Page 385 - Feline Cardiology

Chapter 26: Endocrine Diseases 403

posttreatment values in 24 of the cats from the earlier ation, 11/23 (48%) of hyperthyroid cats had detectable
study were: 5.5 ± 13.2 mm and 4.8 ± 10.1 mm (left ven- circulating levels of cTn-I, whereas after radioiodine
tricular free wall in systole and diastole, respectively) treatment only 3/18 (17%) of cats had detectable circu-
and 5.2 ± 10.4 mm and 4.5 ± 8.9 mm (interventricular lating levels of cTn-I. While not statistically significantly
septum in systole and diastole, respectively). Such sub- different with a small study population, these results
stantial differences, especially with large standard devia- may have identified that myocardial damage both exists
tions, suggest a wide range of ventricular concentric as part of thyrotoxic heart disease, and is reduced when
hypertrophy historically. By comparison, at the present euthyroidism is restored (Connolly et al. 2005). A large,
time the structural cardiac changes induced by hyper- prospective, single-institution study of circulating
thyroidism in cats appear to be modest. Reversion cardiac biomarkers in 425 cats did not reveal a linear
toward normal of even these small changes can occur in relationship between serum thyroxine levels and plasma
some cats after treatment, and a decrease in mean septal NT-proBNP levels; hyperthyroid cats had a mean plasma
thickness of 0.2 mm and left ventricular free wall thick- NTpro-BNP level that was somewhat higher than euthy-
ness of 0.5 mm occurred in 91 hyperthyroid cats treated roid cats, but there was a large degree of overlap between
with radioiodine (Weichselbaum et al. 2005). Other hyperthyroid and euthyroid groups (Ettinger 2010).
reports have described resolution of cardiac enlarge- Therefore, hyperthyroidism may affect plasma NT-
ment radiographically and electrocardiographically but proBNP results in the cat, generally by increasing them.
not echocardiographically (Moïse and Dietze 1986).
Additional abnormal findings, including enlargement Treatment
of the left atrium, aortic root, end-diastolic left ventricu-
lar diameter, and increased fractional shortening, have Antithyroid Therapy
been noted in some hyperthyroid cats (Bond et al. 1988). The treatment of choice for hyperthyroidism is radioio-
Mitral regurgitation due to systolic anterior motion dine therapy, often preceded by a period of oral antithy-
of the mitral valve may be identified echocardiographi- roid drug treatment (see below) to assess renal function
cally. This finding is rarely due to thyrotoxic heart disease during euthyroidism and decide whether to perma-
alone, and finding this abnormality in a hyperthyroid nently eliminate the hyperthyroid state. Treatment with
131
cat should raise the possibility of unrelated, concurrent 3–5 mCi I (dosage based on clinical signs, size of the
hypertrophic cardiomyopathy. thyroid gland, and serum T4 concentration) successfully
lowered the serum T4 concentration to normal or below
Advanced Testing normal in 516/524 (98.5%) of treated cats (Peterson and
In 2–10% of cats with subsequently confirmed hyper- Becker 1995) with few complications (2.1% hypothy-
thyroidism, serum T4 concentration is within the roidism; 2% persistent or recurrent hyperthyroidism).
normal range or in an equivocal zone (e.g., 2–5 µg/dl) at Alternatively, surgical thyroidectomy was preferred prior
the time of first examination for signs of hyperthyroid- to the advent of radioiodine therapy and it remains an
ism (Feldman and Nelson 2004). Possible explanations acceptable option when radioiodine therapy is not pos-
for this paradox include measurement of T4 early during sible and an owner does not wish to, or cannot, admin-
the course of the disease, diurnal variation, and concur- ister daily antithyroid drugs. A substantial drawback is
rent nonthyroidal illness. In this situation, additional the occurrence of thyroid adenoma bilaterally in 70% of Endocrine Diseases
measurement of serum free T4 concentration (by equi- cases, requiring either bilateral adrenalectomy (and daily
librium dialysis) provides diagnostically useful informa- thyroid supplementation, whereas the avoidance of daily
tion: of 26 cats with mild hyperthyroidism, where the medication administration often was the factor that led
cats later developed unmistakable signs of hyperthy- to the decision to perform thyroidectomy instead of
roidism and elevated serum T4 concentrations, initial administering oral antithyroid drugs) or only partial
serum T4 concentration was elevated in 16/26 cats resolution due to removal of only one thyroid lobe. Oral
(60%), but initial serum free T4 concentration was ele- antithyroid drugs are a third treatment option, and
vated above the normal range of 3–10.5 ng/dl in 25/26 while advantages include being able to adjust treatment
cats (96%). based on clinical signs or appearance of comorbid con-
Advanced cardiovascular tests are rarely pursued or ditions (e.g., chronic kidney disease) and lower initial
necessary in feline hyperthyroidism. An intriguing cost to the client, drawbacks include daily pilling, lack
development is the investigation of circulating levels of of eradication of the problem, and the possibility of
a cardiac biomarker, cardiac troponin-I (cTn-I) in adverse drug reactions. Methimazole (Tapazole) 2.5 mg
hyperthyroid cats on initial presentation compared to PO q 12 h is a typical starting dosage, and every-12-hour
levels measured again after treatment. On initial evalu- dosing has been shown to be necessary where previously