<h3>Purpose</h3> Studies on the efficacy of pre-recovery donor thyroxine (T4) treatment for adult heart transplant recipients report inconsistent results. Guidelines for T4 therapy in donors designated for pediatric heart transplantation (PHT) remain undefined. This study investigates the age-dependent impact of donor T4 therapy on the survival of PHT recipients. <h3>Methods</h3> The UNOS Registry was queried for PHT recipients (<18 yrs) from 1/1994-3/2020. Cohort was divided based on donor T4 therapy status. Covariate balanced propensity scores were generated and used in inverse probability treatment weighting (IPTW) Cox-proportional hazards (PH) model for evaluating effectiveness of T4 therapy on recipient survival outcomes. Cubic spline-fitting with hazard ratios (HR) was performed to assess how the relationship between pre-recovery T4 treatment, donor and recipient age impacted survival. <h3>Results</h3> A total of 7,999 PHT recipients were identified. 51.5% received a T4-treated donor heart. Median age of patients receiving a T4-treated heart (5 yrs) was significantly higher than those transplanted with non-treated hearts (3 yrs; p<0.001). Donors that received T4 treatment were significantly older with a median age of 7 vs. 4 yrs for non-treated donors (p < 0.001). Based on IPTW Cox PH modeling controlling for donor age, recipient age and their interactions, infants (<2 yrs; n=3,090) were found to have a significantly better survival when transplanted with T4-treated hearts by 1.71-fold [95% CI: 1.18-2.47]. However, children (2-12 yrs; n=2864) and adolescent (13-17 yrs; n=3171) had no clear benefit from donor treatment of T4 with HR of 1.16 [0.98-1.37] and 0.96 [0.76-1.22], respectively. <h3>Conclusion</h3> Survival benefit of donor T4 treatment in PHT depends mainly on recipient age, demonstrating a nonlinear relationship according to donor age. Better survival is seen in infants transplanted with T4-treated hearts. Future clinical trials are warranted to confirm indications for T4 therapy.