In this study we theoretically examine thermohydraulic effects on dynamic earthquake rupture. We first derive the system of governing equations assuming a thermoporoelastic medium and then conduct numerical calculations based on these equations. Nonlinear feedback between changes in temperature, fluid pressure, and fault slip are shown to play an important role in rupture dynamics. For example, these feedbacks produce a longer duration of fault slip than that predicted by the classical Griffith crack model assumed in an elastic medium; deviation of our results from those of the Griffith crack model increases with increased thickness of the heated fault zone. The feedback effects also produce slip-weakening behavior and gradual slip onset. The slip-weakening distance increases with increased rate of fluid outflow from the heated fault zone. Our simulations demonstrate that smaller events record smaller static stress drops, consistent with seismological observations. This relationship occurs because ongoing fault slip tends to result in increased fluid pressure. Our simulations also indicate that scaling relationships between small and large earthquakes are complicated by thermohydraulic effects.