Background To investigate the potential role of free triiodothyronine (FT3) for predicting long-term heart failure (HF) in patients with acute myocardial infarction (AMI).Methods A total of 269 patients admitted to our hospital with AMI were assessed between 1 January 2016 and 31 December 2018 and followed up for a median of 39 months. Patients were categorized into HF group or non-HF group according to the occurrence of HF after AMI. Cox proportional hazards regression models were used to identify factors independently associated with long-term HF. According to the median level of FT3, the patients were segregated into two groups: the low-level group (FT3 ≤ 4.63pmol/L) and the high-level group (FT3 ≥ 4.64pmol/L) and Kaplan-Meier survival analysis was used to estimate the HF-free survival between the two groups. Receiving operating characteristics (ROC) curves were constructed and area under the ROC curve (AUC) was calculated to evaluate the predictive accuracy of FT3 on long-term HF among patients with AMI.Results Compared to those in non-HF group, patients in HF group were significantly older, had higher levels of creatinine and D-Dimer, but lower levels of hemoglobin and FT3, lower LVEF and a lower proportion of PCI treatment (p < 0.05 for all). Multivariable Cox regression analysis suggested that age (HR 1.07, 95% CI 1.04–1.10; p < 0.001), FT3 (HR 0.67, 95% CI 0.47–0.96; p = 0.028), LVEF (HR 0.95, 95% CI 0.93–0.97; p < 0.001) were independent prognostic factors for long-term HF after AMI. A Kaplan-Meier survival analysis demonstrated significantly decreased HF-free survival among patients with lower FT3 level (log-rank p < 0.01). The ROC analysis revealed FT3 had good performance in prediction long-term HF after AMI (AUC = 0.736, 95% CI 0.676–0.797; p < 0.01).Conclusions Lower level of FT3 is not only an independent risk factor for long-term HF after AMI, but also predicts a higher risk of long-term HF following AMI. Our findings are intriguing for long-term medical management of patients with AMI.