By Rebecca Quinn, DVM, DACVIM (Internal Medicine, Cardiology)
Thyroidal illness is one of the most commonly diagnosed acquired endocrine disorders in both dogs and cats. As a result of this knowledge, most dogs and cats over 7 years of age are screened annually for thyroid disease. Screening typically involves a thyroxine (T4) level, in addition to other baseline labwork including complete blood count, chemistry profile, and urinalysis. In more complicated cases, in which labwork is difficult to interpret, patients may require a thyroid stimulating hormone (TSH) level and free thyroxine (free T4 or fT4) to determine thyroid function accurately. In addition, some cats may benefit from a pertechnetate scans to identify abnormal thyroid function. Once diagnosed, hypothyroidism in dogs and hyperthyroidism in cats can be treated, either with daily supplements, inhibiting enzyme conversion from inactive to active thyroid hormone, radioiodine therapy, gland ablation, or surgical resection of over-active thyroid glands. Treatment is highly recommended and rarely declined given the high success rates of therapy. In cases that are diagnosed late or undertreated, severe secondary effects can occur. Thyroid hormones are known to play a very important role in cardiac function and vascular regulation, and suboptimal patient management can result in severe cardiovascular side effects.
Feline hyperthyroidism is defined as excessive circulating T4, and is most often the result of adenomatous hyperplasia. Thyroxine directly affects cardiac function by several mechanisms. Mainly, T4 increases protein formation and expression of myosin heavy chains. It also increases the calcium handling and the activity of the sodium/potassium/ATPase pump. Less directly, T4 increases the number of adrenergic receptors in cardiac tissue. The consequences of these combined changes include an increase in metabolic rate, increase in tissue oxygen consumption, and decreased peripheral vascular resistance. Excessive T4 can cause potentially reversible cardiomyopathy or decompensation of concurrent heart disease. Patients with chronically elevated T4 develop cardiac remodeling and hypertrophy, increased heart rate, cardiac output, blood pressure, and plasma volume.
Cardiac exam abnormalities reported in hyperthyroid cats include cardiac murmurs, diastolic gallop sounds, tachycardia, supraventricular arrhythmias, ventricular arrhythmias, atrioventricular block, and tachypnea. Cardiac specific testing should include blood pressure measurement, echocardiogram and electrocardiogram. In some cases, thoracic radiographs may be indicated to evaluate for pulmonary edema or pleural effusion. Cardiomegaly in the form of left ventricular hypertrophy and left atrial dilation occur in roughly 50% of hyperthyroid patients. Of those with echocardiographically-confirmed cardiac disease, 30% are categorized as having mild changes, 7% moderate, and 17% severe cardiac disease. In some cases, patients may require concurrent management of hyperthyroidism and congestive heart failure. Heart rate control is important, and cats may benefit from anti-arrhythmic therapy as hyperthyroid treatment is instituted or adjusted. In many circumstances, it is impossible to determine if cardiac changes are solely the result of thyroidal illness, an indication of primary hypertrophic cardiomyopathy, or a combination of both. Oftentimes clients are faced with the decision to treat hyperthyroidism, heart disease, and congestive heart failure with a “wait-and-see” approach. If a patient’s cardiac changes are secondary to hyperthyroidism, recheck cardiac exam, echocardiogram, and electrocardiogram typically improve within 3 – 6 months of a consistent euthyroid state. If primary cardiomyopathy is present, cardiac changes will persist or progress despite appropriate management of hyperthyroidism.
Figure 1: Right parasternal echocardiographic images from a cat with hyperthyroidism. The image on the left is short axis apical view of the left ventricular chamber, and demonstrates left ventricular hypertrophy. The image on the right is a long axis five chamber view, and in addition to left ventricular hypertrophy, highlights left atrial enlargement.
Canine hypothyroidism is associated with decreased production of thyroid hormone. Congenital hypothyroidism has been reported but is very rare. Acquired hypothyroidism occurs most commonly as a primary disorder, and is either the result of lymphocytic thyroiditis or idiopathic thyroid atrophy. The lack of thyroid hormone results in cardiovascular changes and hemodynamic effects, but these consequences tend to be less severe as compared to the effects of hyperthyroidism in cats. In general, dermatologic and metabolic complications of hypothyroidism are much more common than cardiovascular abnormalities.
If present, the most common cardiac complaints in dogs with hypothyroidism include weakness, lethargy, and exercise intolerance. Exam may reveal bradycardia, muffled heart sounds, or weak arterial pulses. If these findings are noted, a cardiac work-up including blood pressure, electrocardiogram, and echocardiogram are recommended. Hypothyroid dogs may be hypotensive as the result of decreased cardiac contractility and decreased cardiac output. Patients may have sinus bradycardia, first degree atrioventricular block or second degree atrioventricular block. In addition, electrocardiogram may identify low-voltage (short) QRS complexes. Echocardiographic findings reported in hypothyroid dogs have included cardiac dilation, thin ventricular walls, and decreased systolic function. Review of current literature suggests that a definitive cause and effect relationship between hypothyroidism and secondary dilated cardiomyopathy has not be established. Congestive heart failure is very rare, and unlike in people, pericardial effusion secondary to hypothyroidism has not been definitively diagnosed in dogs. If cardiovascular abnormalities are noted in dogs with hypothyroidism, the expectation is that the patient will become more hemodynamically stable as thyroid supplement is provided and thyroid levels normalize. Should any cardiovascular abnormalities persist, concurrent systemic or cardiac disease is considered likely.
Feline patients with hyperthyroidism are likely to benefit from cardiac evaluation, especially if abnormal cardiac auscultation is reported or the patient is displaying signs that may be attributed to cardiac disease. As canine hypothyroidism is less severely and conclusively associated with secondary cardiac abnormalities, evaluation by a cardiologist in not as commonly indicated but could be considered on a case-by-case basis.
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