We report results of molecular dynamics simulations and detailed analysis of the local structure of sub- and supercritical ammonia in the range of temperature between 250 and 500 K along the 135 bar isobar. This analysis is based on the behavior of distributions of metric and topological properties of the Voronoi polyhedra (VP). We show that by increasing the temperature, the volume, surface, and face area distributions of the Voronoi polyhedra as well as the vacancy radius distribution broaden, particularly near the temperature T-alpha, where the calculated thermal expansion coefficient has its maximum. Furthermore, the rate of increase of the corresponding mean values and fluctuations increases drastically when approaching T-alpha. This behavior clearly indicates that the local structure, as described by the VP, becomes increasingly heterogeneous upon approaching this temperature. This heterogeneous distribution of ammonia molecules is traced back to the increase of the large voids with increasing temperature, and is also clearly seen in the behavior of the fluctuation of the local density, as measured by the VP. More interestingly, the maximum in the heterogeneity coincides with the maximum of the fluctuation in the density of the VP.