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

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Chapter | 20 Thyroid Function
a doubling of T 4 -RIA at 6 h postinjection in dogs and cats.
Scott-Montcrieff and Nelson (1998) measured TSH at
30min after TRH injection and found a small increase in
TSH in hypothyroid as compared to euthyroid dogs. They
concluded that the test had little advantage over using the
baseline TSH, total T 4 , or fT 4 .
XI . DISEASES OF THE THYROID
Thyroid disease has been most extensively studied in the
dog, whereas in the cat, the number of studies is increasing
as the incidence of hyperthyroidism has increased.
Hyperthyroidism is now the most frequently encountered
endocrinopathy in this species. In the horse, Lowe et al.
(1974) described the clinical effects of experimental thyroidectomy
in mares and stallions, and hypothyroidism
remains as a frequent consideration in breeding problems.
In ruminants, a congenital goiter has been described in
Merino sheep and in Afrikander cattle. Local enlargements
or nodules are also observed in all animals, and these are
usually benign tumors.
A . Goiter
Goiter may be defined as an enlargement of the thyroid
gland, which is not due to inflammation or malignancy.
There are two general types of goiters: (1) nontoxic goiters,
which produce either (a) normal amounts of hormone
(simple goiter) or (b) below normal amounts of hormone
(hypothyroid), and (2) the toxic goiters, which produce
excess amounts of hormone (hypertrophy). Furthermore, a
defect or deficiency at any trophic step can also result in
thyroid disease. Iodine deficiency (endemic goiter) is well
known in iodine-deficient areas of the world. Goitrogenic
materials, either natural substances or drugs, induce goiters
by their blocking effects on steps in the hormonogenic
pathways. There are also rare types of familial goiters
associated with defects in hormone synthesis (dyshormonogenesis)
in humans, which find their counterparts in
Merino sheep ( Rac et al. , 1968 ) and in Afrikander cattle
( Van Zyl et al. , 1965 ). The congenital goiter in the Merino
sheep is fundamentally a failure of thyroglobulin synthesis.
The goiter is inherited as an autosomal recessive and is frequently
seen in Australia. The similar congenital goiter in
the Afrikander cattle ( Ricketts et al. , 1985 ) is also inherited
as an autosomal recessive. These cattle have a thyroglobulin
synthesis defect involving defective gene splicing of the
thyroglobulin gene transcript. In Bongo antelopes, goiter
was observed in a group of adults, which was associated
with synthesis of an abnormal 19S thyroglobulin ( Schiller
et al. , 1995 ). The goitrous antelopes were hormonally
euthyroid but had other manifestations of hypothyroidism
(e.g., reproductive difficulties). Iodine deficiency goiter
was observed in seven Thoroughbred foals in the northern
island of Hokkaido, Japan ( Osame and Ichijo, 1994 ), which
was attributed to iodine deficiency in the soil of the region.
All foals had readily detectable thyroid enlargements, and
four of the seven had clinical signs of thyroid deficiency as
well. The goiters receded after iodine supplementation of
the feed.
Simple goiter is a compensatory increase in thyroid
glandular mass (hyperplasia and hypertrophy) so that the
gland maximizes iodine uptake and is able to synthesize
and release a normal amount of T 4 . At this time, the patient
is physiologically normal, but the gland can become quite
large. Ultimately in iodine deficiency, the goitrous gland
fails to synthesize sufficient T 4 and hypothyroidism occurs.
B . Hypothyroidism
Hypothyroidism may be the result of a variety of causative
factors. Thyroiditis, with similarities to Hashimoto’s
thyroiditis in humans, has been reported in about 12%
of beagle dogs ( Beierwaltes and Nishiyama, 1968 ).
Antithyroglobulin antibodies were found in these dogs. In
the adult dog, follicular atrophy is probably the most common
cause of hypothyroidism. Finally, hypothyroidism may
be secondary to a pituitary insufficiency.
The hypothyroid dog is typically obese, lethargic,
has myxedema, a dry skin, and a sparse hair coat.
Hypothyroidism is therefore an important differential in the
diagnoses of dermatoses. The requirement of T 4 for normal
reproduction, growth, and development is well known, so
hypothyroidism is an important differential in reproductive
failures. Experimentally, thyroidectomized mares and
stallions failed to grow; were lethargic, had coarse, dull
hair coats; and had increased serum cholesterol ( Lowe
et al., 1974 ). Hypothyroid horses tend toward obesity and
crestiness.
In the initial screen, an increased cholesterol is often
the first clue to hypothyroidism. Definitive laboratory
findings in the hypothyroidism of animals are a low T 4
or T 3 with little or no response to the TSH response test.
Therefore, the recommended algorithm is to first obtain
the total T 4 and T 3 (and the fT 4 and fT 3 if available). If the
results are equivocal, this is followed by the TSH response
test. Other definitive studies such as the 99m Tc-pertchnetate
scans can be used in specialized hospital settings for the
identification of isolated thyroid nodules.
In human thyroid disease diagnostics, TSH is now considered
to be the single best test of thyroid status and more
cost effective than T 4 . TSH can now be readily assayed
with a functional sensitivity of 0.01 to 0.02 mU/l. Because
of this sensitivity, human thyroidologists now recommend
that TSH be used as the initial test and that T 4 or preferably
fT 4 be used only on a selected basis. In the dog, however,
Kooistra et al. (2000) found that TSH in hypothyroid
dogs was secreted in a pulsatile pattern. This may explain