Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

X. Thyroid Function Tests
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thyroglobulin antibodies (TgAA) is the ELISA method.
Haines et al. (1984) , using ELISA, detected TgAA in a
high percentage of dogs with hypothyroidism, in dogs
with other endocrine diseases, and in those dogs that were
closely related to the TgAA-positive dogs but found a low
percentage in healthy unrelated dogs. They concluded that
thyroid autoimmunity is strongly genetically influenced in
the dog. An enzyme immunoassay (EIA) method has also
been developed for the detection of autoantibodies to thyroxine
and triiodothyronine ( Patzi and Mostl, 2003) .
6 . Thyroid-Stimulating Hormone
Serum TSH is measured by radioimmunoassay (TSH-RIA)
using an anticanine TSH antibody, which is commercially
available for use in dogs. The human assay method, which
is not suitable for dogs ( Larsson, 1981 ), has been used in
monkeys ( M. mulatta ) with 0.2 to 2.6 μ U/ml (1.53 μ U/ml)
( Belchetz et al. , 1978 ). Marca et al. (2001) compared three
assay methods for canine serum: RIA, EIA, and chemiluminescent.
They found that the chemiluminescent assay
had the best within- and between-run precision, and
that all three had satisfactory correlations to serum TSH
concentrations.
7 . Thyrotrophin-Releasing Hormone
Although as yet no reliable thyrotrophin-releasing factor
(TRH) assay has been developed for use in domestic animals,
a highly purified TRH is available and is used in the
TRH response test.
C . Radionuclide Uptake Tests
1 . Radionuclide Scintigraphy
Radioiodine is taken up by the thyroid gland in exactly the
same manner as the nonradioactive isotope, and its uptake
remains as one of the most definitive tests for thyroid function.
Radioactive 99m Tc-pertechnetate is also taken up in
the same manner and is now widely used for scintigraphy.
Pertechnetate scintigraphy is used for detecting asymmetry
of the thyroids, for localizing of active thyroid nodules and
“ hot spots ” (Scriviani et al. , 2007). Quantitative pertechnetate
uptake was found to have the highest discriminatory
power between primary hypothyroidism and nonthyroidal
illness when compared to other methods of evaluating thyroid
function ( Diaz-Espineira et al. , 2007) .
D . Thyroid Ultrasonography
Ultrasonographic evaluation of the thyroid gland has been
proposed as a means to differentiate between hypothyroid
and euthyroid dogs. Two studies have reached essentially
the same conclusion that the estimated thyroid volume by
ultrasound has a highly specific predictive value for canine
hypothyroidism ( Bromel et al., 2005 ; Reese et al., 2005 ).
E . Trophic Hormone Response Tests
1 . Thyroid-Stimulating Hormone Response Test
The response of the thyroid to TSH injection is a means
of evaluating thyroid activity as well as to differentiate a
primary hypothyroidism because of a thyroid lesion from
a hypothyroidism secondary to a pituitary lesion. The
responsiveness of the thyroid to the TSH injection is evaluated
by an increase (or failure to increase) as evidence of
thyroid activity (or inactivity). In the general procedure,
thyroidal activity (or lack of activity) is first established
by the measurement of serum hormones, and this is followed
by the injection of TSH. In a primary hypothyroidism
where the lesion is localized in the thyroid, there is no
response to the exogenous TSH. If the hypothyroidism is
due to a pituitary hypofunction with a deficiency of TSH
or a hypothalamic lesion with a lack of TRH, there will be
a response of the thyroid to the exogenous TSH as seen by
a significant increase in serum hormone concentrations.
Glucocorticoids and phenylbutazone are also well known
to depress thyroid activity so that the TSH response test is
useful in detecting low hormone concentrations due to thyroid
disease. Useful procedure for the TSH response test is
to first to obtain a serum sample for baseline T 4 or T 3 , then
inject 10IU of bovine TSH intravenously. Held and Oliver
(1984) and Oliver and Waldrop (1983) have recommended
a minimum of 5IU for the dog and horse, respectively.
After 4 h, a second serum sample is taken and hormone
again measured. The normal response in dogs is a doubling
or more of the hormone above baseline level. In the primary
hypothyroid individual, there is a virtual absence of
a response. In the drug-induced or Cushing’s patient with
low hormone, there will be a response to well within the
normal hormone concentrations. The secondary (TSH) or
tertiary (TRH) hypothyroid patient will have a response
similar to the drug-induced or Cushing’s patient. In cats,
Peterson et al. (1983) found T 4 to increase almost threefold
above the baseline at 4 h post TSH. In the baboon,
the TSH response test had peaks of more than double the
baseline at 8 and 12 h for T 3 and T 4 , respectively ( Maul
et al., 1982 ). In horses, Messer et al. (1995) , at 6 h after
TSH administration, found significant increases in T 4 ,
T 3 , and fT 4 but not in fT 3 . They suggested that the TSH
response test may not be as valuable for thyroid disease
diagnostics in the horse as it is in the dog and cat.
2 . Thyrotrophin-Releasing Hormone Response
The response to thyrotrophin-releasing hormone (TRH)
has been used in dogs and cats. Lothrop et al. (1984) found