Cardiac Complications of Feline Hyperthyroidism: Clinical Assessment and Diagnostic Challenges

An older cat diagnosed with hyperthyroidism presents with tachycardia, a systolic murmur and increased activity. These findings may appear to reflect excess thyroid hormone alone, but they can also indicate cardiovascular involvement. Assessing the heart is therefore an important part of managing feline hyperthyroidism. 

How Excess Thyroid Hormones Affect the Heart^[1],[2],[3] 

Elevated thyroxine (T4) and triiodothyronine (T3) increase metabolic activity and tissue demand for blood supply. This produces a high-output circulatory state, with increased cardiac output and reduced systemic vascular resistance. Compensatory mechanisms can lead to cardiac chamber dilation, while thyroid hormones also contribute directly to myocardial hypertrophy. 

Common structural changes include left ventricular concentric hypertrophy (LVCH), left ventricular chamber dilation, and left atrial enlargement. In some cats, these abnormalities may progress to congestive heart failure (CHF), reported in up to 10–15% of hyperthyroid cats with cardiac abnormalities. 

Clinical and Diagnostic Findings^[2],[4] 

Cardiac assessment should begin with auscultation. Systolic murmurs are frequently reported, with prevalence varying across reports and reaching up to 90%. Gallop sounds, tachycardia, bounding pulses and arrhythmias may also be detected. However, a murmur does not necessarily indicate an abnormal echocardiographic finding. 

Electrocardiography may identify sinus tachycardia and tall R-waves, among other less frequent rhythm and conduction abnormalities. Thoracic radiography can reveal cardiomegaly or evidence of CHF, while echocardiography helps assess ventricular wall thickness, chamber dimensions and cardiac function. 

Distinguishing Hyperthyroid Changes from Primary Heart Disease^[2] 

A major diagnostic challenge is differentiating thyroid-associated LVCH from hypertrophic cardiomyopathy (HCM). Echocardiography cannot reliably establish the underlying cause of ventricular thickening. Systemic hypertension and other conditions may also contribute to LVCH, while HCM can coexist with hyperthyroidism. 

Therefore, thyroid status and cardiac findings should be evaluated together. Cats with an HCM-like echocardiographic pattern or CHF should also be assessed for hyperthyroidism. Following treatment, repeat cardiac assessment can help determine whether the abnormalities improve with restoration of normal thyroid hormone levels. 

Clinical Takeaway: Cardiac abnormalities in hyperthyroid cats may be reversible, but concurrent primary heart disease cannot be excluded through echocardiography alone. Combining thyroid assessment with appropriate cardiac investigations and follow-up is essential for clinical decision-making.

References:

1. Janus I, Noszczyk-Nowak A, Bubak J, Tursi M, Vercelli C, Nowak M. Comparative cardiac macroscopic and microscopic study in cats with hyperthyroidism vs. cats with hypertrophic cardiomyopathy. Veterinary Quarterly. 2023 Dec 31;43(1):1-1. https://www.tandfonline.com/doi/pdf/10.1080/01652176.2023.2234436
2. van Zuiden B, Santarelli G, Galac S, Kooistra HS, Szatmári V. Cardiac Abnormalities in Feline Hyperthyroidism. Veterinary Sciences. 2025 Nov 23;12(12):1115. https://www.mdpi.com/2306-7381/12/12/1115
3. Luis Fuentes V, Abbott J, Chetboul V, Côté E, Fox PR, Häggström J, Kittleson MD, Schober K, Stern JA. ACVIM consensus statement guidelines for the classification, diagnosis, and management of cardiomyopathies in cats. Journal of veterinary internal medicine. 2020 May;34(3):1062-77. https://academic.oup.com/jvim/article-pdf/34/3/1062/66660467/jvim15745.pdf
4. Kormpou F, Gil-Morales C, Warren-Smith C, Hibbert A. Concurrent thoracic pathology identified with radiology in hyperthyroid cats referred for radioiodine therapy. Journal of feline medicine and surgery. 2020 Oct;22(10):966-76. https://journals.sagepub.com/doi/pdf/10.1177/1098612X19900408