This paper investigates the total ionizing dose (TID) responses of forward body bias (FBB) ultrathin body and buried oxide fully depleted silicon-on-insulator (UTBB FD-SOI) transistors, which are commonly used in commercial foundries. The experimental results demonstrate that TID-induced threshold voltage shift in FBB nMOSFET cannot be mitigated by applying biasing to back-gate (back-gate biasing). Bias condition dependence has also been explored, revealing that building up of the oxide-trapped charge (Not) in the buried oxide is affected by the electric field induced by the space charge. Moreover, the effect of threshold voltage option on TID responses of UTBB FD-SOI transistors with the undoped channel was first explored. Finally, the TID-enhanced back-gate biasing controlling of the threshold voltage was observed, which is a new phenomenon for the TID responses of UTBB FD-SOI transistors.