We measured the thermal distortion induced on an optical probe beam passing through a diode-end-pumped Nd:YVO4 laser crystal under lasing and non-lasing conditions. A Hartmann wavefront analyser was used to measure the probe beam wavefront, which was analysed with a Zernike polynomial basis. The resulting thermal distortions were compared with a model of thermal lensing. For high pump powers, agreement was reasonable. Some limitations in the use of the Hartmann analyser were identified. Our ultimate goal is to use a deformable mirror to compensate in real time for the beam distortions.