We analyze Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectra in the 750-790 Å wavelength range from 12 different solar regions on the disk to measure the thermal structure of the lower transition region (LTR). We considered four coronal hole (CH), four quiet Sun (QS), and four active region (AR) data sets observed by SUMER during the rising phase of the solar cycle, and we analyzed the emission of seven different ions formed between 2 × 104 and 2 × 105 K. We study the spatial variation of line radiances along the slit within each observation, as well as their relative radiances in different data sets. We also use them to determine the differential emission measure of the LTR. We find that all lines behave in the same way both along the slit within the same observation, and between different data sets from different regions of the Sun. We also find that while the absolute value of the differential emission measure of LTR plasmas changes from region to region, its distribution with temperature is fairly constant, suggesting that the thermal structure of LTR plasmas is the same in CH, QS, and AR regions, and as a function of time along the solar cycle. We discuss the implications of our results for studies of coronal heating and of the solar cycle.