Thirsk was in the family medicine residency program at the Queen Elizabeth Hospital in Montreal when he was selected in December 1983 by the National Research Council of Canada to join the Canadian astronaut program.
Thirsk oli erikoistumassa perhelääkäriksi Queen Elizabeth Hospitalissa, Montrealissa, kun National Research Council of Canada valitsi hänet joulukuussa 1983 Kanadan astronauttiohjelmaan.