Page 313 - Problem-Based Feline Medicine

17 – THE CAT WITH WEIGHT LOSS AND A GOOD APPETITE 305

Cats may have a history of restlessness, aggression ● T4 concentrations varying during the day.
and a lack of grooming. ● Severe systemic illness causing a reduction in T4
(euthyroid sick syndrome).
Cats may also show tachycardia ± a gallop rhythm,
vomiting and/or diarrhea (feces may become bulky) If hyperthyroidism is suspected despite a high normal
and/or polyuria/polydipsia. T4 concentration:
● Retest the cat – either immediately, or in a few
GIT signs may result from polyphagia, malabsorption
weeks time.
or intestinal hypermotility.
● Check free T4 – by equilibrium dialysis.
Polyuria/polydipsia may result from diuretic effects of ● T3 suppression test – Collect blood sample, give
T4, increased renal blood flow, associated renal insuffi- 7 × 25 mg doses of T3 PO every 8 h, then collect
ciency or compulsive polydipsia. blood 2–4 h after the 7th dose (i.e. on day 3). An
increase in T3 concentration confirms successful
Associated cardiac hypertrophy may cause congestive
medication. Suppression of T4 concentration
heart failure with tachycardia, a gallop rhythm, systolic
(below 50% of baseline, <20 nmol/l [1.5 ug/dl]
murmurs, dyspnea, apathy, hindlimb weakness due to
does not usually occur in hyperthyroid cats.
aortic thromboemboli or collapse.
● Thyrotropin-releasing hormone (TRH) stimula-
Associated hypertension may be seen as ocular hemor- tion test – Collect blood, give 0.1 mg/kg TRH IV,
rhage, or may cause clinical signs associated with cere- then collect second blood sample 4 hours later.
brovascular accidents, dementia and/or renal failure. Assess both samples for serum T4 concentration.
● Up to 85% of cats with hyperthyroidism may have Stimulation to > 50% does not occur in hyperthy-
signs of systemic hypertension. roid cats. Side effects of TRH include transient
salivation, vomiting, tachypnea and defecation.
Many hyperthyroid cats are presented for their routine
● Thyroid-stimulating hormone (TSH) response
vaccination with no owner complaints.
test – The usefulness of this test has been ques-
tioned and TSH may be difficult to obtain.
Diagnosis ● Nuclear isotope scanning detects hyperactive thy-
roid tissue. Procedure is relatively safe and simple
Hyperthyroidism should be suspected when any older
to perform, but requires access to a licensed facility,
cat presents with weight loss, and especially when the
which is not always available.
weight loss is associated with a good appetite.
● Trial course of anti-thyroid therapy (see below)
However, inappetence should not rule out hyperthy-
of approximately 30 days is not without risk of
roidism.
toxic side effects.
Physical examination usually reveals a thyroid nodule
on either or both sides of the trachea in the ventral cer-
Differential diagnosis
vical region (80–90%).
These include most other causes of weight loss with a
Tachycardia and/or a gallop rhythm are usually present.
good appetite. However, hyperthyroidism typically
Liver enzymes (ALT and/or SAP) are often raised. occurs in older cats and presents with polyuria, polydip-
Hepatopathy may be secondary to a direct toxic effect sia, vomiting and/or diarrhea, so diabetes, renal disease,
of thyroid hormones, hepatic lipidosis, malnutrition or malassimilation syndromes (including IBD, pancreati-
hepatic hypoxia resulting from cardiac failure. tis/exocrine pancreatic insufficiency, and early intestinal
lymphosarcoma), acromegaly and hyperadrenocorti-
Definitive diagnosis is based on detecting elevated
cism are perhaps the most important differentials.
serum concentrations of total T4 (and/or T3).
Some cats with hyperthyroidism have a T4 concen-
Treatment
tration that is within the normal range. This may be
due to: Hyperthyroidism can be treated medically, surgically,
131
● Early or mild hyperthyroidism. or with radioiodine (I ).