This paper describes the time-varying nature of terahertz communication links between mobile nanodevices, targeting a realistic use case for nanodevice communication within human vessels. We consider a communication link through dipole-like nanoantennas, which flow and rotate in the bloodstream. Such a dynamic scenario causes random glitches in the received power level, resembling a fading-like channel. We present an analytic formulation for the time-variant impulse response and calculate performance metrics like the level crossing rate and the average fade duration. Our findings reveal crossings in the millisecond order and an average duration of fades on the same scale. Our study is the basis for designing robust decoders and error-correcting codes that mitigate the impact of variability on the received power level.