Thersia studied at the University of the Free State, where she received a BA degree in 1977, followed by a Higher Teaching Diploma and then also a Diploma in teaching for Gr R (Reception year). She taught Gr 4 at Oranje Girls School in Bloemfontein in 1979 and 1980.
She worked for the South African Broadcasting Corporation from 1981 – 1983, where she was the floor manager for the Children's Programme Department, and also a part time radio presenter of a children's Bible programme.
She has started several Pre-Schools. One for a church in South Hills Johannesburg, where after she taught Gr R at a Pre-School in the same city. In 1991, she started another Pre-School for a church. Later she accepted the position at Sentraal Primary School in Bloemfontein, where she started yet another Pre-School. In 2006 she taught Gr 3 at Sydenham Primary School in Port Elizabeth and then taught Gr R there until 2020.