A theoretical model has been adopted to analysis the unsteady thermal effects in a laser medium. It is based on the fact that the internal dissipation power is always unequal to the heat removed from the surface. According to this, the case in a laser diode pumped solid-state laser has been studied. The result obtained shows that the fluctuations in the laser beam strongly depend on those of the temperature in the heat sink. It was also shown that the dissymmetric temperature distribution of the heat sink distorts the shape of the laser beam. A set of experiments has been carefully designed to proved these conclusions.