Page 810 - Small Animal Internal Medicine, 6th Edition

782 PART VI Endocrine Disorders

but not TSH concentration is lower in sighthounds, most function, can be used as the initial screening test for hypo-
notably Greyhounds, and in Nordic breeds such as the Sibe- thyroidism, and should be used when serum T 4 concentra-
VetBooks.ir rian Husky, and may be lower in other breeds as well (see tion alone fails to establish the diagnosis. Low serum T 4 and
fT 4 and increased serum TSH concentrations in a dog with
Table 48.2). The lower end of the reference range for serum
T 4 and fT 4 in these breeds may be as low as 0.4 µg/dL
ties strongly support the diagnosis of hypothyroidism. Con-
(5 nmol/L) and 0.4 ng/dL (5 pmol/L), respectively. Serum T 4 appropriate clinical signs and clinicopathologic abnormali-
and fT 4 concentrations consistent with hypothyroidism current presence of Tg autoantibodies suggests lymphocytic
according to standard reference ranges may actually be thyroiditis as the underlying cause.
normal in these breeds. Establishing the diagnosis of hypo- Unfortunately, discordant thyroid hormone test results
thyroidism in these breeds must rely on the strength of clini- are common. When this occurs, the appropriateness of clini-
cal signs, physical examination findings, and results of cal signs, clinicopathologic abnormalities, and clinician
routine blood work, as well as on documentation of extremely index of suspicion become the most important parameters
low T 4 and fT 4 concentrations and ideally an increased serum in determining whether to treat the dog with levothyroxine
TSH concentration. or repeat testing in 3 to 6 months.
Normal serum T 4 or fT 4 and high TSH may suggest early
Diagnosis compensated hypothyroidism, but one has to wonder why
The diagnosis of hypothyroidism is based on a combination clinical signs would develop when the serum T 4 and fT 4
of clinical signs; findings on physical examination; and concentration are normal. Positive Tg autoantibody findings
results of complete blood count (CBC), serum biochemistry merely suggest the possibility of lymphocytic thyroiditis; Tg
panel, and tests of thyroid gland function. The presence of autoantibody determination is not a thyroid function test.
appropriate clinical signs is imperative, especially when one Positive results increase the suspicion for hypothyroidism if
is relying on baseline thyroid hormone concentrations for a serum T 4 and fT 4 concentrations are low but have no bearing
diagnosis. In the adult dog the most consistent clinical signs on the generation of clinical signs if serum T 4 and fT 4 con-
include lethargy, weight gain, and abnormalities affecting the centrations are normal.
skin (e.g., alopecia, seborrhea, pyoderma) and neuromuscu- Interpretation of serum T 4 , fT 4 , and TSH concentra-
lar system (e.g., weakness). Other organ systems may be tions is not always simple. Because of the expense and frus-
affected by thyroid hormone deficiency, but clinical signs tration of working with tests that are not always reliable,
related to these other systems are not usually the primary many veterinarians and some clients prefer trial therapy as
reason for presentation of the dog to the veterinarian. Iden- a diagnostic test. Trial therapy should be done only when
tification of a mild nonregenerative anemia on the CBC, and thyroid hormone supplementation does not pose a risk to
especially lipemia (hypertriglyceridemia) in the blood the patient. Response to trial therapy with sodium levothy-
sample and an increased serum cholesterol concentration on roxine is nonspecific. A dog that has a positive response to
a serum biochemistry panel, adds further evidence for therapy has either hypothyroidism or “thyroid hormone–
hypothyroidism. responsive disease.” Because of its anabolic nature, thyroid
Baseline serum T 4 concentration is often used as the hormone supplementation can create an effect in a dog
initial screening test for thyroid gland function because it is without thyroid dysfunction, especially regarding quality of
widely available at low cost and can be measured in-house. the haircoat. Therefore if a positive response to trial therapy
It is important to remember that serum T 4 concentrations is observed, thyroid supplementation should be gradually
can be suppressed by a variety of factors, most notably non- discontinued once clinical signs have resolved. If clinical
thyroidal illness and medications such as prednisone and signs recur, hypothyroidism is likely and the supplement
phenobarbital. As such, measurement of the serum T 4 con- should be reinitiated. If clinical signs do not recur, a thyroid
centration should be used to confirm normal thyroid gland hormone–responsive disorder or a beneficial response to
function, not hypothyroidism per se. A normal serum T 4 concurrent therapy (e.g., antibiotics, flea control) should be
concentration establishes normal thyroid gland function suspected.
unless serum T 4 autoantibodies are present and are interfer-
ing with the assay. A low serum T 4 concentration (ideally DIAGNOSIS IN A PREVIOUSLY
<0.5 µg/dL [6 nmol/L]) in conjunction with hypercholester- TREATED DOG
olemia and clinical signs strongly suggestive of the disease Occasionally, a clinician wishes to determine whether
supports the diagnosis of hypothyroidism, especially if sys- a dog receiving thyroid hormone supplementation is in
temic illness is not present. The definitive diagnosis must fact hypothyroid. Exogenous administration of thyroid
then rely on response to trial therapy with synthetic levothy- hormone, either T 4 or T 3 , will suppress pituitary TSH secre-
roxine. Additional tests of thyroid gland function (i.e., TSH tion and cause pituitary thyrotroph atrophy and subsequent
and fT 4 ) are warranted if the serum T 4 concentration is thyroid gland atrophy in a healthy euthyroid dog. Serum
equivocal. T 4 , fT 4 , and TSH concentrations are decreased or unde-
Evaluation of a thyroid panel that includes serum T 4 , tectable; the severity of the decrease is dependent on the
fT 4 , TSH, and Tg autoantibody provides a more informa- severity of thyroid gland atrophy induced by the thyroid
tive analysis of the pituitary-thyroid axis and thyroid gland supplement. Serum T 4 and fT 4 results are often suggestive