We investigate the asynchronous detection of terahertz radiation from a quantum cascade laser using an electro-optic sampling apparatus. The signal does not vary substantially upon rotating the detection crystal, while a characteristic angle dependence is exhibited for synchronized time-domain pulses from a photoconductive emitter. Upon increasing the electrical modulation frequency of the cascade laser the unsynchronized signal decreases in good agreement with a thermal detector. Rather than being electro-optic in origin, we therefore ascribe the unsynchronized signal to a thermo-optic modulation of the refractive index. A simple model is in good agreement with the effect’s frequency dependence.