It is well‐known that increased thyroid hormone (TH) levels induce cardiomyocyte growth. MicroRNAs (miRNAs) have been identified as key players in cardiomyocyte hypertrophy, which is associated with increased risk of heart failure. In this study, we evaluated the miR‐1 expression in TH‐induced cardiac hypertrophy, as well as the potential involvement of miR‐1 in cardiomyocyte hypertrophy elicited by TH in vitro. The possible role of type 1 angiotensin II receptor (AT1R) in the effect promoted by TH in miR‐1 expression was also evaluated. Neonatal rat cardiac myocytes (NRCMs) were treated with T3 for 24 hr and Wistar rats were subjected to hyperthyroidism for 14 days combined or not with AT1R blocker. Real Time RT‐PCR analysis indicated that miR‐1 expression was decreased in cardiac hypertrophy in response to TH in vitro and in vivo, and this effect was unchanged by AT1R blocker. In addition, HDAC4, which is target of miR‐1, was increased in NRCMs after T3 treatment. A gain‐of‐function study revealed that overexpression of miR‐1 prevented T3‐induced cardiomyocyte hypertrophy and reduced HADC4 mRNA levels in NRCMs. In vivo experiments confirmed the downregulation of miR‐1 in cardiac tissue from hyperthyroid animals, which was accompanied by increased HDAC4 mRNA levels. In addition, HDAC inhibitor prevented T3‐induced cardiomyocyte hypertrophy. Our data reveal a new mechanistic insight into cardiomyocyte growth in response to TH, suggesting that miR‐1 plays a role in cardiomyocyte hypertrophy induced by TH potentially via targeting HADC4. A gain‐of‐function study revealed that overexpression of miR‐1 prevented T3‐induced cardiomyocyte hypertrophy and reduced HADC4 mRNA levels in neonatal cardiomyocytes. In vivo experiments confirmed the downregulation of miR‐1 in cardiac tissue from hyperthyroid animals, which was accompanied by increased HDAC4 mRNA levels. HDAC inhibition prevented T3‐induced cardiomyocyte hypertrophy revealing a new mechanistic insight into cardiomyocyte growth in response to TH.