Page 354 - Problem-Based Feline Medicine

346 PART 6 CAT WITH WEIGHT LOSS OR CHRONIC ILLNESS

Serum biochemistry frequently shows elevations of
APATHETIC HYPERTHYROIDISM*
hepatic enzymes (alanine amino transferase and alka-
line phosphatase).
Classical signs
A definitive diagnosis of hyperthyroidism is made on
● Older cats (usually > 8 years).
the basis of high serum basal total thyroxine (T )
● Weight loss and inappetence. 4
concentrations.
● Weakness and depression.
● Palpable goiter. Normal serum thyroid hormone concentrations are
● Tachycardia ± gallop rhythm. occasionally found in hyperthyroid cats due to:
● Day-to-day fluctuation in hormone levels.
See main references on page 304 for details (The Cat ● Early or mild hyperthyroidism.
With Weight Loss and a Good Appetite). ● The presence of concurrent non-thyroidal disease
that suppresses serum thyroid levels.
Clinical signs Given that concurrent disease is common in cats with
Hyperthyroidism is a disease of older cats, with only apathetic hyperthyroidism, suspect hyperthyroidism
5% being under 10 years of age at first diagnosis. despite normal Total T concentrations if the cat is
4
older than 8 years and has signs consistent with hyper-
The majority of cats with hyperthyroidism present with thyroidism. e.g. tachycardia or thyroid goitre.
weight loss and polyphagia.
If hyperthyroidism is suspected despite normal total
In approximately 5% of cases, apathetic or masked T concentrations, diagnostic possibilities include:
4
hyperthyroidism occurs. ● Repeat basal total T measurement in 1–2 weeks.
4
● Weight loss is accompanied by a decreased appetite. ● Identify and treat concurrent disease, then
● Cats tend to be depressed. repeat basal total T measurement.
4
● Muscle weakness may be evident as ventroflexion ● Measure basal free T concentrations, although
4
of the neck. euthyriod cats with non-thyroidal illness may have
● Cats with apathetic hyperthyroidism may show an elevated concentration.
signs of concurrent disease including congestive ● Dynamic thyroid testing may also be used:
cardiac failure, renal failure or neoplasia. – Triiodothyronine (T ) suppression test.
3
Other clinical signs are those typical for hyperthyroidism. – Thyrotropin (TRH) stimulation test, although the
● Unilateral or bilateral palpably enlarged thyroid usefulness of this test in cats with non-thyroidal
(goiter). illness to distinguish those with and without
● Polyuria and polydipsia. hyperthyroidism has recently been questioned.
● Vomiting and diarrhea (bulky feces). ● Radionucleotide uptake and imaging.
● Tachycardia, gallop rhythm, murmur and arrhyth- Concurrent disease may be present and should be
mia may be noted. investigated, in particular:
● Poor coat condition with decreased grooming. ● Evidence of concurrent renal failure with increased
creatinine and urea concentration may also be pres-
Diagnosis ent. Urine specific gravity will also provide further
information regarding renal function.
Although clinical signs of inappetence and depression
● Hypertrophic cardiomyopathy secondary to hyper-
are atypical for hyperthyroidism, the presence of a
thyroidism (thyrotoxic cardiac disease) is frequently
thyroid goiter or tachycardia should alert the clini-
seen on echocardiography. This may be reversible
cian to the possibility of hyperthyroidism.
with resolution of the hyperthyroid state but may
As apathetic hyperthyroidism typically occurs in progress to congestive cardiac failure.
hyperthyroid cats with significant concurrent
disease, a thorough investigation is required to estab- Differential diagnosis
lish the presence of such disease.
These include most other causes of inappetence and
Routine hematology may reveal a mild erythrocytosis. weight loss.