With the introduction of the new computing curriculum in England, teachers are facing many challenges, among them the teaching of computer programming. Literature suggests that the conceptual understanding of this subject contributes to its difficulty and that threshold concepts, as a source of troublesome knowledge, have a significant role in this. This paper explores computing teachers' perspectives on the Threshold Concept framework and suggests potential threshold concepts in the area of Functions and, more generally, in Procedural Abstraction. A study was conducted, using the Delphi method, including both computing teachers with experience teaching at upper secondary/high school and computing teachers with experience practicing programming in a professional environment for more than 7 years. The results indicate that the majority of the participants support that the Threshold Concept framework can explain students' difficulties in programming and agreed on 11 potential threshold concepts in the area of Functions and Procedural Abstraction. The participants focused more on the troublesome characteristic of threshold concepts and less on the transformative and integrative. Most of the participants also specified that they would change the way they teach a concept if they knew that this is a threshold one. Finally, the paper discusses the findings and how these will shape our future research.