The overall steady-state energy balance with two phases in a flow domain requires that the change in energy of the domain is equal to the difference between the total energy entering the domain and that leaving the domain. From the condition, the integral thermal flux across the surface is studied for a steady thermocapillary drop migration in a flow field with uniform temperature gradient at small and large Marangoni (Reynolds) numbers. The drop is assumed to have only a slight axisymmetric deformation from a sphere. It is identified that a conservative/nonconservative integral thermal flux across the surface in the steady thermocapillary drop migration at small/large Marangoni (Reynolds) numbers. The conservative flux confirms the assumption of quasi-steady state in the thermocapillary drop migration at small Marangoni (Reynolds) numbers. The nonconservative flux may well result from the invalid assumption of quasi-steady state, which indicates that the thermocapillary drop migration at large Marangoni (Reynolds) numbers cannot reach steady state and is thus a unsteady process.