Hyperthyroid cats only get hypertrophic cardiomyopathy: True or False?
We often think than hyperthyroid cats only get HCM, but it’s not that simple. Cats with hyperthyroidism predominantly get HCM, but restrictive cardiomyopathy is also seen and may reflect more severe hyperthyroidism
Journal Article
Hypertrophic cardiomyopathy is not the sole echocardiographic phenotype associated with hyperthyroidism in cats: a retrospective study in 147 cats (2005-2025) Open Access
Journal of Veterinary Internal Medicine, Volume 40, Issue 1, January-February 2026, aalag006, https://doi.org/10.1093/jvimsj/aalag006
Published:
16 February 2026
Abstract
Background
A hypertrophic cardiomyopathy phenotype (HCMP) can occur in cats with hyperthyroidism. However, it remains unclear whether other cardiomyopathy phenotypes are also associated with hyperthyroidism in cats.
Hypothesis/Objectives
Describe the epidemiological, clinical, and echocardiographic findings and cardiomyopathy phenotypes in a large sample of hyperthyroid cats. Compare the echocardiographic features of hyperthyroid cats with HCMP to those of a contemporaneous sample of normotensive euthyroid cats with primary hypertrophic cardiomyopathy (HCM).
Animals
A total of 147 hyperthyroid cats and 112 cats with primary HCM.
Methods
Retrospective study with review of internal medicine and cardiology service databases (2005-2025).
Results
Most hyperthyroid cats (117/147, 80%) exhibited 1 of the 3 cardiomyopathy phenotypes: HCMP (94/147, 64%), restrictive cardiomyopathy phenotype (RCMP; 20/147, 14%), or nonspecific (3/147, 2%). Hyperthyroid cats with RCMP had significantly higher total thyroxine concentrations (median, 154 nmol/L vs 95 nmol/L) and more dyspnea related to congestive heart failure (80% vs 11%) than those with HCMP (P < .01). A gallop sound was detected in 10% of hyperthyroid cats (14/147), exclusively in those with HCMP (10%, 9/94) or RCMP (25%, 5/20). The end-diastolic left ventricular diameter was higher in hyperthyroid cats with HCMP than in those with primary HCM (P < .01). Subaortic septal hypertrophy was more frequent (95% vs 67%) in cats with primary HCM than in those with hyperthyroidism-associated HCMP (P < .01).
Conclusions and clinical importance
The HCMP is the predominant, but not sole, echocardiographic phenotype observed in hyperthyroid cats. The RCMP is the second most frequent phenotype and may reflect a more severe form of hyperthyroidism.
