Thesleff, who works at the University of Helsinki's Institute of Biotechnology has studied the molecular mechanisms that regulate the development of teeth and other organs. She created a mammalian tooth model that has enabled researchers to study fetal development, the mechanisms that lead to developmental disorders as well as the evolution of species.
Irma Thesleff received her doctorate in dentistry in 1975. She served as professor of paediatric dentistry and orthodontics 1990–1995, as Academy Professor 1998–2003, as the director of the Centre of Excellence in Developmental Biology at the University of Helsinki 1999–2001 and as the director of the Academy of Finland Developmental Biology Research Programme Centre of Excellence 2002–2007.
Thesleff received the honorary title of Academician from the Academy of Finland in 2014.
Thesleff has honorary doctorates from McGill University in Montreal, the University of Leuven in Belgium, the University of Debrecen in Hungary the University of Gothenburg and the Karolinska Institutet in Sweden, the University of Oslo in Norway and the University of Copenhagen in Denmark. Thesleff has been made a member of the Finnish Academy of Science and Letters, the Finnish Society of Sciences and Letters, the European Molecular Biology Organization (EMBO), the American Association for the Advancement of Science (AAAS) and the National Academy of Medicine (NAM).
Other Finnish NAS foreign associates include the late Academician Ilkka Hanski from the University of Helsinki, Academy Professor Kari Alitalo from the University of Helsinki, Academician Riitta Hari from Aalto University, Professor Tuija I. Pulkkinen from Aalto University as well as Academician Olli V. Lounasmaa (1930–2002) from the Helsinki University of Technology which later became part of Aalto University.
Other foreign associates who began their research careers at the University of Helsinki but have since emigrated abroad include Albert de la Chapelle, Ari Helenius, Erkki Ruoslahti and Kai Simons.
Established in 1863, the National Academy of Sciences currently has 2,290 active members and 475 foreign associates. Approximately 200 of them are Nobel laureates.