NOODI and its widely used estimation toolbox assume the intrinsic diffusivity to a fixed value suitable for healthy adult brains. For broader applicability of the model in neurological diseases it is important to understand the validity of assumed fixed intrinsic diffusivity. Using multi-shell diffusion data we investigated the variability of estimated NODDI indices as well as the model residuals with respect to variations in intrinsic diffusivity. The results suggest significant differences between optimum intrinsic diffusivity for white and gray matter regions as derived from intrinsic diffusivity values that generate smallest model residuals. The variability analysis indicates appreciable differences in the estimated parameters in the range of probable diffusivities predicted by the residual analysis.