
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Thyroid Profile 4 (TP4)
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Canine TSH is measured with Free T4 ED. This
is the most useful combination for single sample analysis. |
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Anti
T4/T3 Antibodies |
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The presence of antibodies, although rare,
will lead to falsely high thyroid hormone levels due to interference in
the assay systems. If a very high tT4 (> 100 nmol/L) or fT4 (> 50 pmol/L)
are found in a dog suspected of hypothyroidism, it is worth testing for
thyroid antibodies. |
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Thyroglobulin Auto-antibody
(TGAA) |
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The presence of TGAAs is strongly suggestive
of immune mediated lymphocytic thyroiditis which leads, in some dogs, to
the destruction of the thyroid gland and subsequent hypothyroidism.
Analysis of TGAA provides strong evidence of thyroiditis. In some breeds
of dog it has been suggested there may be a genetic predisposition to immune
mediated thyroiditis and screening of TGAA along with a thyroid screen
may be useful in identifying these dogs. It is important to be aware that
positive TGAAs have been reported in normal dogs who do not have hypothyroidism
so care must be taken in interpretation of results and TGAA must always
be analysed in conjunction with other thyroid parameters such as tT4, fT4,
fT4ED and cTSH. |
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TSH
Stimulation Test |
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Please note a pharmaceutical grade TSH is
no longer available, chemical grade TSH is available but extreme care should
be taken if this product is used as there may be a risk of adverse reaction.
1. Take blood sample for basal T4 concentration.
2. Inject 0.1IU/kg TSH i/v.
3. Take a second blood sample 4-6 hours later for post T4 concentration.
Interpretation T4 levels in a normal dog should increase by 1.5 - 2.0
times the basal concentration to reach a value above 26 nmol/L. |
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TRH Stimulation
Test |
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1. Take blood sample for basal T4 concentration.
2. Inject TRH (Cambridge Laboratories*) i/v slowly over one minute.
3. 1-5 kg 100ug TRH 5 - 30 kg 200ug TRH > 30 kg 300ug TRH
4. Take a second blood sample 4-6 hours later for post T4 concentration.
*Cambridge Laboratories Tel no: 0191 296 9369
Interpretation
T4 levels in a normal dog should increase by about 1.2 times the basal
concentration to reach a value above 25 nmol/L. If the pre stimulation
sample is above 25 nmol/L the dog is very likely to be normal, regardless
of the post stimulation tT4. Hypothyroid dogs usually show low basal T4
levels which fail to respond to TRH or stimulate to a value below 25 nmol/L.
Sometimes a high/normal basal concentration may be seen which fail to increase
1.2 times. On these occasions it is likely that the thyroid gland is being
stimulated maximally and cannot respond further.
Diagnosis of secondary hypothyroidism can be done by measuring cTSH
at zero and 30 minutes post TRH stimulation. Normal dogs should increase
cTSH by at least 0.4 ng/mL. |
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Monitoring
Thyroid Therapy |
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Soloxine (Arnolds) is the only licensed veterinary
preparation of levothyroxine. Dose is normally 20ug/kg given twice daily
(Dr I.K.Ramsey). tT4 levels should be monitored after 1 month of therapy,
a sample is taken 4 - 6 hours post-pill. tT4 levels should be above 50
nmol/L 4 - 6 hours after administering the drug. |
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