Advances in ultrasonic guided waves make it possible to combine the penetration power of guided waves with the early damage detection capabilities of nonlinear ultrasound to interrogate a fairly large area. However, there is a need to re-examine the definition of the relative nonlinearity parameter in the context of SHM. It is known that the definition of the relative nonlinearity parameter relies on the amplitudes of the fundamental and higher harmonic modes and a change in any of these can affect the relative nonlinearity parameter. For SHM methodologies incorporating nonlinear ultrasound, factors such as transducer operating conditions and environmental effects come into play and may affect the nonlinearity parameter even without significant microstructural changes in the material being interrogated. To bridge this gap, the current study aims to investigate the effect of load and temperature changes on guided wave third harmonic generation from shear horizontal (SH) waves in aluminum plates. SH waves were excited using magnetostrictive transducers and third harmonic measurements were made under increasing static-tensile loads at constant temperature and increasing temperature under no load. The results obtained, along with the implications for SHM incorporating nonlinear ultrasound are discussed.