Page 569 - Problem-Based Feline Medicine

25 – THE CAT WITH POLYCYTHEMIA 561

See main reference The thin cat (or cat with weight Hyperthyroidism is commonly treated with oral daily
loss) and a good appetite on page 304 for details. methimazole, which is generally continued for the life
of the cat.
Pathogenesis
Surgical treatment by unilateral or bilateral thy-
Many hyperthyroid cats have mild increases in PCV, roidectomy may be recommended if the cat has accept-
which are thought to be a direct effect of thyroid hor- able anesthetic risk factors. Careful surgical technique
mone stimulating red cell precursors. Erythropoietin is necessary to avoid post-operative hypoparathy-
production may also be increased in hyperthyroidism, roidism and life threatening hypocalcemia.
because of the increased metabolic rate.
Adjunctive treatment with beta-blockers or other
hypotensive agents may be necessary to manage the
Clinical signs cardiovascular complications of hyperthyroidism.

There is usually a history of weight loss despite
polyphagia.
Prognosis
Cats are often hyperexcitable and the coat is often
unkempt from poor grooming. The prognosis for successful management of hyperthy-
roidism is generally very good provided associated
Vomiting and polyuria/polydipsia occur in about one
complicating factors such as thyrotoxic heart dis-
third of cats.
ease are well controlled.
On physical examination, there may be palpable thy-
roid nodules, and tachycardia or a heart murmur on
cardiac auscultation.
CARDIAC DISEASE INCLUDING
CONGENITAL DISEASE*
Diagnosis
Classical signs
Serum total thyroxine (T ) is usually increased, unless
4
there is an accompanying medical condition that may ● Abnormal cardiac rhythm or rate, often
suppress total T to the upper half of the reference accompanied by a cardiac murmur.
4
range (“sick euthyroid syndrome”). In these cases, ● Blood O saturation is reduced (<92%).
2
a diagnosis of hyperthyroidism can be made by detec- ● Advanced cases may present with acute
tion of increased serum free T or by a triiodothyrox- dyspnea.
4
inine (T ) suppression test. ● Usually cardiomegaly on thoracic
3
radiography.
Differential diagnosis
See main references The Cat With Abnormal Heart
Other causes of mild absolute erythrocytosis with Sounds and/or an Enlarged Heart and The Cat With
normal blood O saturation, such as stress-induced Tachycardia, Bradycardia, or an Irregular Rhythm on
2
splenic contraction, must be differentiated from ery- pages 140, 157 for details.
throcytosis associated with hyperthyroidism.
● Demonstration of increased thyroxine is diagnostic
for hyperthyroidism, although stress-induced Pathogenesis
splenic contraction could occur concurrently and
Cardiac failure causes chronic hypoxia, stimulating
compound the mild polycythemia.
increased secretion of erythropoietin, resulting in
polycythemia.
Treatment
The most dramatic increases in PCV occur in young
The treatment of choice for hyperthyroidism is cats with congenital heart disease with right to left
radioactive I 131 therapy where available. shunting (e.g. tetralogy of Fallot, reverse patent ductus