Page 383 - Feline Cardiology

Chapter 26: Endocrine Diseases 401

signs. Given their adult or advanced age, such cats often may in fact worsen thyroid storm simply through han-
have concurrent diseases contributing to the observed dling of the patient, through incomplete monitoring, or
clinical signs. Comprehensive diagnostic testing is indi- both. Therefore, cats experiencing thyroid storm should
cated to rule in or rule out the disorders mentioned be allowed to ambulate freely in a cage if they were pre-
above because nonthyroidal illness is known to reduce viously being manually restrained, should be moved to
circulating thyroxine concentrations (“euthyroid sick a quiet and nonthreatening location if such was not
syndrome”) and could explain a serum thyroxine level already present, and should be monitored closely but in
that is only minimally elevated despite prominent clini- a manner that the cats tolerate and that does not antago-
cal signs of hyperthyroidism. If uncertainty continues to nize or worsen the signs of thyroid storm. Although data
persist, diagnostic tests that help to confirm or refute are lacking to confirm this observation, prevention of
hyperthyroidism may be indicated (see “Advanced thyroid storm is likely the best strategy, and such preven-
Testing,” below). From a cardiovascular perspective, cats tion begins with awareness that this complication occurs
that have mildly elevated serum thyroxine levels and in hyperthyroidism. Specific precautions should include
equivocal, if any, systemic signs of hyperthyroidism careful and conscientious handling of all hyperthyroid
almost never have deleterious cardiovascular effects of cats with attention paid to surroundings (e.g., presence
hyperthyroidism; the heart and circulation seem to be of dogs or other cats should be avoided) and adjustment
affected later in the disease process. Therefore, a patient of one’s handling and restraint of the patient based on
with mild hyperthyroidism and substantial cardiovascu- signs of tolerance.
lar abnormalities (e.g., target organ damage from per-
sistent, severe systemic hypertension, or marked left Other Complications of Hyperthyroidism
ventricular thickening) should be suspected of having a Reversion to euthyroidism is known to reveal chronic
concurrent, unrelated problem as the cause of these kidney disease in a proportion of the hyperthyroid cat
abnormalities.
population, and complications related to antithyroid
treatment are well-recognized. These aspects of hyper-
thyroidism have been reviewed elsewhere (Feldman and
Complication of Hyperthyroidism:
Thyroid Storm Nelson 2004).
“Thyroid storm” refers to an acute exacerbation of thy-
rotoxicosis in hyperthyroid cats. It may be triggered by Diagnostic Testing
the cat’s presence in a stressful environment such as the Initial Tests
veterinary clinical setting, restraint for diagnostic proce- The diagnosis of hyperthyroidism is established by iden-
dures, or other environmental factors, the intensity of tifying a high concentration of circulating thyroxine
which is potentiated by the effects of excess thyroid (T4). Although exact normal ranges are laboratory- and
hormone. Four factors are thought to contribute to its population-dependent, a typical range of normal serum
pathogenesis (Ward 2009): high levels of circulating T4 is 0.8–2 µg/dl (Feldman and Nelson 2004).
thyroid hormones; rapid, acute further increases in cir- In hyperthyroid cats, cardiovascular diagnostic testing
culating thyroid hormones; hyperactivity of the sympa- is inconsistently pursued and specific guidelines for
thetic nervous system; and an increased intracellular which tests to perform under which conditions do not Endocrine Diseases
response to thyroid hormones. Thyroid storm occurs exist at present.
variably and may cause cardiovascular abnormalities At a minimum, all cats with hyperthyroidism should
that can be fatal. Such abnormalities include sinus tachy- have blood pressure measured via Doppler ultrasonic
cardia, premature ventricular complexes and ventricular sphygmomanometry (Syme and Elliott 2003) initially
tachycardia, systemic hypertension, acute cardiogenic and then throughout antithyroid treatment and periodi-
pulmonary edema/congestive heart failure, and cardiac cally thereafter (see Chapter 21 for technique). Additional
arrest. Diagnosis is circumstantial: signs of acute onset testing in the form of electrocardiography (Chapter 9),
of these abnormalities occur in a hyperthyroid cat, typi- radiography (Chapter 6), and echocardiography
cally in the context of a triggering event. Treatment is (Chapter 7) may be implemented as dictated by clinical
generally conservative because the rapidly fluctuating findings. Specifically, hyperthyroid cats with a heart
thyroid hormone and sympathetic effects make phar- murmur, gallop sound, or cardiomegaly on radiographs
maceutical choices and dosage selection challenging. For should undergo echocardiography to rule in or rule
example, while a hypertensive crisis or hemodynami- out concurrent forms of heart disease; cats with an
cally destabilizing arrhythmia may be treated with drugs, arrhythmia or persistent tachycardia should undergo an
administering and monitoring the effects of the drugs electrocardiogram; and cats with dyspnea, auscultatory