This study explores the challenges faced by tertiary education instructors in teaching thermodynamics, with a specific focus on entropy. This study completes and adds to the current literature by providing self-reported perceptions of difficulty by teachers, which is lacking, compared to students' opinions, in this area of research. Through a survey of 41 French-speaking thermodynamics teachers in Belgium (N = 26 full answers, N = 15 partial answers), we report various obstacles in thermodynamics identified by instructors, including content aspects such as their abstract nature, or diverging opinions on mathematical difficulties. For teaching management aspects, instructors report that they have enough resources to teach, but that maybe they have too many non-teaching tasks. For entropy specifically, instructors report that it is challenging due to its abstractness, to its lack of perceptibility, and to the disconnection between macroscopic and microscopic perspectives, among others. 77% of respondents use the disorder metaphor. Invoked reasons include helping students visualize entropy, making it simpler, or making it more concrete. Comparison between teachers' opinions and students' opinions reported by Sözbilir (2004) shows agreements on some difficulties, but also tensions, such as the pedagogical relevance of the lecture format, the need for links with everyday life, or the promotion of conceptual understanding.