The behavior of dielectric loss in SrTiO3 thin films at different microwave frequencies under the dc bias voltage applied is analyzed. Two types of differently processed SrTiO3 thin films (as deposited and oxygen-annealed) deposited by the pulse laser deposition technique onto MgO single crystal substrate are investigated. The half-wave microstrip resonator with the resonance frequencies being 8 and 16 GHz at the first and second resonance modes respectively was used for the microwave characterization of the investigated thin films. The experimental data obtained strongly suggest the occurrence of a crossover between different dielectric loss contributions (extrinsic and intrinsic) in the films. The crossover is driven by the dc bias field: at weak fields the loss is governed by an extrinsic mechanism(s) whereas, at higher fields, the contribution of an intrinsic mechanism (dc field-induced quasi-Debye loss) becomes predominant. The results obtained allowed us to conclude that the quality of tunable ferroelectric films can be judged from the shape of the field dependence of the loss tangent.