Resonant cavities are fundamental to and versatile for terahertz integrated systems. So far, integrated resonant cavities have been implemented in relatively lossy terahertz platforms. In this Letter, we propose a series of integrated disk resonators built into a low-loss substrateless silicon waveguide platform, where the resonances and associated quality factor (Q-factor) can be controlled via an effective medium. The measurement results demonstrate that the Q-factor can reach up to 9146 at 274.4 GHz due to the low dissipation of the platform. Additionally, these resonators show strong tunability of the resonance under moderate optical power. These terahertz integrated disk resonators can be employed in sensing and communications.