Sepsis is often a cause of mortality in patients admitted to the intensive care unit. Notably, the heart is the organ most susceptible to the impact of sepsis and this condition is referred to as sepsis‑induced cardiomyopathy (SIC). Low triiodothyronine (T3) syndrome frequently occurs in patients with sepsis, and the heart is one of the most important target organs for the action of T3. Phospholamban (PLN) is a key protein associated with Ca2+‑pump‑mediated cardiac diastolic function in the myocardium of mice with SIC, and PLN is negatively regulated by T3. The present study aimed to explore whether T3 can protect cardiac function during sepsis and to investigate the specific molecular mechanism underlying the regulation of PLN by T3. C57BL/6J mice and H9C2 cells were used to establish in vivo and in vitro models, respectively. Myocardial damage was detected via pathological tissue sections, a Cell Counting Kit-8 assay, an apoptosis assay and crystal violet staining. Intracellular calcium levels and reactive oxygen species were detected by Fluo‑4AM and DHE fluorescence. The protein and mRNA expression levels of JNK and c‑Jun were measured by western blotting and reverse transcription‑quantitative PCR to investigate the molecular mechanisms involved. Subsequently, 100 clinical patients were recruited to verify the clinical application value of PLN in SIC. The results revealed a significant negative correlation between PLN and T3 in the animal disease model. Furthermore, the expression levels of genes and proteins in the JNK/c‑Jun signaling pathway and PLN expression levels were decreased, whereas the expression levels of sarcoplasmic reticulum calcium ATPase were increased after T3 treatment. These results indicated that T3 alleviated myocardial injury in SIC by inhibiting PLN expression and its phosphorylation, which may be related to the JNK/c‑Jun signaling pathway. Accordingly, PLN may have clinical diagnostic value in patients with SIC.